[PATCH v2 0/3] namei: implement various lookup restriction AT_* flags

From: Aleksa Sarai
Date: Tue Oct 09 2018 - 02:53:33 EST

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.

As per the discussion in the AT_NO_JUMPS thread, AT_NO_JUMPS has been
split into separate flags.

* AT_XDEV blocks mountpoint crossings (both upwards and downwards).
openat("/", "tmp", AT_XDEV); // blocked
openat("/tmp", "..", AT_XDEV); // blocked
openat("/tmp", "/", AT_XDEV); // blocked

* AT_NO_PROCLINKS blocks all resolution through /proc/$pid/fd/$fd
"symlinks". Specifically, this blocks all jumps caused by a
filesystem using nd_jump_link() to shove you around in the
filesystem tree (these are referred to as "proclinks" in lieu of a
better name).
openat(AT_FDCWD, "/proc/self/root", AT_NO_PROCLINKS); // blocked
openat(AT_FDCWD, "/proc/self/fd/0", AT_NO_PROCLINKS); // blocked
openat(AT_FDCWD, "/proc/self/ns/mnt", AT_NO_PROCLINKS); // blocked

* AT_BENEATH disallows escapes from the starting dirfd using ".." or
absolute paths (either in the path or during symlink resolution).
Conceptually this flag ensures that you "stay below" the starting
point in the filesystem tree. ".." resolution is allowed if it
doesn't land you outside of the starting point (this is made safe
against races by patch 3 in this series).
openat("/root", "foo", AT_BENEATH); // *not* blocked
openat("/root", "a/../b", AT_BENEATH); // *not* blocked
openat("/root", "a/../../root/b", AT_BENEATH); // blocked
openat("/root", "/root", AT_BENEATH); // blocked

AT_BENEATH also currently disallows all "proclink" resolution
because they can trivially throw you outside of the starting point.
In a future patch we might allow such resolution (as long as it
stays within the root).
openat("/", "proc/self/exe", AT_BENEATH); // blocked

In addition, two more flags have been added to the series:

* AT_NO_SYMLINKS disallows *all* symlink resolution, and thus implies
AT_NO_PROCLINKS. Linus mentioned this is something that git would
like to have in the original discussion[5].
// assuming 'ln -s / /usr'
openat("/", "/usr/bin", AT_NO_SYMLINKS); // blocked
openat("/", "/proc/self/root", AT_NO_PROCLINKS); // blocked

* AT_THIS_ROOT is a very similar idea to AT_BENEATH, but it serves a
very different purpose. Rather than blocking resolutions if they
would go outside of the starting point, it treats the starting point
as a form of chroot(2). Container runtimes are one of the primary
justifications for this flag, as they currently have to implement
this sort of path handling racily in userspace[6].

The restrictions on "proclink" resolution are the same as with
AT_BENEATH (though in AT_THIS_ROOT's case it's not really clear how
"proclink" jumps outside of the root should be handled), and patch 3
in this series was also required to make ".." resolution safe.

Currently all of these flags are only enabled for openat(2) (and thus
have their own O_* flag names), but the corresponding AT_* flags have
been reserved so they can be added to syscalls where openat(O_PATH) is
not sufficient.

Patch changelog:
* 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).

[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

Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Christian Brauner <christian@xxxxxxxxxx>
Cc: David Drysdale <drysdale@xxxxxxxxxx>
Cc: <containers@xxxxxxxxxxxxxxxxxxxxxxxxxx>
Cc: <linux-fsdevel@xxxxxxxxxxxxxxx>
Cc: <linux-api@xxxxxxxxxxxxxxx>

Aleksa Sarai (3):
namei: implement O_BENEATH-style AT_* flags
namei: implement AT_THIS_ROOT chroot-like path resolution
namei: aggressively check for nd->root escape on ".." resolution

fs/fcntl.c | 2 +-
fs/namei.c | 241 +++++++++++++++++++++++--------
fs/open.c | 10 ++
fs/stat.c | 4 +-
include/linux/fcntl.h | 3 +-
include/linux/namei.h | 8 +
include/uapi/asm-generic/fcntl.h | 20 +++
include/uapi/linux/fcntl.h | 10 ++
8 files changed, 230 insertions(+), 68 deletions(-)