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

[Max Kellermann <max.kellermann@ionos.com>]

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>
---
 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