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Received: from [50.53.46.231] (helo=[192.168.254.15]) by bombadil.infradead.org with esmtpsa (Exim 4.96 #2 (Red Hat Linux)) id 1rNlYY-00Fadl-3B; Thu, 11 Jan 2024 03:16:35 +0000 Message-ID: Date: Wed, 10 Jan 2024 19:16:34 -0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v5 4/4] mseal:add documentation Content-Language: en-US To: jeffxu@chromium.org, 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 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 References: <20240109154547.1839886-1-jeffxu@chromium.org> <20240109154547.1839886-5-jeffxu@chromium.org> From: Randy Dunlap In-Reply-To: <20240109154547.1839886-5-jeffxu@chromium.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 467C41C0018 X-Rspam-User: X-Stat-Signature: 1mawiogxmjtju5pd8kc84pa611ywot5p X-Rspamd-Server: rspam03 X-HE-Tag: 1704943004-566947 X-HE-Meta: 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 b4yn3IsP 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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: On 1/9/24 07:45, jeffxu@chromium.org wrote: > 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..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 > + > +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@mail.gmail.com > + > +[4] https://docs.google.com/document/d/1O2jwK4dxI3nRcOJuPYkonhTkNQfbmwdvxQMyXgeaRHo/edit#heading=h.bvaojj9fu6hc -- #Randy