[1/3] bpf: verifier: Accept dynptr mem as mem in helpers

This allows using memory retrieved from dynptrs with helper functions
that accept ARG_PTR_TO_MEM. For instance, results from bpf_dynptr_data
can be passed along to bpf_strncmp.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
---
 kernel/bpf/verifier.c | 4 ++++
 1 file changed, 4 insertions(+)

On Thu, Apr 6, 2023 at 1:55 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Wed, Apr 5, 2023 at 5:40 PM Daniel Rosenberg <drosen@google.com> wrote:
> >
> > This allows using memory retrieved from dynptrs with helper functions
> > that accept ARG_PTR_TO_MEM. For instance, results from bpf_dynptr_data
> > can be passed along to bpf_strncmp.
> >
> > Signed-off-by: Daniel Rosenberg <drosen@google.com>
> > ---
>
> I think patches like this should be targeted against bpf-next, so for
> next revision please send them against bpf-next tree.
>
> >  kernel/bpf/verifier.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 56f569811f70..20beab52812a 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -7164,12 +7164,16 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
> >          * ARG_PTR_TO_MEM + MAYBE_NULL is compatible with PTR_TO_MEM and PTR_TO_MEM + MAYBE_NULL,
> >          * but ARG_PTR_TO_MEM is compatible only with PTR_TO_MEM but NOT with PTR_TO_MEM + MAYBE_NULL
> >          *
> > +        * ARG_PTR_TO_MEM is compatible with PTR_TO_MEM that is tagged with a dynptr type.
> > +        *
> >          * Therefore we fold these flags depending on the arg_type before comparison.
> >          */
> >         if (arg_type & MEM_RDONLY)
> >                 type &= ~MEM_RDONLY;
> >         if (arg_type & PTR_MAYBE_NULL)
> >                 type &= ~PTR_MAYBE_NULL;
> > +       if (base_type(arg_type) == ARG_PTR_TO_MEM)
> > +               type &= ~DYNPTR_TYPE_FLAG_MASK;
>
> Something feels off here. Can you paste a bit of verifier log for the
> failure you were getting. And let's have a selftest for this situation
> as well.
>
> ARG_PTR_TO_MEM shouldn't be qualified with the DYNPTR_TYPE flag, it's
> just memory, there is no need to know what type of dynptr it was
> derived from. So if that happens, the problem is somewhere else. Let's
> root cause and fix that. Having a selftest that demonstrates the
> problem will help with that.

+1
All of the DYNPTR_TYPE_FLAG_MASK flags cannot appear in type == reg->type
here.
They are either dynamic flags inside bpf_dynptr_kern->size
or in arg_type.
Like in bpf_dynptr_from_mem_proto.

On Thu, Apr 6, 2023 at 1:55 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> Something feels off here. Can you paste a bit of verifier log for the
> failure you were getting. And let's have a selftest for this situation
> as well.
>
> ARG_PTR_TO_MEM shouldn't be qualified with the DYNPTR_TYPE flag, it's
> just memory, there is no need to know what type of dynptr it was
> derived from. So if that happens, the problem is somewhere else. Let's
> root cause and fix that. Having a selftest that demonstrates the
> problem will help with that.
>
>
This label is added by dynptr_slice(_rdwr)

if (meta.func_id == special_kfunc_list[KF_bpf_dynptr_slice] ||
   meta.func_id == special_kfunc_list[KF_bpf_dynptr_slice_rdwr]) {
enum bpf_type_flag type_flag =
get_dynptr_type_flag(meta.initialized_dynptr.type);
...
regs[BPF_REG_0].type = PTR_TO_MEM | type_flag;

That extra flag was causing the type to be unexpected later on.
I'll add a selftest for this as well.

On Thu, Apr 6, 2023 at 3:13 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> +1
> All of the DYNPTR_TYPE_FLAG_MASK flags cannot appear in type == reg->type
> here.
> They are either dynamic flags inside bpf_dynptr_kern->size
> or in arg_type.
> Like in bpf_dynptr_from_mem_proto.

Looking at this a bit more, I believe this is to enforce packet
restrictions for DYNPTR_TYPE_SKB and DYNPTR_TYPE_XDP. When a helper
function alters a packet, dynptr slices of it are invalidated. If I
remove that annotation entirely, then the invalid_data_slice family of
tests fail. bpf_dynptr_from_mem_proto is fine since that's just local
dynptrs, which don't have any extra limitations.

On Mon, May 1, 2023 at 6:12 PM Daniel Rosenberg <drosen@google.com> wrote:
>
> On Thu, Apr 6, 2023 at 3:13 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > +1
> > All of the DYNPTR_TYPE_FLAG_MASK flags cannot appear in type == reg->type
> > here.
> > They are either dynamic flags inside bpf_dynptr_kern->size
> > or in arg_type.
> > Like in bpf_dynptr_from_mem_proto.
>
> Looking at this a bit more, I believe this is to enforce packet
> restrictions for DYNPTR_TYPE_SKB and DYNPTR_TYPE_XDP. When a helper
> function alters a packet, dynptr slices of it are invalidated. If I
> remove that annotation entirely, then the invalid_data_slice family of
> tests fail. bpf_dynptr_from_mem_proto is fine since that's just local
> dynptrs, which don't have any extra limitations.


Ah, ok, thanks for investigating!