This patch is being developed here (with snapshots of each series version being stashed in separate branches with names of the form "resolveat/vX-summary"): https://github.com/cyphar/linux/tree/resolveat/master
Patch changelog: v11: * Fix checkpatch.pl errors and warnings where reasonable. * Minor cleanup to pr_warn logging for may_open_magiclink(). * Drop kselftests patch to handle %m formatting correctly, and send it through the kselftests tree directly. [Shuah Khan] v10: * Ensure that unlazy_walk() will fail if we are in a scoped walk and the caller has zeroed nd->root (this happens in a few places, I'm not sure why because unlazy_walk() does legitimize_path() already). In this case we need to go through path_init() again to reset it (otherwise we will have a breakout because set_root() will breakout). * Also add a WARN_ON (and return -ENOTRECOVERABLE) if LOOKUP_IN_ROOT is set and we are in set_root() -- which should never happen and will cause a breakout. * Make changes suggested by Al Viro: * Remove nd->{opath_mask,acc_mode} by moving all of the magic-link permission logic be done after trailing_symlink() (with trailing_magiclink()) only within path_openat(). * Introduce LOOKUP_MAGICLINK_JUMPED to be able to detect magic-link jumps done with nd_jump_link() (so we don't end up blocking other LOOKUP_JUMPED cases). * Simplify all of the path_init() changes to make the code far less confusing. dirfd_path_init() turns out to be un-necessary. * Make openat2(2) also -EINVAL on unknown how->flags. [Dmitry V. Levin] * Clean up bad definitions of O_EMPTYPATH on architectures where O_* flags are subtly different to <asm-generic/fcntl.h>. * Switch away from passing a struct to build_open_flags() and instead just copy the one field we need to temporarily modify (how->flags). Also fix a bug in OPENHOW_MODE. [Rasmus Villemoes] * Fix syscall linkages and switch to 437. [Arnd Bergmann] * Clean up text in commit messages and the cover-letter. [Rolf Eike Beer] * Fix openat2 selftest makefile. [Michael Ellerman]
The need for some sort of control over VFS's path resolution (to avoid malicious paths resulting in inadvertent breakouts) has been a very long-standing desire of many userspace applications. This patchset is a revival of Al Viro's old AT_NO_JUMPS[1,2] patchset (which was a variant of David Drysdale's O_BENEATH patchset[3] which was a spin-off of the Capsicum project[4]) with a few additions and changes made based on the previous discussion within [5] as well as others I felt were useful.
In line with the conclusions of the original discussion of AT_NO_JUMPS, the flag has been split up into separate flags. However, instead of being an openat(2) flag it is provided through a new syscall openat2(2) which provides several other improvements to the openat(2) interface (see the patch description for more details). The following new LOOKUP_* flags are added:
* LOOKUP_NO_XDEV blocks all mountpoint crossings (upwards, downwards, or through absolute links). Absolute pathnames alone in openat(2) do not trigger this.
* LOOKUP_NO_MAGICLINKS blocks resolution through /proc/$pid/fd-style links. This is done by blocking the usage of nd_jump_link() during resolution in a filesystem. The term "magic-links" is used to match with the only reference to these links in Documentation/, but I'm happy to change the name.
It should be noted that this is different to the scope of ~LOOKUP_FOLLOW in that it applies to all path components. However, you can do openat2(NO_FOLLOW|NO_MAGICLINKS) on a magic-link and it will *not* fail (assuming that no parent component was a magic-link), and you will have an fd for the magic-link.
* LOOKUP_BENEATH disallows escapes to outside the starting dirfd's tree, using techniques such as ".." or absolute links. Absolute paths in openat(2) are also disallowed. Conceptually this flag is to ensure you "stay below" a certain point in the filesystem tree -- but this requires some additional to protect against various races that would allow escape using "..".
Currently LOOKUP_BENEATH implies LOOKUP_NO_MAGICLINKS, because it can trivially beam you around the filesystem (breaking the protection). In future, there might be similar safety checks done as in LOOKUP_IN_ROOT, but that requires more discussion.
In addition, two new flags are added that expand on the above ideas:
* LOOKUP_NO_SYMLINKS does what it says on the tin. No symlink resolution is allowed at all, including magic-links. Just as with LOOKUP_NO_MAGICLINKS this can still be used with NOFOLLOW to open an fd for the symlink as long as no parent path had a symlink component.
* LOOKUP_IN_ROOT is an extension of LOOKUP_BENEATH that, rather than blocking attempts to move past the root, forces all such movements to be scoped to the starting point. This provides chroot(2)-like protection but without the cost of a chroot(2) for each filesystem operation, as well as being safe against race attacks that chroot(2) is not.
If a race is detected (as with LOOKUP_BENEATH) then an error is generated, and similar to LOOKUP_BENEATH it is not permitted to cross magic-links with LOOKUP_IN_ROOT.
The primary need for this is from container runtimes, which currently need to do symlink scoping in userspace[6] when opening paths in a potentially malicious container. There is a long list of CVEs that could have bene mitigated by having RESOLVE_THIS_ROOT (such as CVE-2017-1002101, CVE-2017-1002102, CVE-2018-15664, and CVE-2019-5736, just to name a few).
And further, several semantics of file descriptor "re-opening" are now changed to prevent attacks like CVE-2019-5736 by restricting how magic-links can be resolved (based on their mode). This required some other changes to the semantics of the modes of O_PATH file descriptor's associated /proc/self/fd magic-links. openat2(2) has the ability to further restrict re-opening of its own O_PATH fds, so that users can make even better use of this feature.
Finally, O_EMPTYPATH was added so that users can do /proc/self/fd-style re-opening without depending on procfs. The new restricted semantics for magic-links are applied here too.
In order to make all of the above more usable, I'm working on libpathrs[7] which is a C-friendly library for safe path resolution. It features a userspace-emulated backend if the kernel doesn't support openat2(2). Hopefully we can get userspace to switch to using it, and thus get openat2(2) support for free once it's ready.
Cc: Al Viro viro@zeniv.linux.org.uk Cc: Eric Biederman ebiederm@xmission.com Cc: Andy Lutomirski luto@kernel.org Cc: David Howells dhowells@redhat.com Cc: Jann Horn jannh@google.com Cc: Christian Brauner christian@brauner.io Cc: David Drysdale drysdale@google.com Cc: Tycho Andersen tycho@tycho.ws Cc: Kees Cook keescook@chromium.org Cc: Linus Torvalds torvalds@linux-foundation.org Cc: containers@lists.linux-foundation.org Cc: linux-fsdevel@vger.kernel.org Cc: linux-api@vger.kernel.org
[1]: https://lwn.net/Articles/721443/ [2]: https://lore.kernel.org/patchwork/patch/784221/ [3]: https://lwn.net/Articles/619151/ [4]: https://lwn.net/Articles/603929/ [5]: https://lwn.net/Articles/723057/ [6]: https://github.com/cyphar/filepath-securejoin [7]: https://github.com/openSUSE/libpathrs
Aleksa Sarai (8): namei: obey trailing magic-link DAC permissions procfs: switch magic-link modes to be more sane open: O_EMPTYPATH: procfs-less file descriptor re-opening namei: O_BENEATH-style path resolution flags namei: LOOKUP_IN_ROOT: chroot-like path resolution namei: aggressively check for nd->root escape on ".." resolution open: openat2(2) syscall selftests: add openat2(2) selftests
Documentation/filesystems/path-lookup.rst | 12 +- arch/alpha/include/uapi/asm/fcntl.h | 1 + arch/alpha/kernel/syscalls/syscall.tbl | 1 + arch/arm/tools/syscall.tbl | 1 + arch/arm64/include/asm/unistd.h | 2 +- arch/arm64/include/asm/unistd32.h | 2 + arch/ia64/kernel/syscalls/syscall.tbl | 1 + arch/m68k/kernel/syscalls/syscall.tbl | 1 + arch/microblaze/kernel/syscalls/syscall.tbl | 1 + arch/mips/kernel/syscalls/syscall_n32.tbl | 1 + arch/mips/kernel/syscalls/syscall_n64.tbl | 1 + arch/mips/kernel/syscalls/syscall_o32.tbl | 1 + arch/parisc/include/uapi/asm/fcntl.h | 39 +- arch/parisc/kernel/syscalls/syscall.tbl | 1 + arch/powerpc/kernel/syscalls/syscall.tbl | 1 + arch/s390/kernel/syscalls/syscall.tbl | 1 + arch/sh/kernel/syscalls/syscall.tbl | 1 + arch/sparc/include/uapi/asm/fcntl.h | 1 + arch/sparc/kernel/syscalls/syscall.tbl | 1 + arch/x86/entry/syscalls/syscall_32.tbl | 1 + arch/x86/entry/syscalls/syscall_64.tbl | 1 + arch/xtensa/kernel/syscalls/syscall.tbl | 1 + fs/fcntl.c | 2 +- fs/internal.h | 1 + fs/namei.c | 270 ++++++++++-- fs/open.c | 112 ++++- fs/proc/base.c | 20 +- fs/proc/fd.c | 23 +- fs/proc/namespaces.c | 2 +- include/linux/fcntl.h | 17 +- include/linux/fs.h | 8 +- include/linux/namei.h | 9 + include/linux/syscalls.h | 17 +- include/uapi/asm-generic/fcntl.h | 4 + include/uapi/asm-generic/unistd.h | 5 +- include/uapi/linux/fcntl.h | 42 ++ tools/testing/selftests/Makefile | 1 + tools/testing/selftests/memfd/memfd_test.c | 7 +- tools/testing/selftests/openat2/.gitignore | 1 + tools/testing/selftests/openat2/Makefile | 8 + tools/testing/selftests/openat2/helpers.c | 162 +++++++ tools/testing/selftests/openat2/helpers.h | 116 +++++ .../testing/selftests/openat2/linkmode_test.c | 333 +++++++++++++++ .../selftests/openat2/rename_attack_test.c | 127 ++++++ .../testing/selftests/openat2/resolve_test.c | 402 ++++++++++++++++++ 45 files changed, 1655 insertions(+), 107 deletions(-) create mode 100644 tools/testing/selftests/openat2/.gitignore create mode 100644 tools/testing/selftests/openat2/Makefile create mode 100644 tools/testing/selftests/openat2/helpers.c create mode 100644 tools/testing/selftests/openat2/helpers.h create mode 100644 tools/testing/selftests/openat2/linkmode_test.c create mode 100644 tools/testing/selftests/openat2/rename_attack_test.c create mode 100644 tools/testing/selftests/openat2/resolve_test.c