| Message ID | 20220729145525.1729066-1-Jason@zx2c4.com |
|---|---|
| State | New |
| Headers | show |
| Series | [RFC,v1] random: implement getrandom() in vDSO | expand |
* Jason A. Donenfeld: > diff --git a/lib/vdso/getrandom.c b/lib/vdso/getrandom.c > new file mode 100644 > index 000000000000..3ffc900f31ff > --- /dev/null > +++ b/lib/vdso/getrandom.c > +static struct getrandom_state *find_free_bucket(struct getrandom_state *buckets) > +{ > + unsigned int start = 0, i; > + > + if (getcpu(&start, NULL, NULL) == 0) > + start %= NUM_BUCKETS; getcpu is not available everywhere. Userspace/libc should probably provide a CPU number hint as an additional argument during the vDSO call. We can load that easily enough from rseq. That's going to be faster on x86, too (the LSL instruction is quite slow). The only advantage of using getcpu like this is that it's compatible with a libc that isn't rseq-enabled. > + for (i = start;;) { > + struct getrandom_state *state = &buckets[i]; > + > + if (cmpxchg(&state->in_use, false, true) == false) > + return state; > + > + i = i == NUM_BUCKETS - 1 ? 0 : i + 1; > + if (i == start) > + break; > + } Surely this scales very badly once the number of buckets is smaller than the system processor count? > +static ssize_t __always_inline > +__cvdso_getrandom(void **state, void *buffer, size_t len, unsigned long flags) > +more_batch: > + batch_len = min_t(size_t, sizeof(s->batch) - s->pos, len); > + if (batch_len) { > + memcpy_and_zero(buffer, s->batch, batch_len); > + s->pos += batch_len; > + buffer += batch_len; > + len -= batch_len; > + if (!len) { > + WRITE_ONCE(s->in_use, false); > + return ret; > + } > + } I expect the main performance benefit comes from not doing any ChaCha20 work except on batch boundaries, not the vDSO acceleration as such. Maybe that's something that should be tried first within the system call implementation. (Even for getrandom with a small buffer, it's not the system call overhead that dominates, but something related to ChaCha20.) Thanks, Florian
Hey Florian, Thanks for the feedback. On Fri, Jul 29, 2022 at 10:19:05PM +0200, Florian Weimer wrote: > > + if (getcpu(&start, NULL, NULL) == 0) > > + start %= NUM_BUCKETS; > > getcpu is not available everywhere. Userspace/libc should probably > provide a CPU number hint as an additional argument during the vDSO > call. We can load that easily enough from rseq. That's going to be > faster on x86, too (the LSL instruction is quite slow). The only > advantage of using getcpu like this is that it's compatible with a libc > that isn't rseq-enabled. Actually, the only requirement is that it's somewhat stable and somehow separates threads most of the time. So a per-thread ID or even a per-thread address would work fine too. Adhemerval suggested on IRC this afternoon that there's a thread pointer register value that would be usable for this purpose. I think what I'll do for v2 is abstract this out to a __arch_get_bucket_hint() function, or similar, which the different archs can fill in. > > + for (i = start;;) { > > + struct getrandom_state *state = &buckets[i]; > > + > > + if (cmpxchg(&state->in_use, false, true) == false) > > + return state; > > + > > + i = i == NUM_BUCKETS - 1 ? 0 : i + 1; > > + if (i == start) > > + break; > > + } > > Surely this scales very badly once the number of buckets is smaller than > the system processor count? Right, and there are a few ways that observation can go: 1) It doesn't matter, because who has > 28 threads all churning at once here? Is that something real? 2) The state variable is controllable by userspace, so in theory different ones could be passed. I don't like this idea though - hard to manage and not enough information to do it well. 3) Since we know when this kind of contention is hit, it should be possible to just expand the map size. Seems a bit complicated. 4) Simply allocate a number of pages relative to the number of CPUs, so that this isn't actually a problem. This seems like the simplest approach and seems fine. Jason
On Fri, Jul 29, 2022 at 3:32 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote: > > Two statements: > > 1) Userspace wants faster cryptographically secure numbers of > arbitrary size, big or small. > > 2) Userspace is currently unable to safely roll its own RNG with the > same security profile as getrandom(). So I'm really not convinced that this kind of thing is something the kernel should work that hard to help. And that state allocation in particular looks very random in all the wrong ways, with various "if I run out of resources I'll just do a system call" things. Which just makes me go "just do the system call". People who are _that_ sensitive to performance can't use this anyway, unless they know the exact rules of "ok, I only need X state buffers" (ok, the buckets are probably the more real "sometimes it might fail" thing). So they basically need to know about the implementation details - even if it's only about that kind of "only a limited number of states" thing. Not to mention that I don't think your patch can work anyway, with things like "cmpxchg()" not being something you can do in the vdso because it might have the kernel instrumentation in it. So this all smells really fragile to me, and honestly, unlike the vdso things we _do_ have, I don't think I've ever seen getrandom() be a huge deal performance-wise. It's just too specialized, and the people who care about performance can - and do - do special things anyway. Your patch fundamentally seems to be about "make it easy to not have to care, and still get high performance", but that's _such_ a small use-case (the intersection between "don't care" and "really really need high performance" would seem to be basically NIL). Linus
Hey Linus, Thanks a bunch for chiming in. Indeed this whole thing is kind of crazy, so your input is particularly useful here. On Sat, Jul 30, 2022 at 08:48:42AM -0700, Linus Torvalds wrote: > It's just too specialized, and the people who care about performance > can - and do - do special things anyway. I followed most of your email, but I just wanted to point out that the "can" part of this isn't quite right, though the "do" part is. Specifically, I don't think there's currently a good way for userspace to do this kind of thing and get the same kind of security guarantees that the syscall has. They "do" it anyway, though (openssl, libgcrypt, glibc's arc4random() implementation before I tamed it last week, etc), and this is somewhat concerning. So my larger intent is, assuming that people will continue to attempt such things, to just nip the issue in the bud by providing an actually safe way for it to be done. To be clear, I really would rather not do this. I'm not really looking for more stuff to do, and I don't tend to write (public) code "just 'cuz". My worry is that by /not/ doing it, footguns will proliferate. The glibc thing was what finally motivated me to want to at least sketch out a potential action to make this kind of (apparently common) urge of writing a userspace RNG safer. (Actually coding it up didn't really take much time, which perhaps shows: that `if (!len)` check needs to be hoisted out of the inner block!) > So I'm really not convinced that this kind of thing is something the > kernel should work that hard to help. > > Your patch fundamentally seems to be about "make it easy to not have > to care, and still get high performance", but that's _such_ a small > use-case (the intersection between "don't care" and "really really > need high performance" would seem to be basically NIL). So this is "statement (1)" stuff. Namely, userspace apparently wants faster random numbers. Is this desire justified? Has anybody aside from Phoronix even benchmarked getrandom() since I did the neat lockless stuff to it? Is this just for some artificial card shuffling unit tests, or is generating TLS CBC nonces at scale using getrandom() a real bottleneck for a real use case? I'm honestly not quite sure. But I do know that people are building these userspace RNGs anyway, and will keep building them, and that kind of worries me. So either this is a useful thing to have, and people are building it anyway, so maybe the kernel should get involved. Or it's not a useful thing to have, BUT people are building it anyway, so maybe the kernel should [not?] get involved? The latter case is a bit decisionally hairier. Anyway, onto the technical feedback: > And that state allocation in particular looks very random in all the > wrong ways, with various "if I run out of resources I'll just do a > system call" things. > > Not to mention that I don't think your patch can work anyway, with > things like "cmpxchg()" not being something you can do in the vdso > because it might have the kernel instrumentation in it. Yea this sharding thing is somewhat faulty. In its current inception, it also falls over during fork, since the cmpxchg pseudo trylock is dropped, among other problems Andy and I discussed on IRC. Andy also suggested not doing the allocation inside of the same function. Florian brought up the difficulty of even determining the CPU number on arm64. And also that's a good point about instrumentation on cmpxchg. So, anyway, if I do muster a v2 of this (perhaps just to see the idea through), the API might split in two to something like: void *getrandom_allocate_states([inout] size_t *number_of_states, [out] size_t *length_per_state); ssize_t getrandom(void *state, void *buffer, size_t len, unsigned long flags); User code will call getrandom_allocate_state(), which will allocate enough pages to hold *number_of_states, and return the size of each one in length_per_state and the number actually allocated back in number_of_states. The result can then be sliced up by that size, and passed to getrandom(). So glibc or whatever would presumably allocate one per thread, and handle any reentrancy/locking around it. Or some other variation on that. I'm sure you hate those function signatures. Everybody loves to bikeshed APIs, right? There's plenty to be tweaked. But that's anyhow about where my thinking is for a potential v2. Jason
On Sun, Jul 31, 2022 at 01:45:43AM +0200, Jason A. Donenfeld wrote: > So, anyway, if I do muster a v2 of this (perhaps just to see the idea > through), the API might split in two to something like: > > void *getrandom_allocate_states([inout] size_t *number_of_states, [out] size_t *length_per_state); > ssize_t getrandom(void *state, void *buffer, size_t len, unsigned long flags); > > User code will call getrandom_allocate_state(), which will allocate > enough pages to hold *number_of_states, and return the size of each one > in length_per_state and the number actually allocated back in > number_of_states. The result can then be sliced up by that size, and > passed to getrandom(). So glibc or whatever would presumably allocate > one per thread, and handle any reentrancy/locking around it. > > Or some other variation on that. I'm sure you hate those function > signatures. Everybody loves to bikeshed APIs, right? There's plenty to > be tweaked. But that's anyhow about where my thinking is for a potential > v2. Doing this also doubled performance, perhaps unsurprisingly, as that getcpu() operation wasn't free. For uint32_t generation: vdso: 25000000 times in 0.289876265 seconds syscall: 25000000 times in 4.296636025 seconds
Jason! On Sun, Jul 31 2022 at 01:45, Jason A. Donenfeld wrote: > Thanks a bunch for chiming in. Indeed this whole thing is kind of crazy, > so your input is particularly useful here. > > On Sat, Jul 30, 2022 at 08:48:42AM -0700, Linus Torvalds wrote: >> It's just too specialized, and the people who care about performance >> can - and do - do special things anyway. > > To be clear, I really would rather not do this. I'm not really looking > for more stuff to do, and I don't tend to write (public) code "just > 'cuz". My worry is that by /not/ doing it, footguns will proliferate. > The glibc thing was what finally motivated me to want to at least sketch > out a potential action to make this kind of (apparently common) urge of > writing a userspace RNG safer. But the user space tinkering will continue no matter what. They might then just use the vdso to get access to the ready/generation bits. I've seen "better" VDSO implementations to access time. :) > So, anyway, if I do muster a v2 of this (perhaps just to see the idea > through), the API might split in two to something like: > > void *getrandom_allocate_states([inout] size_t *number_of_states, [out] size_t *length_per_state); > ssize_t getrandom(void *state, void *buffer, size_t len, unsigned long flags); I'm not seeing any reason to have those functions at all. The only thing which would be VDSO worthy here is the access to random_state->ready and random_state->generation as that's the information which is otherwise not available to userspace. So you can just have: int random_check_and_update_generation(u64 *generation); Everything else is library material, really. Thanks, tglx
On Mon, Aug 01, 2022 at 09:30:20PM +0200, Thomas Gleixner wrote: > Jason! Thomas! > > So, anyway, if I do muster a v2 of this (perhaps just to see the idea > > through), the API might split in two to something like: > > > > void *getrandom_allocate_states([inout] size_t *number_of_states, [out] size_t *length_per_state); > > ssize_t getrandom(void *state, void *buffer, size_t len, unsigned long flags); > > I'm not seeing any reason to have those functions at all. > > The only thing which would be VDSO worthy here is the access to > random_state->ready and random_state->generation as that's the > information which is otherwise not available to userspace. I think you might have missed the part of the patch message where I discuss this. I'm happy to talk about that more, but it might help the discussion to refer to the parts already addressed. Reproduced here: | How do we rectify this? By putting a safe implementation of getrandom() | in the vDSO, which has access to whatever information a | particular iteration of random.c is using to make its decisions. I use | that careful language of "particular iteration of random.c", because the | set of things that a vDSO getrandom() implementation might need for making | decisions as good as the kernel's will likely change over time. This | isn't just a matter of exporting certain *data* to userspace. We're not | going to commit to a "data API" where the various heuristics used are | exposed, locking in how the kernel works for decades to come, and then | leave it to various userspaces to roll something on top and shoot | themselves in the foot and have all sorts of complexity disasters. | Rather, vDSO getrandom() is supposed to be the *same exact algorithm* | that runs in the kernel, except it's been hoisted into userspace as | much as possible. And so vDSO getrandom() and kernel getrandom() will | always mirror each other hermetically. To reiterate, I don't want to commit to a particular data API, or even to an ideal interplay between kernel random and user random. I'd like to retain the latitude to change the semantics there considerably, so that Linux isn't locked into one RNG design forever. I think that kind of lock in would be a mistake. For example, just the generation counter alone won't do it (as I mentioned later on in the message; the RFC patch is somewhat incomplete). Rather, the interface I'm fine committing to would be the higher level getrandom(), with maybe an added state parameter, which doesn't expose any guts about what it's actually doing. Comex (CC'd) described in a forum comment the idea (and perhaps vDSO in general?) as a little more akin to system libraries on Windows or macOS, which represent the OS barrier, rather than the raw system call. Such libraries then can operate on private data as necessary. So in that sense, this patch here isn't very Linuxy (which Comex described as a potentially positive thing, but I assume you disagree). Anyway, I guess it in large part isn't so dissimilar to decisions you made around other vDSO functions, where to draw the barrier, etc. Why not just have an accessor for each vvar struct member and leave it to userspaces to implement? Well, that'd probably be a terrible idea for various reasons, and I feel the same way about exposing too many getrandom() guts. > So you can just have: > > int random_check_and_update_generation(u64 *generation); > > Everything else is library material, really. Not very appealing for the reasons mentioned above, but also for the record, I may like this idea for a closely related thing, vmgenid, but that's a different conversation I'll get back to another time. Jason
Jason! On Tue, Aug 02 2022 at 01:16, Jason A. Donenfeld wrote: > On Mon, Aug 01, 2022 at 09:30:20PM +0200, Thomas Gleixner wrote: >> > So, anyway, if I do muster a v2 of this (perhaps just to see the idea >> > through), the API might split in two to something like: >> > >> > void *getrandom_allocate_states([inout] size_t *number_of_states, [out] size_t *length_per_state); >> > ssize_t getrandom(void *state, void *buffer, size_t len, unsigned long flags); >> >> I'm not seeing any reason to have those functions at all. >> >> The only thing which would be VDSO worthy here is the access to >> random_state->ready and random_state->generation as that's the >> information which is otherwise not available to userspace. > > I think you might have missed the part of the patch message where I > discuss this. I'm happy to talk about that more, but it might help the > discussion to refer to the parts already addressed. Reproduced here: I did not miss this. I carefully read it. > To reiterate, I don't want to commit to a particular data API, or even > to an ideal interplay between kernel random and user random. I'd like to > retain the latitude to change the semantics there considerably, so that > Linux isn't locked into one RNG design forever. I think that kind of > lock in would be a mistake. For example, just the generation counter > alone won't do it (as I mentioned later on in the message; the RFC patch > is somewhat incomplete). Rather, the interface I'm fine committing to > would be the higher level getrandom(), with maybe an added state > parameter, which doesn't expose any guts about what it's actually doing. > > Comex (CC'd) described in a forum comment the idea (and perhaps vDSO in > general?) as a little more akin to system libraries on Windows or macOS, > which represent the OS barrier, rather than the raw system call. Such > libraries then can operate on private data as necessary. So in that > sense, this patch here isn't very Linuxy (which Comex described as a > potentially positive thing, but I assume you disagree). > > Anyway, I guess it in large part isn't so dissimilar to decisions you > made around other vDSO functions, where to draw the barrier, etc. Why > not just have an accessor for each vvar struct member and leave it to > userspaces to implement? Well, that'd probably be a terrible idea for > various reasons, and I feel the same way about exposing too many > getrandom() guts. I surely understand your goal, but the real question is where we draw the line and what kind of functionality should go into such a library and what's the required justification for it. The concept of system libraries on Windows NT was to provide different APIs for application programmers: Win32, OS/2, Posix. That allowed to change the actual syscalls without breaking existing applications. IOW, it's just a glue layer which translates between application API and syscall API. Right now the Linux VDSO functions are 1:1 replacements for system calls and not adding a magic pile of functionality which is otherwise not available. What you are proposing is to have an implementation which is not available via a regular syscall. Which means you are creating a VDSO only syscall which still has the same problem as any other syscall in terms of API design and functionality which needs to be supported forever. Thanks, tglx
Hi Thomas, On Tue, Aug 02, 2022 at 03:46:27PM +0200, Thomas Gleixner wrote: > Right now the Linux VDSO functions are 1:1 replacements for system calls > and not adding a magic pile of functionality which is otherwise not > available. > > What you are proposing is to have an implementation which is not > available via a regular syscall. Which means you are creating a VDSO > only syscall which still has the same problem as any other syscall in > terms of API design and functionality which needs to be supported > forever. Wait, what? That's not correct. The WHOLE point is that vdso getrandom() will generate bytes in the same way as the ordinary syscall, without differences. Same function name, same algorithm. But just faster, because vDSO. I explicitly don't want to dip into introducing something different. That's the big selling point: that vDSO getrandom() and syscall getrandom() are the same thing. If you trust one, you can trust the other. If you expect properties of one, you get that from the other. If you know the API of one, you can use the other. There might be other valid objections to this whole thing, but "this is different from the syscall" really isn't one of them. It's the same ideaspace that motivated gettimeofday() and such. Jason
On Tue, Aug 02 2022 at 15:59, Jason A. Donenfeld wrote: > On Tue, Aug 02, 2022 at 03:46:27PM +0200, Thomas Gleixner wrote: >> Right now the Linux VDSO functions are 1:1 replacements for system calls >> and not adding a magic pile of functionality which is otherwise not >> available. >> >> What you are proposing is to have an implementation which is not >> available via a regular syscall. Which means you are creating a VDSO >> only syscall which still has the same problem as any other syscall in >> terms of API design and functionality which needs to be supported >> forever. > > Wait, what? That's not correct. The WHOLE point is that vdso getrandom() > will generate bytes in the same way as the ordinary syscall, without > differences. Same function name, same algorithm. But just faster, > because vDSO. I explicitly don't want to dip into introducing something > different. That's the big selling point: that vDSO getrandom() and > syscall getrandom() are the same thing. If you trust one, you can trust > the other. If you expect properties of one, you get that from the other. > If you know the API of one, you can use the other. Seriously no. All existing VDSO functions have exactly the same function signature and semantics as their syscall counterparts. So they are drop in equivalent. But: ssize_t getrandom(void *, void *, size_t, unsigned int); is very much different than ssize_t getrandom(void *, size_t, unsigned int); Different signature and different semantics. So you have to go through the whole process of a new ABI whether you like it or not. It does not matter whether they both produce random numbers. If your argument would hold true, then you can also claim that openat(2) and openat2(2) are the same thing because they both open a file. Thanks, tglx
Hi Thomas, On Tue, Aug 2, 2022 at 5:14 PM Thomas Gleixner <tglx@linutronix.de> wrote: > Seriously no. Why so serious all at once? :-) > All existing VDSO functions have exactly the same function > signature and semantics as their syscall counterparts. So they are drop > in equivalent. > > But: > > ssize_t getrandom(void *, void *, size_t, unsigned int); > > is very much different than > > ssize_t getrandom(void *, size_t, unsigned int); > > Different signature and different semantics. Different signature, but basically the same semantics. > So you have to go through the whole process of a new ABI whether you > like it or not. Ahh, in that sense. Yea, I'd rather not have to do that too, with the additional opaque handle passed as the first argument. It'd be nice if there were some private place where I could store the necessary state, but I'm not really sure where that might be at the moment. If you have any ideas, please let me know. Jason
Jason! On Tue, Aug 02 2022 at 17:26, Jason A. Donenfeld wrote: > On Tue, Aug 2, 2022 at 5:14 PM Thomas Gleixner <tglx@linutronix.de> wrote: >> Seriously no. > > Why so serious all at once? :-) Because you triggered the 'now it gets serious' button with your "it's the same" sentiment. >> All existing VDSO functions have exactly the same function >> signature and semantics as their syscall counterparts. So they are drop >> in equivalent. >> >> But: >> >> ssize_t getrandom(void *, void *, size_t, unsigned int); >> >> is very much different than >> >> ssize_t getrandom(void *, size_t, unsigned int); >> >> Different signature and different semantics. > > Different signature, but basically the same semantics. Not at all. The concept of 'basically same semantics' is a delusion. It does not exist. Either it's the same or it's not. I really want to see your reaction on a claim that some RNG implementation is basically the same as the existing one. I'm sure you buy that without complaints. >> So you have to go through the whole process of a new ABI whether you >> like it or not. > > Ahh, in that sense. Yea, I'd rather not have to do that too, with the > additional opaque handle passed as the first argument. It'd be nice if > there were some private place where I could store the necessary state, > but I'm not really sure where that might be at the moment. If you have > any ideas, please let me know. That's exactly the problem. VDSO is a stateless syscall wrapper which has to be self contained for obvious reasons. My previous statement: Everything else is library material, really. is based on that fact and not on the unwillingness to add magic muck to the VDSO. The unwillingness part is just the question: Is there a sensible usecase? Assumed that there is a sensible usecase, there is a way out and that's exactly the library part. You can make that VDSO interface versioned and provide a library in tools/random/ which goes in lockstep with the VDSO changes. If the RNG tinkerers abuse that, then so be it. You can't do anything about it whatever you try. They can abuse your magic vdso functionality too. That's very much the same as we have with e.g. perf. The old perf binary still works, but it does not have access to the latest and greatest features. You can do very much the same in a kernel supplied helper library which either can cope with the version change or falls back to sys_getrandom(). Vs. the storage problem. That yells TLS, but that makes your process wide sharing moot, which might not be the worst of all things IMO. Thanks, tglx
On Wed, Aug 03, 2022 at 12:27:43AM +0200, Thomas Gleixner wrote: > Jason! Thomas!! > Not at all. The concept of 'basically same semantics' is a delusion. It > does not exist. Either it's the same or it's not. > > I really want to see your reaction on a claim that some RNG > implementation is basically the same as the existing one. I'm sure you > buy that without complaints. I mean there are some undeniable similarities here. We might be arguing over semantics of semantics at this point, but... Barring the additional `void *state` argument, they take the same inputs and produce the same outputs. Then, most importantly, the way they each produce randomness is the same, using the same algorithms and same lifetime rules. For all measures that are meaningful to me, they're the "same". Yes yes technically that extra `void *state` means something is slightly different. But does that matter? I guess your point is that somehow it does matter, and maybe that's where we disagree. > >> So you have to go through the whole process of a new ABI whether you > >> like it or not. > > > > Ahh, in that sense. Yea, I'd rather not have to do that too, with the > > additional opaque handle passed as the first argument. It'd be nice if > > there were some private place where I could store the necessary state, > > but I'm not really sure where that might be at the moment. If you have > > any ideas, please let me know. > > That's exactly the problem. VDSO is a stateless syscall wrapper which > has to be self contained for obvious reasons. Unless each call can import and export any potential state. That is, unless there's a `void *state` argument like what I added. Then the code itself doesn't refer to any global state, but can still behave in a stateful manner. We could even establish this as a "convention" if necessary, and document it as such. That way upgrading a syscall ABI to a vDSO stateful ABI always follows some rule, and then this doesn't feel as ad-hoc. Heck, it could even be an opaque type, `vdso_handle_t state`, which would just be a unsigned long. > My previous statement: > > Everything else is library material, really. > > is based on that fact and not on the unwillingness to add magic muck to > the VDSO. > > The unwillingness part is just the question: > > Is there a sensible usecase? In the use case department, by the way, apparently there really is. arc4random() is too slow for chronyd: https://sourceware.org/bugzilla/show_bug.cgi?id=29437 And that's on a kernel even with the "newer faster getrandom()". > Assumed that there is a sensible usecase, there is a way out and that's > exactly the library part. You can make that VDSO interface versioned and > provide a library in tools/random/ which goes in lockstep with the VDSO > changes. That sounds absolutely dreadful; no way jose. Then we wind up having to maintain the data in the vDSO as a particular version, and keep that working into the future. That's not going to fly. As I wrote before, I don't want to commit the RNG to preserving certain internal semantics over time. That's not something I feel comfortable committing to. And even if we can "add" new ones with a new version in your hypothetical scheme, we'd still have to keep the old ones working, and that could prove prohibitive of improvements. So that's not going to work. And again, I don't see how this is any different than gettimeofday() and such. Why didn't you just make versioned accessor functions for each one of the various struct fields, and then stick some library code into tools/timekeeping/ for glibc to copy and paste once and never update again? Why isn't this a nightmare you chose to have each time the moon is full? Clearly because doing so would be a maintenance disaster and would impede future meaningful progress, not to mention proliferating wrong means of reading the time. Just as gettimeofday benefits from being an actual function in the vDSO, so too does getrandom(). > Vs. the storage problem. That yells TLS, but that makes your process > wide sharing moot, which might not be the worst of all things IMO. Yea, TLS is what we want here. The `void *state` argument thing is meant for this. You allocate an array of states using that alloc function, and then you divvy them up per-thread. Jason
On Thu, Aug 4, 2022 at 11:40 AM Jason A. Donenfeld <Jason@zx2c4.com> wrote: > > On Wed, Aug 03, 2022 at 12:27:43AM +0200, Thomas Gleixner wrote: > > Jason! > > ... > > Vs. the storage problem. That yells TLS, but that makes your process > > wide sharing moot, which might not be the worst of all things IMO. > > Yea, TLS is what we want here. The `void *state` argument thing is meant > for this. You allocate an array of states using that alloc function, and > then you divvy them up per-thread. I think it would be wise to give each thread its own state. It will simplify locking and help avoid contention. Jeff
Jason! On Thu, Aug 04 2022 at 17:23, Jason A. Donenfeld wrote: > On Wed, Aug 03, 2022 at 12:27:43AM +0200, Thomas Gleixner wrote: >> Not at all. The concept of 'basically same semantics' is a delusion. It >> does not exist. Either it's the same or it's not. >> >> I really want to see your reaction on a claim that some RNG >> implementation is basically the same as the existing one. I'm sure you >> buy that without complaints. > > I mean there are some undeniable similarities here. Similarity yes. Semantically in the sense of an API definitely not. > We might be arguing over semantics of semantics at this point, > but... Barring the additional `void *state` argument, they take the > same inputs and produce the same outputs. Then, most importantly, the > way they each produce randomness is the same, using the same > algorithms and same lifetime rules. For all measures that are > meaningful to me, they're the "same". Yes yes technically that extra > `void *state` means something is slightly different. But does that > matter? I guess your point is that somehow it does matter, and maybe > that's where we disagree. The point is that the API is different and you can't provide a API compatible variant in the VDSO ever. This matters because it requires auxilliary code outside of the VDSO or even application changes. >> >> So you have to go through the whole process of a new ABI whether you >> >> like it or not. >> > >> > Ahh, in that sense. Yea, I'd rather not have to do that too, with the >> > additional opaque handle passed as the first argument. It'd be nice if >> > there were some private place where I could store the necessary state, >> > but I'm not really sure where that might be at the moment. If you have >> > any ideas, please let me know. >> >> That's exactly the problem. VDSO is a stateless syscall wrapper which >> has to be self contained for obvious reasons. > > Unless each call can import and export any potential state. That is, > unless there's a `void *state` argument like what I added. Then the code > itself doesn't refer to any global state, but can still behave in a > stateful manner. Which makes it an incompatible API with an untyped pointer which is sloppy at best. >> The unwillingness part is just the question: >> >> Is there a sensible usecase? > > In the use case department, by the way, apparently there really is. > arc4random() is too slow for chronyd: > https://sourceware.org/bugzilla/show_bug.cgi?id=29437 > And that's on a kernel even with the "newer faster getrandom()". For a problem which does not require cryptographically secure random numbers. Truly convincing. >> Assumed that there is a sensible usecase, there is a way out and that's >> exactly the library part. You can make that VDSO interface versioned and >> provide a library in tools/random/ which goes in lockstep with the VDSO >> changes. > > That sounds absolutely dreadful; no way jose. Then we wind up having to > maintain the data in the vDSO as a particular version, and keep that > working into the future. That's not going to fly. As I wrote before, I > don't want to commit the RNG to preserving certain internal semantics > over time. That's not something I feel comfortable committing to. And > even if we can "add" new ones with a new version in your hypothetical > scheme, we'd still have to keep the old ones working, and that could > prove prohibitive of improvements. So that's not going to work. That's a complete nonsensical argumentation. If you install a new kernel, then the new library which exposes the application interface is installed too. So where is the problem? That needs obviously some infrastructure so that the kernel install handles such libraries similar to modules, but that would be a worthwhile exercise if we want to expose more information through VDSO. Such a library exposes the API and the interface between the library and the VDSO is kernel dependent. They are shipped in lockstep. That way you avoid to implement a ton of syscall fallbacks for this muck and just treat it as any other regular library. You even can hide the whole allocation completely and make it a drop in replacement: getrandom(....) if (!tls->random_state) alloc_tls_random_state() .... A real library can do that, VDSO not. The only version information in that data interface would be: struct vdso_random_data { unsigned int version; .... }; All other fields are version dependent and you can change them as you want. If some tinkerer uses the data interface directly and ignores the version check, then he can keep the pieces. That's not any different than the timekeeping data interface of the VDSO. I've seen code which accesses the timekeeping VDSO data directly just because some performance expert decided that rdtsc_ordered() is too slow and a trivial rdtsc() is good enough. That exploded in their face when we overhauled the VDSO. Can you prevent that? No. Does it matter? No. The data interface is documented to be for the kernel supplied library only and the version field is only for paranoia reasons to validate that kernel and library agree. That means if some tinkerer uses the data interface directly then he either does a version check and bails out when the version is different from the expected version or he falls flat on his face. That's a very simple contract. Versioned interfaces are not required to be backwards compatible. That's a matter of definition and documentation. > And again, I don't see how this is any different than gettimeofday() and > such. Why didn't you just make versioned accessor functions for each one > of the various struct fields, and then stick some library code into > tools/timekeeping/ for glibc to copy and paste once and never update > again? Why isn't this a nightmare you chose to have each time the moon > is full? Clearly because doing so would be a maintenance disaster and > would impede future meaningful progress, not to mention proliferating > wrong means of reading the time. The time accessors have been proven to be performance critical and the implementation is a drop in replacement for the syscall so that an extra library is pointless and less performant. But you are neither providing a convincing use case nor a drop in replacement simply because it's technically impossible in the VDSO to provide that with the features you want to add. > Just as gettimeofday benefits from being an actual function in the vDSO, > so too does getrandom(). time*() == vsdo_time*() but getrandom() != vdso_magic_getrandom() Can you spot the difference? >> Vs. the storage problem. That yells TLS, but that makes your process >> wide sharing moot, which might not be the worst of all things IMO. > > Yea, TLS is what we want here. The `void *state` argument thing is meant > for this. You allocate an array of states using that alloc function, and > then you divvy them up per-thread. How does the first caller know how many threads are going to use this and how is the pointer handed over to another thread?. TLS == Thread Local Storage which unsurprisingly means that such a pointer is thread private. Which part of the code is going to do that pointer sharing? Are you going to hide that in the VDSO library too? You can't. If you want to use TLS which is the only sensible solution then you have to do the allocation per thread which avoids all the sharding issues and screams even more for a lib_getrandom_$kernel_version.so solution. But before we go there, please provide a sensible use case. Thanks, tglx
* Thomas Gleixner: > On Tue, Aug 02 2022 at 15:59, Jason A. Donenfeld wrote: >> On Tue, Aug 02, 2022 at 03:46:27PM +0200, Thomas Gleixner wrote: >>> Right now the Linux VDSO functions are 1:1 replacements for system calls >>> and not adding a magic pile of functionality which is otherwise not >>> available. >>> >>> What you are proposing is to have an implementation which is not >>> available via a regular syscall. Which means you are creating a VDSO >>> only syscall which still has the same problem as any other syscall in >>> terms of API design and functionality which needs to be supported >>> forever. >> >> Wait, what? That's not correct. The WHOLE point is that vdso getrandom() >> will generate bytes in the same way as the ordinary syscall, without >> differences. Same function name, same algorithm. But just faster, >> because vDSO. I explicitly don't want to dip into introducing something >> different. That's the big selling point: that vDSO getrandom() and >> syscall getrandom() are the same thing. If you trust one, you can trust >> the other. If you expect properties of one, you get that from the other. >> If you know the API of one, you can use the other. > > Seriously no. All existing VDSO functions have exactly the same function > signature and semantics as their syscall counterparts. So they are drop > in equivalent. > > But: > > ssize_t getrandom(void *, void *, size_t, unsigned int); > > is very much different than > > ssize_t getrandom(void *, size_t, unsigned int); > > Different signature and different semantics. Just use ssize_t getrandom(size_t, unsigned int, void *); then and have the system call ignore the argument. There is recent precedent for adding additional arguments to system calls, see membarrier. If we want to be super-conservative, we could add a new flag and have the vDSO version always call into the kernel if the flag isn't set. *This* part is far less problematic compared to the approach to per-thread memory allocation. We now have: * Explicit donation of memory areas to the kernel (set_robust_list, rseq). * This getrandom_alloc vDSO call which does something unspecified and may return pointers which are or are not abstract. (How is CRIU expected to handle this?) * There's also userspace shadow stack coming. I think the kernel moved away from implicit allocation, to something mmap-based. It's not clear to me why that would be okay here, but not for shadow stacks. Does io_uring have to handle a similar problem, too? As long as the vDSO doesn't use private system calls, I don't expect any practical problems, but this optimization doesn't really look to me like something that intrinsically benefits from a completely new way of allocating userspace memory for use by the kernel. Thanks, Florian
On Wed, Aug 03, 2022 at 12:27:43AM +0200, Thomas Gleixner wrote: > Assumed that there is a sensible usecase, there is a way out and that's > exactly the library part. You can make that VDSO interface versioned and > provide a library in tools/random/ which goes in lockstep with the VDSO > changes. Well.... there is still the old idea of making a genuine libkernel.so which is part of the kernel and provided as a virtual file (say in /sys/lib/). Since that's perfectly in lock-step with the kerne image, it can be used to avoid the vdso data layout compat issues. Only the actual symbols provided by the library are ABI constrained, not their implementation.
diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile index 76cd790ed0bd..a60d4771d500 100644 --- a/arch/x86/entry/vdso/Makefile +++ b/arch/x86/entry/vdso/Makefile @@ -24,7 +24,7 @@ VDSO32-$(CONFIG_X86_32) := y VDSO32-$(CONFIG_IA32_EMULATION) := y # files to link into the vdso -vobjs-y := vdso-note.o vclock_gettime.o vgetcpu.o +vobjs-y := vdso-note.o vclock_gettime.o vgetcpu.o vgetrandom.o vobjs32-y := vdso32/note.o vdso32/system_call.o vdso32/sigreturn.o vobjs32-y += vdso32/vclock_gettime.o vobjs-$(CONFIG_X86_SGX) += vsgx.o diff --git a/arch/x86/entry/vdso/vdso.lds.S b/arch/x86/entry/vdso/vdso.lds.S index 4bf48462fca7..1919cc39277e 100644 --- a/arch/x86/entry/vdso/vdso.lds.S +++ b/arch/x86/entry/vdso/vdso.lds.S @@ -28,6 +28,8 @@ VERSION { clock_getres; __vdso_clock_getres; __vdso_sgx_enter_enclave; + getrandom; + __vdso_getrandom; local: *; }; } diff --git a/arch/x86/entry/vdso/vgetrandom.c b/arch/x86/entry/vdso/vgetrandom.c new file mode 100644 index 000000000000..40389c399c6a --- /dev/null +++ b/arch/x86/entry/vdso/vgetrandom.c @@ -0,0 +1,16 @@ +// SPDX-License-Identifier: GPL-2.0-only +/* + * Copyright (C) 2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved. + */ +#include <linux/kernel.h> +#include <linux/types.h> + +#include "../../../../lib/vdso/getrandom.c" + +ssize_t __vdso_getrandom(void **state, void *buffer, size_t len, unsigned long flags) +{ + return __cvdso_getrandom(state, buffer, len, flags); +} + +ssize_t getrandom(void **, void *, size_t, unsigned long) + __attribute__((weak, alias("__vdso_getrandom"))); diff --git a/arch/x86/include/asm/vdso/getrandom.h b/arch/x86/include/asm/vdso/getrandom.h new file mode 100644 index 000000000000..83cb483aab74 --- /dev/null +++ b/arch/x86/include/asm/vdso/getrandom.h @@ -0,0 +1,74 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +/* + * Copyright (C) 2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved. + */ +#ifndef __ASM_VDSO_GETRANDOM_H +#define __ASM_VDSO_GETRANDOM_H + +#ifndef __ASSEMBLY__ + +#include <asm/unistd.h> +#include <asm/vvar.h> + +static __always_inline ssize_t +getrandom_syscall(void *buffer, size_t len, unsigned long flags) +{ + long ret; + + asm ("syscall" : "=a" (ret) : + "0" (__NR_getrandom), "D" (buffer), "S" (len), "d" (flags) : + "rcx", "r11", "memory"); + + return ret; +} + +static __always_inline void * +mmap_syscall(void *addr, size_t len, int prot, int flags, int fd, off_t offset) +{ + long ret; + register long r10 asm("r10") = flags; + register long r8 asm("r8") = fd; + register long r9 asm("r9") = offset; + + asm ("syscall" : "=a" (ret) : + "0" (__NR_mmap), "D" (addr), "S" (len), "d" (prot), + "r" (r10), "r" (r8), "r" (r9) : + "rcx", "r11"); + + return (void *)ret; +} + +static __always_inline int +munmap_syscall(void *addr, size_t len) +{ + long ret; + + asm ("syscall" : "=a" (ret) : + "0" (__NR_munmap), "D" (addr), "S" (len) : + "rcx", "r11"); + + return ret; +} + +static __always_inline int +madvise_syscall(void *addr, size_t len, int advice) +{ + long ret; + + asm ("syscall" : "=a" (ret) : + "0" (__NR_madvise), "D" (addr), "S" (len), "d" (advice) : + "rcx", "r11"); + + return ret; +} + +#define __vdso_rng_data (VVAR(_vdso_rng_data)) + +static __always_inline const struct vdso_rng_data *__arch_get_vdso_rng_data(void) +{ + return &__vdso_rng_data; +} + +#endif /* !__ASSEMBLY__ */ + +#endif /* __ASM_VDSO_GETRANDOM_H */ diff --git a/arch/x86/include/asm/vvar.h b/arch/x86/include/asm/vvar.h index 183e98e49ab9..9d9af37f7cab 100644 --- a/arch/x86/include/asm/vvar.h +++ b/arch/x86/include/asm/vvar.h @@ -26,6 +26,8 @@ */ #define DECLARE_VVAR(offset, type, name) \ EMIT_VVAR(name, offset) +#define DECLARE_VVAR_SINGLE(offset, type, name) \ + EMIT_VVAR(name, offset) #else @@ -37,6 +39,10 @@ extern char __vvar_page; extern type timens_ ## name[CS_BASES] \ __attribute__((visibility("hidden"))); \ +#define DECLARE_VVAR_SINGLE(offset, type, name) \ + extern type vvar_ ## name \ + __attribute__((visibility("hidden"))); \ + #define VVAR(name) (vvar_ ## name) #define TIMENS(name) (timens_ ## name) @@ -44,12 +50,22 @@ extern char __vvar_page; type name[CS_BASES] \ __attribute__((section(".vvar_" #name), aligned(16))) __visible +#define DEFINE_VVAR_SINGLE(type, name) \ + type name \ + __attribute__((section(".vvar_" #name), aligned(16))) __visible + #endif /* DECLARE_VVAR(offset, type, name) */ DECLARE_VVAR(128, struct vdso_data, _vdso_data) +#if !defined(_SINGLE_DATA) +#define _SINGLE_DATA +DECLARE_VVAR_SINGLE(640, struct vdso_rng_data, _vdso_rng_data) +#endif + #undef DECLARE_VVAR +#undef DECLARE_VVAR_SINGLE #endif diff --git a/drivers/char/random.c b/drivers/char/random.c index 7bf11fa66265..a18ff41713d7 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -59,6 +59,7 @@ #include <asm/irq.h> #include <asm/irq_regs.h> #include <asm/io.h> +#include <vdso/datapage.h> /********************************************************************* * @@ -84,6 +85,7 @@ static DEFINE_STATIC_KEY_FALSE(crng_is_ready); /* Various types of waiters for crng_init->CRNG_READY transition. */ static DECLARE_WAIT_QUEUE_HEAD(crng_init_wait); static struct fasync_struct *fasync; +DEFINE_VVAR_SINGLE(struct vdso_rng_data, _vdso_rng_data); /* Control how we warn userspace. */ static struct ratelimit_state urandom_warning = @@ -221,6 +223,7 @@ static void crng_reseed(void) ++next_gen; WRITE_ONCE(base_crng.generation, next_gen); WRITE_ONCE(base_crng.birth, jiffies); + WRITE_ONCE(_vdso_rng_data.generation, next_gen + 1); if (!static_branch_likely(&crng_is_ready)) crng_init = CRNG_READY; spin_unlock_irqrestore(&base_crng.lock, flags); @@ -660,6 +663,7 @@ static void __cold _credit_init_bits(size_t bits) crng_reseed(); /* Sets crng_init to CRNG_READY under base_crng.lock. */ if (static_key_initialized) execute_in_process_context(crng_set_ready, &set_ready); + _vdso_rng_data.is_ready = true; wake_up_interruptible(&crng_init_wait); kill_fasync(&fasync, SIGIO, POLL_IN); pr_notice("crng init done\n"); diff --git a/include/vdso/datapage.h b/include/vdso/datapage.h index 73eb622e7663..cbacfd923a5c 100644 --- a/include/vdso/datapage.h +++ b/include/vdso/datapage.h @@ -109,6 +109,11 @@ struct vdso_data { struct arch_vdso_data arch_data; }; +struct vdso_rng_data { + unsigned long generation; + bool is_ready; +}; + /* * We use the hidden visibility to prevent the compiler from generating a GOT * relocation. Not only is going through a GOT useless (the entry couldn't and @@ -120,6 +125,7 @@ struct vdso_data { */ extern struct vdso_data _vdso_data[CS_BASES] __attribute__((visibility("hidden"))); extern struct vdso_data _timens_data[CS_BASES] __attribute__((visibility("hidden"))); +extern struct vdso_rng_data _vdso_rng_data __attribute__((visibility("hidden"))); /* * The generic vDSO implementation requires that gettimeofday.h diff --git a/lib/vdso/getrandom.c b/lib/vdso/getrandom.c new file mode 100644 index 000000000000..3ffc900f31ff --- /dev/null +++ b/lib/vdso/getrandom.c @@ -0,0 +1,176 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * Copyright (C) 2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved. + */ + +#include <linux/kernel.h> +#include <linux/atomic.h> +#include <linux/fs.h> +#include <vdso/datapage.h> +#include <asm/vdso/getrandom.h> +#include <asm/vdso/vsyscall.h> +#include <asm/page.h> +#include <uapi/linux/mman.h> +#include "../crypto/chacha.c" + +#define NEG_NULL ((void *)~0UL) + +struct getrandom_state { + unsigned long generation; + union { + struct { + u8 key[CHACHA_KEY_SIZE]; + u8 batch[CHACHA_BLOCK_SIZE * 3 / 2]; + }; + u8 key_batch[CHACHA_BLOCK_SIZE * 2]; + }; + u8 pos; + bool in_use; +}; + +enum { NUM_BUCKETS = PAGE_SIZE / sizeof(struct getrandom_state) }; + +static bool allocate_new_state(void **state) +{ + void *new_state; + + if (cmpxchg(state, NULL, NEG_NULL) != NULL) + return false; + + new_state = mmap_syscall(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE, + MAP_PRIVATE | MAP_ANONYMOUS | MAP_LOCKED, -1, 0); + if (new_state == NEG_NULL) + goto err_unlock; + + if (madvise_syscall(new_state, PAGE_SIZE, MADV_WIPEONFORK)) + goto err_unmap; + + WRITE_ONCE(*state, new_state); + return true; + +err_unmap: + munmap_syscall(new_state, PAGE_SIZE); +err_unlock: + WRITE_ONCE(*state, NULL); + return false; +} + +extern long getcpu(unsigned int *cpu, unsigned int *node, void *unused); + +static struct getrandom_state *find_free_bucket(struct getrandom_state *buckets) +{ + unsigned int start = 0, i; + + if (getcpu(&start, NULL, NULL) == 0) + start %= NUM_BUCKETS; + + for (i = start;;) { + struct getrandom_state *state = &buckets[i]; + + if (cmpxchg(&state->in_use, false, true) == false) + return state; + + i = i == NUM_BUCKETS - 1 ? 0 : i + 1; + if (i == start) + break; + } + + return NULL; +} + +static void memcpy_and_zero(void *dst, void *src, size_t len) +{ +#define CASCADE(type) \ + while (len >= sizeof(type)) { \ + *(type *)dst = *(type *)src; \ + *(type *)src = 0; \ + dst += sizeof(type); \ + src += sizeof(type); \ + len -= sizeof(type); \ + } +#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS +#if BITS_PER_LONG == 64 + CASCADE(u64); +#endif + CASCADE(u32); + CASCADE(u16); +#endif + CASCADE(u8); +#undef CASCADE +} + +static ssize_t __always_inline +__cvdso_getrandom(void **state, void *buffer, size_t len, unsigned long flags) +{ + const struct vdso_rng_data *v = __arch_get_vdso_rng_data(); + struct getrandom_state *buckets = *state, *s; + u32 chacha_state[CHACHA_STATE_WORDS]; + unsigned long generation; + ssize_t ret = min_t(size_t, MAX_RW_COUNT, len); + size_t batch_len; + + if (unlikely(!v->is_ready)) + return getrandom_syscall(buffer, len, flags); + + if (unlikely(!len)) + return 0; + + if (unlikely(!buckets)) { + if (!allocate_new_state(state)) + return getrandom_syscall(buffer, len, flags); + buckets = *state; + } else if (unlikely(buckets == NEG_NULL)) { + return getrandom_syscall(buffer, len, flags); + } + + s = find_free_bucket(buckets); + if (unlikely(!s)) + return getrandom_syscall(buffer, len, flags); + +retry_generation: + generation = READ_ONCE(v->generation); + if (unlikely(s->generation != generation)) { + if (getrandom_syscall(s->key, sizeof(s->key), 0) != sizeof(s->key)) { + WRITE_ONCE(s->in_use, false); + return getrandom_syscall(buffer, len, flags); + } + s->pos = sizeof(s->batch); + s->generation = generation; + if (generation != READ_ONCE(v->generation)) + goto retry_generation; + } + + len = ret; +more_batch: + batch_len = min_t(size_t, sizeof(s->batch) - s->pos, len); + if (batch_len) { + memcpy_and_zero(buffer, s->batch, batch_len); + s->pos += batch_len; + buffer += batch_len; + len -= batch_len; + if (!len) { + WRITE_ONCE(s->in_use, false); + return ret; + } + } + + chacha_init_consts(chacha_state); + memcpy(&chacha_state[4], s->key, CHACHA_KEY_SIZE); + memset(&chacha_state[12], 0, sizeof(u32) * 4); + + while (len >= CHACHA_BLOCK_SIZE) { + chacha20_block(chacha_state, buffer); + if (unlikely(chacha_state[12] == 0)) + ++chacha_state[13]; + buffer += CHACHA_BLOCK_SIZE; + len -= CHACHA_BLOCK_SIZE; + } + + chacha20_block(chacha_state, s->key_batch); + if (unlikely(chacha_state[12] == 0)) + ++chacha_state[13]; + chacha20_block(chacha_state, s->key_batch + CHACHA_BLOCK_SIZE); + s->pos = 0; + memzero_explicit(chacha_state, sizeof(chacha_state)); + goto more_batch; +}
Two statements: 1) Userspace wants faster cryptographically secure numbers of arbitrary size, big or small. 2) Userspace is currently unable to safely roll its own RNG with the same security profile as getrandom(). Statement (1) has been debated for years, with arguments ranging from "we need faster cryptographically secure card shuffling!" to "the only things that actually need good randomness are keys, which are few and far between" to "actually, TLS CBC nonces are frequent" and so on. I don't intend to wade into that debate substantially, except to note that recently glibc added arc4random(), whose goal is to return a cryptographically secure uint32_t. So here we are. Statement (2) is more interesting. The kernel is the nexus of all entropic inputs that influence the RNG. It is in the best position, and probably the only position, to decide anything at all about the current state of the RNG and of its entropy. One of the things it uniquely knows about is when reseeding is necessary. For example, when a virtual machine is forked, restored, or duplicated, it's imparative that the RNG doesn't generate the same outputs. For this reason, there's a small protocol between hypervisors and the kernel that indicates this has happened, alongside some ID, which the RNG uses to immediately reseed, so as not to return the same numbers. Were userspace to expand a getrandom() seed from time T1 for the next hour, and at some point T2 < hour, the virtual machine forked, userspace would continue to provide the same numbers to two (or more) different virtual machines, resulting in potential cryptographic catastrophe. Something similar happens on resuming from hibernation (or even suspend), with various compromise scenarios there in mind. There's a more general reason why userspace rolling its own RNG from a getrandom() seed is fraught. There's a lot of attention paid to this particular Linuxism we have of the RNG being initialized and thus non-blocking or uninitialized and thus blocking until it is initialized. These are our Two Big States that many hold to be the holy differentiating factor between safe and not safe, between cryptographically secure and garbage. The fact is, however, that the distinction between these two states is a hand-wavy wishy-washy inexact approximation. Outside of a few exceptional cases (e.g. a HW RNG is available), we actually don't really ever know with any rigor at all when the RNG is safe and ready (nor when it's compromised). We do the best we can to "estimate" it, but entropy estimation is fundamentally impossible in the general case. So really, we're just doing guess work, and hoping it's good and conservative enough. Let's then assume that there's always some potential error involved in this differentiator. In fact, under the surface, the RNG is engineered around a different principal, and that is trying to *use* new entropic inputs regularly and at the right specific moments in time. For example, close to boot time, the RNG reseeds itself more often than later. At certain events, like VM fork, the RNG reseeds itself immediately. The various heuristics for when the RNG will use new entropy and how often is really a core aspect of what the RNG has some potential to do decently enough (and something that will probably continue to improve in the future from random.c's present set of algorithms). So in your mind, put away the metal attachment to the Two Big States, which represent an approximation with a potential margin of error. Instead keep in mind that the RNG's primary operating heuristic is how often and exactly when it's going to reseed. So, if userspace takes a seed from getrandom() at point T1, and uses it for the next hour (or N megabytes or some other meaningless metric), during that time, potential errors in the Two Big States approximation are amplified. During that time potential reseeds are being lost, forgotten, not reflected in the output stream. That's not good. The simplest statement you could is that userspace RNGs that expand a getrandom() seed at some point T1 are nearly always *worse*, in some way, than just calling getrandom() every time a random number is desired. For those reasons, after some discussion on libc-alpha, glibc's arc4random() now just calls getrandom() on each invocation. That's trivially safe, and gives us latitude to then make the safe thing faster without becoming unsafe at our leasure. Card shuffling isn't particularly fast, however. How do we rectify this? By putting a safe implementation of getrandom() in the vDSO, which has access to whatever information a particular iteration of random.c is using to make its decisions. I use that careful language of "particular iteration of random.c", because the set of things that a vDSO getrandom() implementation might need for making decisions as good as the kernel's will likely change over time. This isn't just a matter of exporting certain *data* to userspace. We're not going to commit to a "data API" where the various heuristics used are exposed, locking in how the kernel works for decades to come, and then leave it to various userspaces to roll something on top and shoot themselves in the foot and have all sorts of complexity disasters. Rather, vDSO getrandom() is supposed to be the *same exact algorithm* that runs in the kernel, except it's been hoisted into userspace as much as possible. And so vDSO getrandom() and kernel getrandom() will always mirror each other hermetically. API-wise, vDSO getrandom has this signature: ssize_t getrandom(void **state, void *buffer, size_t len, unsigned long flags); The return value and the latter 3 arguments are the same as ordinary getrandom(). The first argument is a double pointer to some state that vDSO allocates and manages. Call it first with *&my_state==NULL, and subsequently with the same &my_state, and only that first call will allocate. We very intentionally do *not* leave state memory management up to the caller. There are too many weird things that can go wrong, and it's important that vDSO does not provide too generic of a mechanism. It's not going to store its state in just any old memory address. It'll do it only in ones it allocates. Right now this means it's a mlock'd page with WIPEONFORK set. In the future maybe there will be other interesting page flags or anti-heartbleed measures, or other platform-specific kernel-specific things that can be set. Again, it's important that the vDSO has a say in how this works rather than agreeing to operate on any old address; memory isn't neutral. Because WIPEONFORK implies a whole page, vDSO getrandom() itself uses vDSO getcpu() in order to shard into various buckets, so that this remains fast from multiple threads. The interesting meat of the implementation is in lib/vdso/getrandom.c, as generic C code, and it aims to mainly follow random.c's buffered fast key erasure logic. Before the RNG is initialized, it falls back to the syscall. Right now it uses a simple generation counter to make its decisions on reseeding; this covers many cases, but not all, so this RFC still has a little bit of improvement work to do. But it should give you the general idea. The actual place that has the most work to do is in all of the other files. Most of the vDSO shared page infrastructure is centered around gettimeofday, and so the main structs are all in arrays for different timestamp types, and attached to time namespaces, and so forth. I've done the best I could to add onto this in an unintrusive way, but you'll notice almost immediately from glancing at the code that it still needs some untangling work. This also only works on x86 at the moment. I could certainly use a hand with this part. So far in my test results, performance is pretty stellar, and it seems to be working. But this is very, very young, immature code, suitable for an RFC and no more, so expect dragons. Cc: linux-crypto@vger.kernel.org Cc: x86@kernel.org Cc: Nadia Heninger <nadiah@cs.ucsd.edu> Cc: Thomas Ristenpart <ristenpart@cornell.edu> Cc: Theodore Ts'o <tytso@mit.edu> Cc: Vincenzo Frascino <vincenzo.frascino@arm.com> Cc: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org> Cc: Florian Weimer <fweimer@redhat.com> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> --- arch/x86/entry/vdso/Makefile | 2 +- arch/x86/entry/vdso/vdso.lds.S | 2 + arch/x86/entry/vdso/vgetrandom.c | 16 +++ arch/x86/include/asm/vdso/getrandom.h | 74 +++++++++++ arch/x86/include/asm/vvar.h | 16 +++ drivers/char/random.c | 4 + include/vdso/datapage.h | 6 + lib/vdso/getrandom.c | 176 ++++++++++++++++++++++++++ 8 files changed, 295 insertions(+), 1 deletion(-) create mode 100644 arch/x86/entry/vdso/vgetrandom.c create mode 100644 arch/x86/include/asm/vdso/getrandom.h create mode 100644 lib/vdso/getrandom.c