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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Jeff Layton <jlayton@kernel.org>,
        Ilya Dryomov <idryomov@gmail.com>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 4.9 096/121] ceph: fix potential race in ceph_check_caps
Date:   Tue, 29 Sep 2020 13:00:40 +0200
Message-Id: <20200929105934.940310959@linuxfoundation.org>
In-Reply-To: <20200929105930.172747117@linuxfoundation.org>
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From: Jeff Layton <jlayton@kernel.org>

[ 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@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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