Patch changelog: v8: * Default to O_CLOEXEC to match other new fd-creation syscalls (users can always disable O_CLOEXEC afterwards). [Christian] * Implement magic-link restrictions based on their mode. This is done through a series of masks and is designed to avoid breaking users -- most users don't have chained O_PATH fd re-opens. * Add O_EMPTYPATH which allows for fd re-opening without needing procfs. This would help some users of fd re-opening, and with the changes to magic-link permissions we now have the right semantics for such a flag. * Add selftests for resolveat(2), O_EMPTYPATH, and the magic-link mode semantics. v7: * Remove execveat(2) support for these flags since it might result in some pretty hairy security issues with setuid binaries. There are other avenues we can go down to solve the issues with CVE-2019-5736. [Jann] * Reserve an additional bit in resolveat(2) for the eXecute access mode if we end up implementing it. v6: * Drop O_* flags API to the new LOOKUP_ path scoping bits and instead introduce resolveat(2) as an alternative method of obtaining an O_PATH. The justification for this is included in patch 6 (though switching back to O_* flags is trivial). v5: * In response to CVE-2019-5736 (one of the vectors showed that open(2)+fexec(3) cannot be used to scope binfmt_script's implicit open_exec()), AT_* flags have been re-added and are now piped through to binfmt_script (and other binfmt_* that use open_exec) but are only supported for execveat(2) for now. v4: * Remove AT_* flag reservations, as they require more discussion. * Switch to path_is_under() over __d_path() for breakout checking. * Make O_XDEV no longer block openat("/tmp", "/", O_XDEV) -- dirfd is now ignored for absolute paths to match other flags. * Improve the dirfd_path_init() refactor and move it to a separate commit. * Remove reference to Linux-capsicum. * Switch "proclink" name to magic-link. v3: [resend] v2: * Made ".." resolution with AT_THIS_ROOT and AT_BENEATH safe(r) with some semi-aggressive __d_path checking (see patch 3). * Disallowed "proclinks" with AT_THIS_ROOT and AT_BENEATH, in the hopes they can be re-enabled once safe. * Removed the selftests as they will be reimplemented as xfstests. * Removed stat(2) support, since you can already get it through O_PATH and fstatat(2).
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 resolveat(2) which provides an alternative way to get an O_PATH file descriptor (the reasoning for doing this is included in patch 6). The following new LOOKUP_ flags are added:
* LOOKUP_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 resolveat(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 ".." (see patch 4 for more detail).
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 as in patch 4, 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 O_THISROOT (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. resolveat(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.
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
Aleksa Sarai (10): 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: split out nd->dfd handling to dirfd_path_init namei: O_BENEATH-style path resolution flags namei: LOOKUP_IN_ROOT: chroot-like path resolution namei: aggressively check for nd->root escape on ".." resolution namei: resolveat(2) syscall kselftest: save-and-restore errno to allow for %m formatting selftests: add resolveat(2) selftests
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 | 3 + 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/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/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 | 397 ++++++++++++++--- fs/open.c | 10 +- fs/proc/base.c | 20 +- fs/proc/fd.c | 16 +- fs/proc/namespaces.c | 2 +- include/linux/fcntl.h | 10 +- include/linux/fs.h | 4 + include/linux/namei.h | 8 + include/linux/types.h | 2 +- include/uapi/asm-generic/fcntl.h | 5 + include/uapi/asm-generic/unistd.h | 5 +- include/uapi/linux/fcntl.h | 10 + tools/testing/selftests/Makefile | 1 + tools/testing/selftests/kselftest.h | 15 + tools/testing/selftests/resolveat/.gitignore | 1 + tools/testing/selftests/resolveat/Makefile | 6 + tools/testing/selftests/resolveat/helpers.h | 195 +++++++++ .../selftests/resolveat/linkmode_test.c | 306 ++++++++++++++ .../selftests/resolveat/resolveat_test.c | 400 ++++++++++++++++++ 39 files changed, 1350 insertions(+), 87 deletions(-) create mode 100644 tools/testing/selftests/resolveat/.gitignore create mode 100644 tools/testing/selftests/resolveat/Makefile create mode 100644 tools/testing/selftests/resolveat/helpers.h create mode 100644 tools/testing/selftests/resolveat/linkmode_test.c create mode 100644 tools/testing/selftests/resolveat/resolveat_test.c