@@ -23,177 +23,159 @@ applications can additionally seal security critical data at runtime.
A similar feature already exists in the XNU kernel with the
VM_FLAGS_PERMANENT flag [1] and on OpenBSD with the mimmutable syscall [2].
-User API
-========
-mseal()
------------
-The mseal() syscall has the following signature:
-
-``int mseal(void addr, size_t len, unsigned long flags)``
-
-**addr/len**: virtual memory address range.
-
-The address range set by ``addr``/``len`` must meet:
- - The start address must be in an allocated VMA.
- - The start address must be page aligned.
- - The end address (``addr`` + ``len``) must be in an allocated VMA.
- - no gap (unallocated memory) between start and end address.
-
-The ``len`` will be paged aligned implicitly by the kernel.
-
-**flags**: reserved for future use.
-
-**return values**:
-
-- ``0``: Success.
-
-- ``-EINVAL``:
- - Invalid input ``flags``.
- - The start address (``addr``) is not page aligned.
- - Address range (``addr`` + ``len``) overflow.
-
-- ``-ENOMEM``:
- - The start address (``addr``) is not allocated.
- - The end address (``addr`` + ``len``) is not allocated.
- - A gap (unallocated memory) between start and end address.
-
-- ``-EPERM``:
- - sealing is supported only on 64-bit CPUs, 32-bit is not supported.
-
-- For above error cases, users can expect the given memory range is
- unmodified, i.e. no partial update.
-
-- There might be other internal errors/cases not listed here, e.g.
- error during merging/splitting VMAs, or the process reaching the max
- number of supported VMAs. In those cases, partial updates to the given
- memory range could happen. However, those cases should be rare.
-
-**Blocked operations after sealing**:
- Unmapping, moving to another location, and shrinking the size,
- via munmap() and mremap(), can leave an empty space, therefore
- can be replaced with a VMA with a new set of attributes.
-
- Moving or expanding a different VMA into the current location,
- via mremap().
-
- Modifying a VMA via mmap(MAP_FIXED).
-
- Size expansion, via mremap(), does not appear to pose any
- specific risks to sealed VMAs. It is included anyway because
- the use case is unclear. In any case, users can rely on
- merging to expand a sealed VMA.
-
- mprotect() and pkey_mprotect().
-
- Some destructive madvice() behaviors (e.g. MADV_DONTNEED)
- for anonymous memory, when users don't have write permission to the
- memory. Those behaviors can alter region contents by discarding pages,
- effectively a memset(0) for anonymous memory.
-
- Kernel will return -EPERM for blocked operations.
-
- For blocked operations, one can expect the given address is unmodified,
- i.e. no partial update. Note, this is different from existing mm
- system call behaviors, where partial updates are made till an error is
- found and returned to userspace. To give an example:
-
- Assume following code sequence:
-
- - ptr = mmap(null, 8192, PROT_NONE);
- - munmap(ptr + 4096, 4096);
- - ret1 = mprotect(ptr, 8192, PROT_READ);
- - mseal(ptr, 4096);
- - ret2 = mprotect(ptr, 8192, PROT_NONE);
-
- ret1 will be -ENOMEM, the page from ptr is updated to PROT_READ.
-
- ret2 will be -EPERM, the page remains to be PROT_READ.
-
-**Note**:
-
-- mseal() only works on 64-bit CPUs, not 32-bit CPU.
-
-- users can call mseal() multiple times, mseal() on an already sealed memory
- is a no-action (not error).
-
-- munseal() is not supported.
+SYSCALL
+=======
+mseal syscall signature
+-----------------------
+ **int** mseal(**void \*** addr, **size_t** len, **unsigned long** flags)
+
+ **addr**/**len**: virtual memory address range.
+ The address range set by **addr**/**len** must meet:
+ - The start address must be in an allocated VMA.
+ - The start address must be page aligned.
+ - The end address (**addr** + **len**) must be in an allocated VMA.
+ - no gap (unallocated memory) between start and end address.
+
+ The ``len`` will be paged aligned implicitly by the kernel.
+
+ **flags**: reserved for future use.
+
+ **Return values**:
+ - **0**: Success.
+ - **-EINVAL**:
+ * Invalid input ``flags``.
+ * The start address (``addr``) is not page aligned.
+ * Address range (``addr`` + ``len``) overflow.
+ - **-ENOMEM**:
+ * The start address (``addr``) is not allocated.
+ * The end address (``addr`` + ``len``) is not allocated.
+ * A gap (unallocated memory) between start and end address.
+ - **-EPERM**:
+ * sealing is supported only on 64-bit CPUs, 32-bit is not supported.
+
+ **Note about error return**:
+ - For above error cases, users can expect the given memory range is
+ unmodified, i.e. no partial update.
+ - There might be other internal errors/cases not listed here, e.g.
+ error during merging/splitting VMAs, or the process reaching the max
+ number of supported VMAs. In those cases, partial updates to the given
+ memory range could happen. However, those cases should be rare.
+
+ **Architecture support**:
+ mseal only works on 64-bit CPUs, not 32-bit CPU.
+
+ **Idempotent**:
+ users can call mseal multiple times, mseal on an already sealed memory
+ is a no-action (not error).
+
+ **no munseal**
+ Once mapping is sealed, it can't be unsealed. kernel should never
+ have munseal, this is consistent with other sealing feature, e.g.
+ F_SEAL_SEAL for file.
+
+Blocked mm syscall for sealed mapping
+-------------------------------------
+ It might be imporant to note: **once the mapping is sealed, it will
+ stay in the process's memory till the process terminates**.
+
+ Example::
+
+ *ptr = mmap(0, 4096, PROT_READ, MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
+ rc = mseal(ptr, 4096, 0);
+ /* munmap will fail */
+ rc = munmap(ptr, 4096);
+ assert(rc < 0);
+
+ Blocked mm syscall:
+ - munmap
+ - mmap
+ - mremap
+ - mprotect and pkey_mprotect
+ - some destructive madvise behaviors: MADV_DONTNEED, MADV_FREE,
+ MADV_DONTNEED_LOCKED, MADV_FREE, MADV_DONTFORK, MADV_WIPEONFORK
+
+ The first set of syscall to block is munmap, mremap, mmap. They can
+ either leave an empty space in the address space, therefore allow
+ replacement with a new mapping with new set of attributes, or can
+ overwrite the existing mapping with another mapping.
+
+ mprotect and pkey_mprotect are blocked because they changes the
+ protection bits (rwx) of the mapping.
+
+ Some destructive madvice behaviors (MADV_DONTNEED, MADV_FREE,
+ MADV_DONTNEED_LOCKED, MADV_FREE, MADV_DONTFORK, MADV_WIPEONFORK)
+ for anonymous memory, when users don't have write permission to the
+ memory. Those behaviors can alter region contents by discarding pages,
+ effectively a memset(0) for anonymous memory.
+
+ Kernel will return -EPERM for blocked syscalls.
+
+ When blocked syscall return -EPERM due to sealing, the memory regions may or may not be changed, depends on the syscall being blocked:
+ - munmap: munmap is atomic. If one of VMAs in the given range is
+ sealed, none of VMAs are updated.
+ - mprotect, pkey_mprotect, madvise: partial update might happen, e.g.
+ when mprotect over multiple VMAs, mprotect might update the beginning
+ VMAs before reaching the sealed VMA and return -EPERM.
+ - mmap and mremap: undefined behavior.
Use cases:
==========
- glibc:
The dynamic linker, during loading ELF executables, can apply sealing to
- non-writable memory segments.
+ mapping segments.
- Chrome browser: protect some security sensitive data-structures.
-Notes on which memory to seal:
-==============================
-
-It might be important to note that sealing changes the lifetime of a mapping,
-i.e. the sealed mapping won’t be unmapped till the process terminates or the
-exec system call is invoked. Applications can apply sealing to any virtual
-memory region from userspace, but it is crucial to thoroughly analyze the
-mapping's lifetime prior to apply the sealing.
+Don't use mseal on:
+===================
+Applications can apply sealing to any virtual memory region from userspace,
+but it is *crucial to thoroughly analyze the mapping's lifetime* prior to
+apply the sealing. This is because the sealed mapping *won’t be unmapped*
+till the process terminates or the exec system call is invoked.
For example:
+ - aio/shm
+ aio/shm can call mmap and munmap on behalf of userspace, e.g.
+ ksys_shmdt() in shm.c. The lifetime of those mapping are not tied to
+ the lifetime of the process. If those memories are sealed from userspace,
+ then munmap will fail, causing leaks in VMA address space during the
+ lifetime of the process.
+
+ - ptr allocated by malloc (heap)
+ Don't use mseal on the memory ptr return from malloc().
+ malloc() is implemented by allocator, e.g. by glibc. Heap manager might
+ allocate a ptr from brk or mapping created by mmap.
+ If app calls mseal on ptr returned from malloc(), this can affect the heap
+ manager's ability to manage the mappings, the outcome is non-deterministic.
+ Example::
+
+ ptr = malloc(size);
+ /* don't call mseal on ptr return from malloc. */
+ mseal(ptr, size);
+ /* free will success, allocator can't shrink heap lower than ptr */
+ free(ptr);
+
+mseal doesn't block:
+====================
+In a nutshell, mseal blocks certain mm syscall from modifying some of VMA's
+attributes, such as protection bits (rwx). Sealed mappings doesn't mean the
+memory is immutable.
-- aio/shm
-
- aio/shm can call mmap()/munmap() on behalf of userspace, e.g. ksys_shmdt() in
- shm.c. The lifetime of those mapping are not tied to the lifetime of the
- process. If those memories are sealed from userspace, then munmap() will fail,
- causing leaks in VMA address space during the lifetime of the process.
-
-- Brk (heap)
-
- Currently, userspace applications can seal parts of the heap by calling
- malloc() and mseal().
- let's assume following calls from user space:
-
- - ptr = malloc(size);
- - mprotect(ptr, size, RO);
- - mseal(ptr, size);
- - free(ptr);
-
- Technically, before mseal() is added, the user can change the protection of
- the heap by calling mprotect(RO). As long as the user changes the protection
- back to RW before free(), the memory range can be reused.
-
- Adding mseal() into the picture, however, the heap is then sealed partially,
- the user can still free it, but the memory remains to be RO. If the address
- is re-used by the heap manager for another malloc, the process might crash
- soon after. Therefore, it is important not to apply sealing to any memory
- that might get recycled.
-
- Furthermore, even if the application never calls the free() for the ptr,
- the heap manager may invoke the brk system call to shrink the size of the
- heap. In the kernel, the brk-shrink will call munmap(). Consequently,
- depending on the location of the ptr, the outcome of brk-shrink is
- nondeterministic.
-
-
-Additional notes:
-=================
As Jann Horn pointed out in [3], there are still a few ways to write
-to RO memory, which is, in a way, by design. Those cases are not covered
-by mseal(). If applications want to block such cases, sandbox tools (such as
-seccomp, LSM, etc) might be considered.
+to RO memory, which is, in a way, by design. And those could be blocked
+by different security measures.
Those cases are:
-
-- Write to read-only memory through /proc/self/mem interface.
-- Write to read-only memory through ptrace (such as PTRACE_POKETEXT).
-- userfaultfd.
+ - Write to read-only memory through /proc/self/mem interface (FOLL_FORCE).
+ - Write to read-only memory through ptrace (such as PTRACE_POKETEXT).
+ - userfaultfd.
The idea that inspired this patch comes from Stephen Röttger’s work in V8
CFI [4]. Chrome browser in ChromeOS will be the first user of this API.
Reference:
==========
-[1] https://github.com/apple-oss-distributions/xnu/blob/1031c584a5e37aff177559b9f69dbd3c8c3fd30a/osfmk/mach/vm_statistics.h#L274
-
-[2] https://man.openbsd.org/mimmutable.2
-
-[3] https://lore.kernel.org/lkml/CAG48ez3ShUYey+ZAFsU2i1RpQn0a5eOs2hzQ426FkcgnfUGLvA@mail.gmail.com
-
-[4] https://docs.google.com/document/d/1O2jwK4dxI3nRcOJuPYkonhTkNQfbmwdvxQMyXgeaRHo/edit#heading=h.bvaojj9fu6hc
+- [1] https://github.com/apple-oss-distributions/xnu/blob/1031c584a5e37aff177559b9f69dbd3c8c3fd30a/osfmk/mach/vm_statistics.h#L274
+- [2] https://man.openbsd.org/mimmutable.2
+- [3] https://lore.kernel.org/lkml/CAG48ez3ShUYey+ZAFsU2i1RpQn0a5eOs2hzQ426FkcgnfUGLvA@mail.gmail.com
+- [4] https://docs.google.com/document/d/1O2jwK4dxI3nRcOJuPYkonhTkNQfbmwdvxQMyXgeaRHo/edit#heading=h.bvaojj9fu6hc