ceph: give up on paths longer than PATH_MAX

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.

Cc: stable@vger.kernel.org
Reported-by: Dario Weißer <dario@cure53.de>
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Reviewed-by: Alex Markuze <amarkuze@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index 219a2cc2bf3cc2f22a0d76d822df3aef6de8649f..785fe489ef4b8e97f2030a47711235f9dce37389 100644 (file)
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -2800,12 +2800,11 @@ retry:
 
        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;