From patchwork Thu Jan 11 23:41:41 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Jeff Xu X-Patchwork-Id: 762592 Received: from mail-pf1-f182.google.com (mail-pf1-f182.google.com [209.85.210.182]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 88D705A10A for ; Thu, 11 Jan 2024 23:42:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=chromium.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=chromium.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="Tr8tWk+Q" Received: by mail-pf1-f182.google.com with SMTP id d2e1a72fcca58-6d9ac5bd128so3995999b3a.0 for ; Thu, 11 Jan 2024 15:42:32 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; t=1705016552; x=1705621352; darn=vger.kernel.org; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=dPOt9smlRtRbYxwBRyVbgWAEdHEPdj5BH7dSKIF5aYg=; b=Tr8tWk+QJJePIQp2HV89Zztn8laajT5b87Dl9vqi+uWu3z2gRoCVn3PZKoSJmXLCT/ 7R+aAFWWmZ3XrFgWy5rV35EotMH8R+PNXgOUbVFvvd+RcY+Z4eS4KtT8kyunpub6dL5U 8mx7BWxfRvjgV4//WSKrK5673Gf21gDKL8e/s= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1705016552; x=1705621352; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=dPOt9smlRtRbYxwBRyVbgWAEdHEPdj5BH7dSKIF5aYg=; b=LfE06xshp4j9dQGhN5qtJcfLo8ayjyoQXCouaipZ0lndf+uGw++ysvDs1i2+PUUOqa TU2rsZdj0P6QhSIZlEvhhOTWOsqf1bI6U7uBv31embmJMTLJI7Ay+o6xGBJZCwNxshfr ETJSumWs1D8bLM9vrUPsjl45WHEEW9MNfO5JNHrFHfNxFuttKGzs+TAInYAHgTCHnp3s blkNu40IjBj9KAdOjo/4l6Dvvk1WQU062LuMCKe6MXCU4myD/uhEt4GFRdqaIM13oSgv wxDc2izXQUpdTXWpSlu04JGySmoy/Tr4y+bepcJdyBASSGGR3RoGgSIzNd7WRl2IGAAX skAg== X-Gm-Message-State: AOJu0YzY6c+rrMfyZAzxqLnMUakyb0FZCIef4+aAWffw47mp8lIvkA7j g0insWqsde1SMKZH+63XWuey9tL1uKum X-Google-Smtp-Source: AGHT+IHJDwdHYyS7Zbj3sKxeCcxONqJ1iFvG2HlgJ22bfBwKo2IZ7jo/F+uXbfaJl4SWskGm+QdBFw== X-Received: by 2002:a05:6a00:22d5:b0:6d9:bf59:462a with SMTP id f21-20020a056a0022d500b006d9bf59462amr98647pfj.65.1705016551711; Thu, 11 Jan 2024 15:42:31 -0800 (PST) Received: from localhost (34.85.168.34.bc.googleusercontent.com. [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> Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 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