Re: Safety of resolving untrusted paths with detached mount dirfd

On 11/19/25 21:18, Aleksa Sarai wrote:

On 2025-11-19, Alyssa Ross [off-list ref] wrote:

quoted

Hello,

As we know, it's not safe to use chroot() for resolving untrusted paths
within some root, as a subdirectory could be moved outside of the
process root while walking the path[1].  On the other hand,
LOOKUP_BENEATH is supposed to be robust against this, and going by [2],
it sounds like resolving with the mount namespace root as dirfd should
also be.

My question is: would resolving an untrusted path against a detached
mount root dirfd opened with OPEN_TREE_CLONE (not necessarily a
filesystem root) also be expected to be robust against traversal issues?
i.e. can I rely on an untrusted path never resolving to a path that
isn't under the mount root?

No, if you hit an absolute symlink or use an absolute path it will
resolve to your current->fs->root (mount namespace root or chroot).
However, OPEN_TREE_CLONE will stop ".." from naively stepping out of the
detached bind-mount. If you are dealing with procfs then magic-links can
also jump out.

Is using open_tree_attr() with MOUNT_ATTR_NOSYMFOLLOW enough to prevent
these?  Will it still provide protection even if someone concurrently
renames one of the files out from under the root?  I know that can
escape a chroot, but I wonder if this provides more guarantees.
https://github.com/QubesOS/qubes-secpack/blob/main/QSBs/qsb-014-2015.txt
was the chroot breakout.

You can always use RESOLVE_BENEATH or RESOLVE_IN_ROOT in combination
with OPEN_TREE_CLONE.

Unfortunately not everything supports that.  For instance, mkdirat()
doesn't.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)