[PATCH stable 5.15/6.1/6.6] af_unix: Clear oob_skb in scan_inflight().

[Kuniyuki Iwashima <kuniyu@amazon.com>]

Embryo socket is not queued in gc_candidates, so we can't drop
a reference held by its oob_skb.

Let's say we create listener and embryo sockets, send the
listener's fd to the embryo as OOB data, and close() them
without recv()ing the OOB data.

There is a self-reference cycle like

  listener -> embryo.oob_skb -> listener

, so this must be cleaned up by GC.  Otherwise, the listener's
refcnt is not released and sockets are leaked:

  # unshare -n
  # cat /proc/net/protocols | grep UNIX-STREAM
  UNIX-STREAM 1024      0      -1   NI       0   yes  kernel ...

  # python3
  >>> from array import array
  >>> from socket import *
  >>>
  >>> s = socket(AF_UNIX, SOCK_STREAM)
  >>> s.bind('\0test\0')
  >>> s.listen()
  >>>
  >>> c = socket(AF_UNIX, SOCK_STREAM)
  >>> c.connect(s.getsockname())
  >>> c.sendmsg([b'x'], [(SOL_SOCKET, SCM_RIGHTS, array('i', [s.fileno()]))], MSG_OOB)
  1
  >>> quit()

  # cat /proc/net/protocols | grep UNIX-STREAM
  UNIX-STREAM 1024      3      -1   NI       0   yes  kernel ...
                        ^^^
                        3 sockets still in use after FDs are close()d

Let's drop the embryo socket's oob_skb ref in scan_inflight().

This also fixes a racy access to oob_skb that commit 9841991a446c
("af_unix: Update unix_sk(sk)->oob_skb under sk_receive_queue
lock.") fixed for the new Tarjan's algo-based GC.

Fixes: 314001f0bf92 ("af_unix: Add OOB support")
Reported-by: Lei Lu <llfamsec@gmail.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
---
This has no upstream commit because I replaced the entire GC in
6.10 and the new GC does not have this bug, and this fix is only
applicable to the old GC (<= 6.9), thus for 5.15/6.1/6.6.
---
---
 net/unix/garbage.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index 2a758531e102..b3fbdf129944 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -102,13 +102,14 @@ static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),
 			/* Process the descriptors of this socket */
 			int nfd = UNIXCB(skb).fp->count;
 			struct file **fp = UNIXCB(skb).fp->fp;
+			struct unix_sock *u;
 
 			while (nfd--) {
 				/* Get the socket the fd matches if it indeed does so */
 				struct sock *sk = unix_get_socket(*fp++);
 
 				if (sk) {
-					struct unix_sock *u = unix_sk(sk);
+					u = unix_sk(sk);
 
 					/* Ignore non-candidates, they could
 					 * have been added to the queues after
@@ -122,6 +123,13 @@ static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),
 				}
 			}
 			if (hit && hitlist != NULL) {
+#if IS_ENABLED(CONFIG_AF_UNIX_OOB)
+				u = unix_sk(x);
+				if (u->oob_skb) {
+					WARN_ON_ONCE(skb_unref(u->oob_skb));
+					u->oob_skb = NULL;
+				}
+#endif
 				__skb_unlink(skb, &x->sk_receive_queue);
 				__skb_queue_tail(hitlist, skb);
 			}
@@ -299,17 +307,9 @@ void unix_gc(void)
 	 * which are creating the cycle(s).
 	 */
 	skb_queue_head_init(&hitlist);
-	list_for_each_entry(u, &gc_candidates, link) {
+	list_for_each_entry(u, &gc_candidates, link)
 		scan_children(&u->sk, inc_inflight, &hitlist);
 
-#if IS_ENABLED(CONFIG_AF_UNIX_OOB)
-		if (u->oob_skb) {
-			kfree_skb(u->oob_skb);
-			u->oob_skb = NULL;
-		}
-#endif
-	}
-
 	/* not_cycle_list contains those sockets which do not make up a
 	 * cycle.  Restore these to the inflight list.
 	 */
-- 
2.39.5 (Apple Git-154)