Re: BUG: d_path() races with do_move_mount() on ->mnt_ns, leading to use-after-free

[Al Viro]

On Tue, Sep 13, 2022 at 07:14:56PM +0200, Jann Horn wrote:
> As the subject says, there's a race between d_path() (specifically
> __prepend_path()) looking at mnt->mnt_ns with is_anon_ns(), and
> do_move_mount() switching out the ->mnt_ns and freeing the old one.
> This can theoretically lead to a use-after-free read, but it doesn't
> seem to be very interesting from a security perspective, since all it
> gets you is a comparison of a value in freed memory with zero.

... with d_absolute_path() being the only caller that might even
theoretically care.

	Anyway, shouldn't be hard to deal with - adding rcu_head to
struct mnt_namespace (anon-unioned with e.g. ->list) and turning kfree()
in free_mnt_ns() into kfree_rcu() ought to do it...