[PATCH v6 0/6] namei: resolveat(2) path resolution restriction API

Aleksa Sarai
2019-05-06T16:54:33Z

Patch changelog:
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_ (and corresponding uapi) 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(NOFOLLOW|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 were 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).

In addition, a mirror set of AT_* flags have been added (though
currently these are only supported for execveat(2) -- and not for any
other syscall). The need for these is explained in patch 5 (it's
motivated by CVE-2019-5736).

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 (6):
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
binfmt_*: scope path resolution of interpreters
namei: resolveat(2) syscall

arch/alpha/kernel/syscalls/syscall.tbl | 1 +
arch/arm/tools/syscall.tbl | 1 +
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/binfmt_elf.c | 2 +-
fs/binfmt_elf_fdpic.c | 2 +-
fs/binfmt_em86.c | 4 +-
fs/binfmt_misc.c | 2 +-
fs/binfmt_script.c | 2 +-
fs/exec.c | 26 +-
fs/namei.c | 251 +++++++++++++++-----
include/linux/binfmts.h | 1 +
include/linux/fs.h | 9 +-
include/linux/namei.h | 8 +
include/uapi/linux/fcntl.h | 18 ++
27 files changed, 270 insertions(+), 71 deletions(-)

--
2.21.0