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[34.82.72.56]) by smtp.gmail.com with UTF8SMTPSA id fa20-20020a056a002d1400b006d70b0d4639sm10172924pfb.107.2024.01.31.09.50.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 31 Jan 2024 09:50:41 -0800 (PST) From: jeffxu@chromium.org To: akpm@linux-foundation.org, keescook@chromium.org, jannh@google.com, sroettger@google.com, willy@infradead.org, gregkh@linuxfoundation.org, torvalds@linux-foundation.org, usama.anjum@collabora.com, rdunlap@infradead.org Cc: jeffxu@google.com, jorgelo@chromium.org, groeck@chromium.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, pedro.falcato@gmail.com, dave.hansen@intel.com, linux-hardening@vger.kernel.org, deraadt@openbsd.org, Jeff Xu Subject: [PATCH v8 4/4] mseal:add documentation Date: Wed, 31 Jan 2024 17:50:26 +0000 Message-ID: <20240131175027.3287009-5-jeffxu@chromium.org> X-Mailer: git-send-email 2.43.0.429.g432eaa2c6b-goog In-Reply-To: <20240131175027.3287009-1-jeffxu@chromium.org> References: <20240131175027.3287009-1-jeffxu@chromium.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable From: Jeff Xu Add documentation for mseal(). Signed-off-by: Jeff Xu --- Documentation/userspace-api/index.rst | 1 + Documentation/userspace-api/mseal.rst | 215 ++++++++++++++++++++++++++ 2 files changed, 216 insertions(+) create mode 100644 Documentation/userspace-api/mseal.rst diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspac= e-api/index.rst index 09f61bd2ac2e..178f6a1d79cb 100644 --- a/Documentation/userspace-api/index.rst +++ b/Documentation/userspace-api/index.rst @@ -26,6 +26,7 @@ place where this information is gathered. iommu iommufd media/index + mseal netlink/index sysfs-platform_profile vduse diff --git a/Documentation/userspace-api/mseal.rst b/Documentation/userspac= e-api/mseal.rst new file mode 100644 index 000000000000..6bfac0622178 --- /dev/null +++ b/Documentation/userspace-api/mseal.rst @@ -0,0 +1,215 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +Introduction of mseal +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +:Author: Jeff Xu + +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=E2=80=99t just write to arbitrary memory and point the co= de 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 +=3D=3D=3D=3D=3D=3D=3D=3D +Two system calls are involved in virtual memory sealing, mseal() and mmap(= ). + +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. + +- ``-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 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. + +**Note**: + +- mseal() only works on 64-bit CPUs, not 32-bit CPU. + +- users can call mseal() multiple times, mseal() on an already sealed memo= ry + 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(). + +Use Case: +=3D=3D=3D=3D=3D=3D=3D=3D=3D +- glibc: + The dynamic linker, during loading ELF executables, can apply sealing to + non-writable memory segments. + +- Chrome browser: protect some security sensitive data-structures. + +Notes On MAP_SEALABLE +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +With the MAP_SEALABLE flag in mmap(), the memory must be mmap() with +MAP_SEALABLE, otherwise mseal() will fail. This raises the bar of +which memory can be sealed. + +Today, in linux, sealing have known side effects if applied in below +two cases: + +- aio/shm + + aio/shm can 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 proce= ss. If those memories are sealed from userspace, then unmap will fail, caus= ing leaks in VMA address space during the lifetime of the process. + +- Brk (heap/stack) + + Currently, userspace applications can seal parts of the heap by calling = malloc() and mseal(). + let's assume following calls from user space: + + - ptr =3D 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 prote= ction back to RW before free(), the memory can be reused. + + Adding mseal() into the picture, however, the heap is then sealed partia= lly, the user can still free it, but the memory remains to be RO. In additi= on, the result of brk-shrink is nondeterministic, depending on if munmap() = will try to free the sealed memory.(brk uses munmap to shrink the heap). + + Given the heap is not marked with MAP_SEALABLE (at the time of this docu= ment's writing), this might discourage the inadvertent sealing on the heap. + + It is noteworthy, nonetheless, for mappings that were created without th= e MAP_SEALABLE flag, a knowledgeable developer who wants to assume ownershi= p of the memory range still has the option of mmap(MAP_FIXED|MAP_SEALABLE),= which is equivalent to invoking munmap() and then mmap(MAP_FIXED). Indeed,= a "not-allow-sealing" feature is not possible without some level of baseli= ne sealing support and is out-of-scope currently. + + In summary, the considerations for having MAP_SEALABLE are as follows: + +- Grants software owners the ability to incrementally incorporate sealing = support for their designated memory ranges, such as brk. +- Raises the bar for which memory can be sealed, and discourages inadverte= nt sealing. +- Such a decision is reversible. In other words, a sysctl could be impleme= nted to render all memory sealable in the future. However, if all memory we= re allowed to be sealable from the beginning, reversing that decision would= be problematic. + +Additional notes: +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +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 R=C3=B6ttger=E2=80=99= s work in V8 +CFI [4]. Chrome browser in ChromeOS will be the first user of this API. + +Reference: +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +[1] https://github.com/apple-oss-distributions/xnu/blob/1031c584a5e37aff17= 7559b9f69dbd3c8c3fd30a/osfmk/mach/vm_statistics.h#L274 + +[2] https://man.openbsd.org/mimmutable.2 + +[3] https://lore.kernel.org/lkml/CAG48ez3ShUYey+ZAFsU2i1RpQn0a5eOs2hzQ426F= kcgnfUGLvA@mail.gmail.com + +[4] https://docs.google.com/document/d/1O2jwK4dxI3nRcOJuPYkonhTkNQfbmwdvxQ= MyXgeaRHo/edit#heading=3Dh.bvaojj9fu6hc --=20 2.43.0.429.g432eaa2c6b-goog