ceph: clear inode pointer when snap realm gets dropped by its inode

commit d95e674c upstream.

snap realm and corresponding inode have pointers to each other.
The two pointer should get clear at the same time. Otherwise,
snap realm's pointer may reference freed inode.

Cc: stable@vger.kernel.org # 4.17+
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Luis Henriques <lhenriques@suse.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/ceph/caps.c

@@ -1030,6 +1030,8 @@ static void drop_inode_snap_realm(struct ceph_inode_info *ci)
 	list_del_init(&ci->i_snap_realm_item);
 	ci->i_snap_realm_counter++;
 	ci->i_snap_realm = NULL;
+	if (realm->ino == ci->i_vino.ino)
+		realm->inode = NULL;
 	spin_unlock(&realm->inodes_with_caps_lock);
 	ceph_put_snap_realm(ceph_sb_to_client(ci->vfs_inode.i_sb)->mdsc,
 			    realm);