From: Jeff Xu jeffxu@chromium.org
Add documentation for mseal().
Signed-off-by: Jeff Xu jeffxu@chromium.org --- Documentation/userspace-api/mseal.rst | 189 ++++++++++++++++++++++++++ 1 file changed, 189 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..651c618d0664 --- /dev/null +++ b/Documentation/userspace-api/mseal.rst @@ -0,0 +1,189 @@ +.. 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()`` is an architecture independent syscall, and with following +signature: + +``int mseal(void addr, size_t len, unsigned long types, unsigned long flags)`` + +**addr/len**: virtual memory address range. + +The address range set by ``addr``/``len`` must meet: + - start (addr) must be in a valid VMA. + - end (addr + len) must be in a valid VMA. + - no gap (unallocated memory) between start and end. + - start (addr) must be page aligned. + +The ``len`` will be paged aligned implicitly by kernel. + +**types**: bit mask to specify the sealing types, they are: + +- The ``MM_SEAL_BASE``: Prevent VMA from: + + 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. + + Move or expand a different vma into the current location, + via mremap(). + + Modifying sealed 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. + + We consider the MM_SEAL_BASE feature, on which other sealing + features will depend. For instance, it probably does not make sense + to seal PROT_PKEY without sealing the BASE, and the kernel will + implicitly add SEAL_BASE for SEAL_PROT_PKEY. (If the application + wants to relax this in future, we could use the “flags” field in + mseal() to overwrite this the behavior.) + +- The ``MM_SEAL_PROT_PKEY``: + + Seal PROT and PKEY of the address range, in other words, + mprotect() and pkey_mprotect() will be denied if the memory is + sealed with MM_SEAL_PROT_PKEY. + +- The ``MM_SEAL_DISCARD_RO_ANON``: + + Certain types of madvise() operations are destructive [3], such + as MADV_DONTNEED, which can effectively alter region contents by + discarding pages, especially when memory is anonymous. This blocks + such operations for anonymous memory which is not writable to the + user. + +- The ``MM_SEAL_SEAL`` + Denies adding a new seal. + +**flags**: reserved for future use. + +**return values**: + +- ``0``: + - Success. + +- ``-EINVAL``: + - Invalid seal type. + - Invalid input flags. + - Start address is not page aligned. + - Address range (``addr`` + ``len``) overflow. + +- ``-ENOMEM``: + - ``addr`` is not a valid address (not allocated). + - End address (``addr`` + ``len``) is not a valid address. + - A gap (unallocated memory) between start and end. + +- ``-EACCES``: + - ``MM_SEAL_SEAL`` is set, adding a new seal is not allowed. + - Address range is not sealable, e.g. ``MAP_SEALABLE`` is not + set during ``mmap()``. + +**Note**: + +- User can call mseal(2) multiple times to add new seal types. +- Adding an already added seal type is a no-action (no error). +- unseal() or removing a seal type is not supported. +- In case of error return, one can expect the memory range is unchanged. + +``mmap()`` +---------- +``void *mmap(void* addr, size_t length, int prot, int flags, int fd, +off_t offset);`` + +We made two changes (``prot`` and ``flags``) to ``mmap()`` related to +memory sealing. + +**prot**: + +- ``PROT_SEAL_SEAL`` +- ``PROT_SEAL_BASE`` +- ``PROT_SEAL_PROT_PKEY`` +- ``PROT_SEAL_DISCARD_RO_ANON`` + +Allow ``mmap()`` to set the sealing type when creating a mapping. This is +useful for 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 type is set via the +``prot`` field in ``mmap()``, we don't require it to be set in the ``prot`` +field in later ``mprotect()`` call. 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. + +**flags** +The ``MAP_SEALABLE`` flag is added to 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`` when creating the map. + +Use Case: +========= +- glibc: + The dymamic linker, during loading ELF executables, can apply sealing to + to non-writeable 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 are not covered by +``mseal()``. If applications want to block such cases, sandboxer +(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...