From patchwork Tue Dec 12 23:17:05 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Jeff Xu X-Patchwork-Id: 13490117 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="Q6rto8wf" Received: from mail-ot1-x32c.google.com (mail-ot1-x32c.google.com [IPv6:2607:f8b0:4864:20::32c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0B5E4E3 for ; Tue, 12 Dec 2023 15:17:22 -0800 (PST) Received: by mail-ot1-x32c.google.com with SMTP id 46e09a7af769-6d84ddd642fso4746040a34.0 for ; Tue, 12 Dec 2023 15:17:22 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; t=1702423042; x=1703027842; 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=uU36FjOIbdJKQPRiJg4nrZQU0fb/SKB6wgyMQ4t+i44=; b=Q6rto8wfztnbMECUp7AIeqx0uu86PE9aG6ZHMHfsjUoqaLNMUDRhZk/SE9OjXCUtFl tyNx8suTCa8z103x5Vnhmo1qX9D6NaCaZE+t4uFYEu19z4tSFEZdam0z9d4iVNwCSfCg LwwbGS3JrdK+T+sSR4ad7VVWE1Nq2hRlTCaGo= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1702423042; x=1703027842; 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=uU36FjOIbdJKQPRiJg4nrZQU0fb/SKB6wgyMQ4t+i44=; b=sDRSl1Nuo+iXCWV02ndSl601MWRxOtsmLt5YASlukwTaQuiuSXsjI9Zj+kFFIuY1z4 xfYImsDnj8cfR+m1Md6mM6HgXDMhQ2GuAHxCZleMHKq50coyBrjAufmhXSrP7BLfVp35 fS/f5yotAVwlNIKI4AJ4yapt6+dMzD1/efkSOU/XYn0lz/JU1CwTwriR9vD/KdDKiZaO V8CvnH0L4fCAKpOJkDefjqY6LLZSS2IP6VjXLOba7moZ+4mOsIRFEa29c6T25k7AS83j eHIM3jeQwCX6m1BdCyPaA5jt1AehsaSAOr450+3GkuLIKbqYPuAywy3aass0WQft3j7a 2o0A== X-Gm-Message-State: AOJu0YyAfJyIFkTAzHpZpmxJl9ro8SUIj+Beq7RGjosTGN1BMDyXadm+ zHPl2wFUtInYtFvHTRTo4CfX0Q== X-Google-Smtp-Source: AGHT+IESnkqDjGhkYEemYfFxmrgB/rWYhFKb6e57jqVPg8rTGimOrJk6IlvAbIgCglInHK6pIykjCw== X-Received: by 2002:a05:6870:ec8b:b0:1fb:75c:400e with SMTP id eo11-20020a056870ec8b00b001fb075c400emr8193744oab.110.1702423042075; Tue, 12 Dec 2023 15:17:22 -0800 (PST) Received: from localhost (34.133.83.34.bc.googleusercontent.com. [34.83.133.34]) by smtp.gmail.com with UTF8SMTPSA id v29-20020a63481d000000b005c19c586cb7sm8685104pga.33.2023.12.12.15.17.21 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 12 Dec 2023 15:17:21 -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 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: [RFC PATCH v3 11/11] mseal:add documentation Date: Tue, 12 Dec 2023 23:17:05 +0000 Message-ID: <20231212231706.2680890-12-jeffxu@chromium.org> X-Mailer: git-send-email 2.43.0.472.g3155946c3a-goog In-Reply-To: <20231212231706.2680890-1-jeffxu@chromium.org> References: <20231212231706.2680890-1-jeffxu@chromium.org> Precedence: bulk X-Mailing-List: linux-hardening@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 | 189 ++++++++++++++++++++++++++ 1 file changed, 189 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..651c618d0664 --- /dev/null +++ b/Documentation/userspace-api/mseal.rst @@ -0,0 +1,189 @@ +.. 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()`` is an architecture independent syscall, and with following +signature: + +``int mseal(void addr, size_t len, unsigned long types, unsigned long flags)`` + +**addr/len**: virtual memory address range. + +The address range set by ``addr``/``len`` must meet: + - start (addr) must be in a valid VMA. + - end (addr + len) must be in a valid VMA. + - no gap (unallocated memory) between start and end. + - start (addr) must be page aligned. + +The ``len`` will be paged aligned implicitly by kernel. + +**types**: bit mask to specify the sealing types, they are: + +- The ``MM_SEAL_BASE``: Prevent VMA from: + + 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. + + Move or expand a different vma into the current location, + via mremap(). + + Modifying sealed 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. + + We consider the MM_SEAL_BASE feature, on which other sealing + features will depend. For instance, it probably does not make sense + to seal PROT_PKEY without sealing the BASE, and the kernel will + implicitly add SEAL_BASE for SEAL_PROT_PKEY. (If the application + wants to relax this in future, we could use the “flags” field in + mseal() to overwrite this the behavior.) + +- The ``MM_SEAL_PROT_PKEY``: + + Seal PROT and PKEY of the address range, in other words, + mprotect() and pkey_mprotect() will be denied if the memory is + sealed with MM_SEAL_PROT_PKEY. + +- The ``MM_SEAL_DISCARD_RO_ANON``: + + Certain types of madvise() operations are destructive [3], such + as MADV_DONTNEED, which can effectively alter region contents by + discarding pages, especially when memory is anonymous. This blocks + such operations for anonymous memory which is not writable to the + user. + +- The ``MM_SEAL_SEAL`` + Denies adding a new seal. + +**flags**: reserved for future use. + +**return values**: + +- ``0``: + - Success. + +- ``-EINVAL``: + - Invalid seal type. + - Invalid input flags. + - Start address is not page aligned. + - Address range (``addr`` + ``len``) overflow. + +- ``-ENOMEM``: + - ``addr`` is not a valid address (not allocated). + - End address (``addr`` + ``len``) is not a valid address. + - A gap (unallocated memory) between start and end. + +- ``-EACCES``: + - ``MM_SEAL_SEAL`` is set, adding a new seal is not allowed. + - Address range is not sealable, e.g. ``MAP_SEALABLE`` is not + set during ``mmap()``. + +**Note**: + +- User can call mseal(2) multiple times to add new seal types. +- Adding an already added seal type is a no-action (no error). +- unseal() or removing a seal type is not supported. +- In case of error return, one can expect the memory range is unchanged. + +``mmap()`` +---------- +``void *mmap(void* addr, size_t length, int prot, int flags, int fd, +off_t offset);`` + +We made two changes (``prot`` and ``flags``) to ``mmap()`` related to +memory sealing. + +**prot**: + +- ``PROT_SEAL_SEAL`` +- ``PROT_SEAL_BASE`` +- ``PROT_SEAL_PROT_PKEY`` +- ``PROT_SEAL_DISCARD_RO_ANON`` + +Allow ``mmap()`` to set the sealing type when creating a mapping. This is +useful for 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 type is set via the +``prot`` field in ``mmap()``, we don't require it to be set in the ``prot`` +field in later ``mprotect()`` call. 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. + +**flags** +The ``MAP_SEALABLE`` flag is added to 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`` when creating the map. + +Use Case: +========= +- glibc: + The dymamic linker, during loading ELF executables, can apply sealing to + to non-writeable 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 are not covered by +``mseal()``. If applications want to block such cases, sandboxer +(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