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[34.168.85.34]) by smtp.gmail.com with UTF8SMTPSA id n40-20020a056a000d6800b006d5929810a6sm1785848pfv.83.2024.01.11.15.42.31 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 11 Jan 2024 15:42:31 -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 v6 4/4] mseal:add documentation Date: Thu, 11 Jan 2024 23:41:41 +0000 Message-ID: <20240111234142.2944934-5-jeffxu@chromium.org> X-Mailer: git-send-email 2.43.0.275.g3460e3d667-goog In-Reply-To: <20240111234142.2944934-1-jeffxu@chromium.org> References: <20240111234142.2944934-1-jeffxu@chromium.org> MIME-Version: 1.0 X-Rspamd-Queue-Id: 1112240016 X-Rspam-User: X-Stat-Signature: aieadr63h98oewimm498r1i41fj3o5zc X-Rspamd-Server: rspam03 X-HE-Tag: 1705016552-37483 X-HE-Meta: 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 rnWjqZ2r 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: Jeff Xu Add documentation for mseal(). Signed-off-by: Jeff Xu --- 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..3ca0c9ce3967 --- /dev/null +++ b/Documentation/userspace-api/mseal.rst @@ -0,0 +1,181 @@ +.. SPDX-License-Identifier: GPL-2.0 + +===================== +Introduction of mseal +===================== + +: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’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 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. + +**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@mail.gmail.com + +[4] https://docs.google.com/document/d/1O2jwK4dxI3nRcOJuPYkonhTkNQfbmwdvxQMyXgeaRHo/edit#heading=h.bvaojj9fu6hc