On Wed, Jan 10, 2024 at 7:16 PM Randy Dunlap <rdunlap@xxxxxxxxxxxxx> wrote: > > > > On 1/9/24 07:45, jeffxu@xxxxxxxxxxxx wrote: > > 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: > > 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/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 > Thanks. Will update in the next version. -Jeff > -- > #Randy
