ceph: fix race in concurrent __ceph_remove_cap invocations

A NULL pointer dereference may occur in __ceph_remove_cap with some of the
callbacks used in ceph_iterate_session_caps, namely trim_caps_cb and
remove_session_caps_cb. Those callers hold the session->s_mutex, so they
are prevented from concurrent execution, but ceph_evict_inode does not.

Since the callers of this function hold the i_ceph_lock, the fix is simply
a matter of returning immediately if caps->ci is NULL.

Cc: stable@vger.kernel.org
URL: https://tracker.ceph.com/issues/43272Suggested-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Luis Henriques <lhenriques@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/caps.c

@@ -1140,12 +1140,19 @@ void __ceph_remove_cap(struct ceph_cap *cap, bool queue_release)
 {
 	struct ceph_mds_session *session = cap->session;
 	struct ceph_inode_info *ci = cap->ci;
-	struct ceph_mds_client *mdsc =
-		ceph_sb_to_client(ci->vfs_inode.i_sb)->mdsc;
+	struct ceph_mds_client *mdsc;
 	int removed = 0;
 
+	/* 'ci' being NULL means the remove have already occurred */
+	if (!ci) {
+		dout("%s: cap inode is NULL\n", __func__);
+		return;
+	}
+
 	dout("__ceph_remove_cap %p from %p\n", cap, &ci->vfs_inode);
 
+	mdsc = ceph_inode_to_client(&ci->vfs_inode)->mdsc;
+
 	/* remove from inode's cap rbtree, and clear auth cap */
 	rb_erase(&cap->ci_node, &ci->i_caps);
 	if (ci->i_auth_cap == cap) {