From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8C1D02BAEF for ; Fri, 24 May 2024 02:40:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1716518453; cv=none; b=p8xJ/WIXNh3hveZ3aaVFxeDp+gC1PvdXY/RUMecgMULz8NkVuYmxe/TX9uDO3JAV9niL7j5BZSybFDE6dMwX0OaUw13tVNoqT4f4Of189dhhKAMxGL7EMcsEJM2Xpd0RCfGhR7FUs9LLczo510YEysaYyl1F7LOu2+zuqVqTPdI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1716518453; c=relaxed/simple; bh=ICKeMTGKYO+17JUPTMFav3G3m3rzYiU1W/8hhVRbXmw=; h=Date:To:From:Subject:Message-Id; b=FvLX4a+C7AYU9wmGiSHfyJvO9SSEt4Qd/Rrm/iK7QS+3KeAo2C1tkVoidEfWWt3FUJwiz0Dgi+dp1xuEoO5x7b846pzOfh0PwomSVW+TitYd1VDVINKs+es4UU2mmkccLPH7jo4Nnl5mWLS57SncJMl1dC+DKbcYDxf4ygYYolg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=KrxLxRD8; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="KrxLxRD8" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0DEA9C32781; Fri, 24 May 2024 02:40:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1716518453; bh=ICKeMTGKYO+17JUPTMFav3G3m3rzYiU1W/8hhVRbXmw=; h=Date:To:From:Subject:From; b=KrxLxRD84VLiDKTHYO+aXvNGuvtWud04kzB2WundLxITpZ/iMIJsZpe6/rVhw7U+J Pb4RML+Y2bQY/PEzW1dvDTxUq+rQKmdq3SgdbS6RCEPZQfl1UWgL4G0tkux38HicRT L4xJx6wXC8KRlWXJ22pJst58ZZe+Mroc4qWzR5s4= Date: Thu, 23 May 2024 19:40:52 -0700 To: mm-commits@vger.kernel.org,willy@infradead.org,usama.anjum@collabora.com,torvalds@linux-foundation.org,surenb@google.com,sroettger@google.com,shuah@kernel.org,pedro.falcato@gmail.com,Liam.Howlett@oracle.com,keescook@chromium.org,jorgelo@chromium.org,jeffxu@google.com,javier.carrasco.cruz@gmail.com,jannh@google.com,groeck@chromium.org,gregkh@linuxfoundation.org,dave.hansen@intel.com,corbet@lwn.net,amer.shanawany@gmail.com,jeffxu@chromium.org,akpm@linux-foundation.org From: Andrew Morton Subject: [merged mm-stable] mseal-add-documentation.patch removed from -mm tree Message-Id: <20240524024053.0DEA9C32781@smtp.kernel.org> Precedence: bulk X-Mailing-List: mm-commits@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The quilt patch titled Subject: mseal: add documentation has been removed from the -mm tree. Its filename was mseal-add-documentation.patch This patch was dropped because it was merged into the mm-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm ------------------------------------------------------ From: Jeff Xu Subject: mseal: add documentation Date: Mon, 15 Apr 2024 16:35:23 +0000 Add documentation for mseal(). Link: https://lkml.kernel.org/r/20240415163527.626541-5-jeffxu@chromium.org Signed-off-by: Jeff Xu Reviewed-by: Kees Cook Reviewed-by: Liam R. Howlett Cc: Dave Hansen Cc: Greg Kroah-Hartman Cc: Guenter Roeck Cc: Jann Horn Cc: Jeff Xu Cc: Jonathan Corbet Cc: Jorge Lucangeli Obes Cc: Linus Torvalds Cc: Matthew Wilcox (Oracle) Cc: Muhammad Usama Anjum Cc: Pedro Falcato Cc: Stephen Röttger Cc: Suren Baghdasaryan Cc: Amer Al Shanawany Cc: Javier Carrasco Cc: Shuah Khan Signed-off-by: Andrew Morton --- Documentation/userspace-api/index.rst | 1 Documentation/userspace-api/mseal.rst | 199 ++++++++++++++++++++++++ 2 files changed, 200 insertions(+) --- a/Documentation/userspace-api/index.rst~mseal-add-documentation +++ a/Documentation/userspace-api/index.rst @@ -20,6 +20,7 @@ System calls futex2 ebpf/index ioctl/index + mseal Security-related interfaces =========================== --- /dev/null +++ a/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 _ Patches currently in -mm which might be from jeffxu@chromium.org are selftest-mm-mseal-read-only-elf-memory-segment.patch