Re: [PATCH] fs/ceph/mds_client: give up on paths longer than PATH_MAX
From: Ilya Dryomov
Date: Tue Nov 19 2024 - 07:39:13 EST
On Mon, Nov 18, 2024 at 11:28 PM Max Kellermann
<max.kellermann@xxxxxxxxx> 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@xxxxxxxxxxxxxxx
> Reported-by: Dario Weißer <dario@xxxxxxxxx>
> Signed-off-by: Max Kellermann <max.kellermann@xxxxxxxxx>
> ---
> 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
>