Re: [PATCH] fs/ceph/mds_client: give up on paths longer than PATH_MAX

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2024-11-19 at 13:38 +0100, Ilya Dryomov wrote:
> On Mon, Nov 18, 2024 at 11:28 PM Max Kellermann
> <max.kellermann@ionos.com> wrote:
> > 
> > If the full path to be built by ceph_mdsc_build_path() happens to be
> > longer than PATH_MAX, then this function will enter an endless (retry)
> > loop, effectively blocking the whole task.  Most of the machine
> > becomes unusable, making this a very simple and effective DoS
> > vulnerability.
> > 
> > I cannot imagine why this retry was ever implemented, but it seems
> > rather useless and harmful to me.  Let's remove it and fail with
> > ENAMETOOLONG instead.
> 
> Hi Max,
> 
> When this was put in place in 2009, I think the idea of a retry was
> copied from CIFS.  Jeff preserved the retry when he massaged this code
> to not warn in case a rename race is detected [1].  CIFS got rid of it
> only a couple of years ago [2][3].
> 
> Adding Patrick and Venky as well, please chime in.
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f5946bcc5e79038f9f7cb66ec25bd3b2d39b2775
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f6a9bc336b600e1266e6eebb0972d75d5b93aea9
> [3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=991e72eb0e99764219865b9a3a07328695148e14
> 
> Thanks,
> 
>                 Ilya
> 
> > 
> > Cc: stable@vger.kernel.org
> > Reported-by: Dario Weißer <dario@cure53.de>
> > Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
> > ---
> >  fs/ceph/mds_client.c | 9 ++++-----
> >  1 file changed, 4 insertions(+), 5 deletions(-)
> > 
> > diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
> > index c4a5fd94bbbb..4f6ac015edcd 100644
> > --- a/fs/ceph/mds_client.c
> > +++ b/fs/ceph/mds_client.c
> > @@ -2808,12 +2808,11 @@ char *ceph_mdsc_build_path(struct ceph_mds_client *mdsc, struct dentry *dentry,
> > 
> >         if (pos < 0) {
> >                 /*
> > -                * A rename didn't occur, but somehow we didn't end up where
> > -                * we thought we would. Throw a warning and try again.
> > +                * The path is longer than PATH_MAX and this function
> > +                * cannot ever succeed.  Creating paths that long is
> > +                * possible with Ceph, but Linux cannot use them.
> >                  */
> > -               pr_warn_client(cl, "did not end path lookup where expected (pos = %d)\n",
> > -                              pos);
> > -               goto retry;
> > +               return ERR_PTR(-ENAMETOOLONG);
> >         }
> > 
> >         *pbase = base;
> > --
> > 2.45.2
> > 

I think the idea was that if we ended up with a path longer than
PATH_MAX that something must have changed in the middle of the reverse
pathwalk to make it too long a path, and we should just go and try it
again. At the very least, this code should cap the retries to a certain
number if you don't just return an error here.

-ENAMETOOLONG could be problematic there. This function is often called
when we have a dentry and need to build a path to it to send to the MDS
in a call. The system call that caused us to generate this path
probably doesn't involve a pathname itself, so the caller may be
confused by an -ENAMETOOLONG return.

You may want to go with a more generic error code here -- -EIO or
something. It might also be worthwhile to leave in a pr_warn_once or
something since there may be users confused by this error return.
-- 
Jeff Layton <jlayton@kernel.org>