From: Jeff Layton <jlayton@kernel.org>
Date: Fri, 20 Mar 2020 20:45:45 +0000 (-0400)
Subject: ceph: fix potential race in ceph_check_caps
X-Git-Tag: v5.8-rc1~88^2~18
X-Git-Url: https://git.kernel.dk/?a=commitdiff_plain;h=dc3da0461cc4b76f2d0c5b12247fcb3b520edbbf;p=linux-block.git

ceph: fix potential race in ceph_check_caps

Nothing ensures that session will still be valid by the time we
dereference the pointer. Take and put a reference.

In principle, we should always be able to get a reference here, but
throw a warning if that's ever not the case.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
---

diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
index f135660a2479..d555d2790619 100644
--- a/fs/ceph/caps.c
+++ b/fs/ceph/caps.c
@@ -2044,12 +2044,24 @@ ack:
 			if (mutex_trylock(&session->s_mutex) == 0) {
 				dout("inverting session/ino locks on %p\n",
 				     session);
+				session = ceph_get_mds_session(session);
 				spin_unlock(&ci->i_ceph_lock);
 				if (took_snap_rwsem) {
 					up_read(&mdsc->snap_rwsem);
 					took_snap_rwsem = 0;
 				}
-				mutex_lock(&session->s_mutex);
+				if (session) {
+					mutex_lock(&session->s_mutex);
+					ceph_put_mds_session(session);
+				} else {
+					/*
+					 * Because we take the reference while
+					 * holding the i_ceph_lock, it should
+					 * never be NULL. Throw a warning if it
+					 * ever is.
+					 */
+					WARN_ON_ONCE(true);
+				}
 				goto retry;
 			}
 		}