On Wed, Jan 10, 2024 at 7:16 PM Randy Dunlap rdunlap@infradead.org wrote:
On 1/9/24 07:45, jeffxu@chromium.org wrote:
From: Jeff Xu jeffxu@chromium.org
Add documentation for mseal().
Signed-off-by: Jeff Xu jeffxu@chromium.org
Documentation/userspace-api/mseal.rst | 181 ++++++++++++++++++++++++++ 1 file changed, 181 insertions(+) create mode 100644 Documentation/userspace-api/mseal.rst
diff --git a/Documentation/userspace-api/mseal.rst b/Documentation/userspace-api/mseal.rst new file mode 100644 index 000000000000..1700ce5af218 --- /dev/null +++ b/Documentation/userspace-api/mseal.rst @@ -0,0 +1,181 @@ +.. SPDX-License-Identifier: GPL-2.0
+===================== +Introduction of mseal +=====================
+:Author: Jeff Xu jeffxu@chromium.org
+Modern CPUs support memory permissions such as RW and NX bits. The memory +permission feature improves security stance on memory corruption bugs, i.e. +the attacker can’t just write to arbitrary memory and point the code to it, +the memory has to be marked with X bit, or else an exception will happen.
+Memory sealing additionally protects the mapping itself against +modifications. This is useful to mitigate memory corruption issues where a +corrupted pointer is passed to a memory management system. For example, +such an attacker primitive can break control-flow integrity guarantees +since read-only memory that is supposed to be trusted can become writable +or .text pages can get remapped. Memory sealing can automatically be +applied by the runtime loader to seal .text and .rodata pages and +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 +======== +Two system calls are involved in virtual memory sealing, mseal() and mmap().
+mseal() +----------- +The mseal() syscall has following signature:
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.
+- ``-EACCES``:
- ``MAP_SEALABLE`` is not set during mmap().
+- ``-EPERM``:
- sealing is supported only on 64 bit CPUs, 32-bit is not supported.
64-bit
+- 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 shall be rare.
s/shall/should/ unless you are predicting the future.
+**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.
+**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.
+mmap() +---------- +``void *mmap(void* addr, size_t length, int prot, int flags, int fd, +off_t offset);``
+We add two changes in ``prot`` and ``flags`` of mmap() related to +memory sealing.
+**prot**
+The ``PROT_SEAL`` bit in ``prot`` field of mmap().
+When present, it marks the memory is sealed since creation.
+This is useful as optimization because it avoids having to make two +system calls: one for mmap() and one for mseal().
+It's worth noting that even though the sealing is set via the +``prot`` field in mmap(), it can't be set in the ``prot`` +field in later mprotect(). This is unlike the ``PROT_READ``, +``PROT_WRITE``, ``PROT_EXEC`` bits, e.g. if ``PROT_WRITE`` is not set in +mprotect(), it means that the region is not writable.
+Setting ``PROT_SEAL`` implies setting ``MAP_SEALABLE`` below.
+**flags**
+The ``MAP_SEALABLE`` bit in the ``flags`` field of mmap().
+When present, it marks the map as sealable. A map created +without ``MAP_SEALABLE`` will not support sealing; In other words,
sealing. In+mseal() will fail for such a map.
+Applications that don't care about sealing will expect their +behavior unchanged. For those that need sealing support, opt-in
opt in+by adding ``MAP_SEALABLE`` in mmap().
+Note: for a map created without ``MAP_SEALABLE`` or a map created +with ``MAP_SEALABLE`` but not sealed yet, mmap(MAP_FIXED) can +change the sealable or sealing bit.
+Use Case: +========= +- glibc:
- The dynamic linker, during loading ELF executables, can apply sealing to
- non-writable memory segments.
+- Chrome browser: protect some security sensitive data-structures.
+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.
+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.
+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/1031c584a5e37aff177559b9...
+[2] https://man.openbsd.org/mimmutable.2
+[3] https://lore.kernel.org/lkml/CAG48ez3ShUYey+ZAFsU2i1RpQn0a5eOs2hzQ426FkcgnfU...
+[4] https://docs.google.com/document/d/1O2jwK4dxI3nRcOJuPYkonhTkNQfbmwdvxQMyXgea...
Thanks. Will update in the next version. -Jeff
-- #Randy