[PATCH AUTOSEL 4.9 84/90] ceph: fix potential race in ceph_check_caps

From: Sasha Levin
Date: Thu Sep 17 2020 - 22:24:26 EST


From: Jeff Layton <jlayton@xxxxxxxxxx>

[ Upstream commit dc3da0461cc4b76f2d0c5b12247fcb3b520edbbf ]

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@xxxxxxxxxx>
Signed-off-by: Ilya Dryomov <idryomov@xxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
fs/ceph/caps.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
index e11aacb35d6b5..cbd92dd89de16 100644
--- a/fs/ceph/caps.c
+++ b/fs/ceph/caps.c
@@ -1807,12 +1807,24 @@ 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;
}
}
--
2.25.1