From: Jeff Xu <jeffxu@xxxxxxxxxxxx> Add documentation for mseal(). Signed-off-by: Jeff Xu <jeffxu@xxxxxxxxxxxx> --- 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@xxxxxxxxxxxx> + +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: + +``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. + +- 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. + +**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, +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 +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/1031c584a5e37aff177559b9f69dbd3c8c3fd30a/osfmk/mach/vm_statistics.h#L274 + +[2] https://man.openbsd.org/mimmutable.2 + +[3] https://lore.kernel.org/lkml/CAG48ez3ShUYey+ZAFsU2i1RpQn0a5eOs2hzQ426FkcgnfUGLvA@xxxxxxxxxxxxxx + +[4] https://docs.google.com/document/d/1O2jwK4dxI3nRcOJuPYkonhTkNQfbmwdvxQMyXgeaRHo/edit#heading=h.bvaojj9fu6hc -- 2.43.0.195.gebba966016-goog
