Re: Safety of resolving untrusted paths with detached mount dirfd

On Wed, 19 Nov 2025 14:46:35 +0100
Alyssa Ross [off-list ref] wrote:

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?

[1]: https://lore.kernel.org/lkml/CAG48ez30WJhbsro2HOc_DR7V91M+hNFzBP5ogRMZaxbAORvqzg@mail.gmail.com/ (local)
[2]: https://lore.kernel.org/lkml/C89D720F-3CC4-4FA9-9CBB-E41A67360A6B@amacapital.net/ (local)

May not be directly relevant, but I found 'pwd' giving the wrong answer
when done inside a chroot (that isn't a filesytem mount point) after
'faffing' [1] with network namespaces.

The basic problem was that two kernel 'inode' structures end up referencing
the base of the chroot - so the pointer equality test fails.

So you could find the path of the chroot without any help from outside. 

[1] Brain thinks it might have been an 'unshare' to leave a network namespace
that cause the problem.

	David