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[104.198.4.15]) by smtp.gmail.com with UTF8SMTPSA id f12-20020a170902684c00b001e3d2314f3csm8299078pln.141.2024.04.15.09.35.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 15 Apr 2024 09:35:40 -0700 (PDT) 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, corbet@lwn.net, Liam.Howlett@oracle.com, surenb@google.com, merimus@google.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 v10 4/5] mseal:add documentation Date: Mon, 15 Apr 2024 16:35:23 +0000 Message-ID: <20240415163527.626541-5-jeffxu@chromium.org> X-Mailer: git-send-email 2.44.0.683.g7961c838ac-goog In-Reply-To: <20240415163527.626541-1-jeffxu@chromium.org> References: <20240415163527.626541-1-jeffxu@chromium.org> MIME-Version: 1.0 X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: C3A6C1C0027 X-Stat-Signature: 7khne38at3t4f6dud3ez9ywfr1zptf8e X-HE-Tag: 1713198941-397812 X-HE-Meta: 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 IVqaHQfG 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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/index.rst | 1 + Documentation/userspace-api/mseal.rst | 199 ++++++++++++++++++++++++++ 2 files changed, 200 insertions(+) create mode 100644 Documentation/userspace-api/mseal.rst diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst index afecfe3cc4a8..5926115ec0ed 100644 --- a/Documentation/userspace-api/index.rst +++ b/Documentation/userspace-api/index.rst @@ -20,6 +20,7 @@ System calls futex2 ebpf/index ioctl/index + mseal Security-related interfaces =========================== diff --git a/Documentation/userspace-api/mseal.rst b/Documentation/userspace-api/mseal.rst new file mode 100644 index 000000000000..4132eec995a3 --- /dev/null +++ b/Documentation/userspace-api/mseal.rst @@ -0,0 +1,199 @@ +.. 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 +======== +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. + +- ``-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. + + For blocked operations, one can expect the given address is unmodified, + i.e. no partial update. Note, this is different from existing mm + system call behaviors, where partial updates are made till an error is + found and returned to userspace. To give an example: + + Assume following code sequence: + + - ptr = mmap(null, 8192, PROT_NONE); + - munmap(ptr + 4096, 4096); + - ret1 = mprotect(ptr, 8192, PROT_READ); + - mseal(ptr, 4096); + - ret2 = mprotect(ptr, 8192, PROT_NONE); + + ret1 will be -ENOMEM, the page from ptr is updated to PROT_READ. + + ret2 will be -EPERM, the page remains to be PROT_READ. + +**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. + +Use cases: +========== +- 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 which memory to seal: +============================== + +It might be important to note that sealing changes the lifetime of a mapping, +i.e. the sealed mapping won’t be unmapped till the process terminates or the +exec system call is invoked. Applications can apply sealing to any virtual +memory region from userspace, but it is crucial to thoroughly analyze the +mapping's lifetime prior to apply the sealing. + +For example: + +- aio/shm + + aio/shm can call 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 + process. If those memories are sealed from userspace, then munmap() will fail, + causing leaks in VMA address space during the lifetime of the process. + +- Brk (heap) + + Currently, userspace applications can seal parts of the heap by calling + malloc() and mseal(). + let's assume following calls from user space: + + - ptr = 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 protection + back to RW before free(), the memory range can be reused. + + Adding mseal() into the picture, however, the heap is then sealed partially, + the user can still free it, but the memory remains to be RO. If the address + is re-used by the heap manager for another malloc, the process might crash + soon after. Therefore, it is important not to apply sealing to any memory + that might get recycled. + + Furthermore, even if the application never calls the free() for the ptr, + the heap manager may invoke the brk system call to shrink the size of the + heap. In the kernel, the brk-shrink will call munmap(). Consequently, + depending on the location of the ptr, the outcome of brk-shrink is + nondeterministic. + + +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