[PATCH] atm: fix skb leak in sigd_send() on a closing listen socket

[Weiming Shi <bestswngs@gmail.com>]

In the as_indicate path, sigd_send() pins the listening socket with
find_get_vcc() and queues the skb on its receive queue under lock_sock().
It does not check whether the socket is being torn down. If the listener
is closed concurrently, vcc_destroy_socket() purges the receive queue
once under lock_sock() and removes the socket from vcc_hash; the final
free goes __sk_destruct() -> vcc_sock_destruct(), which does not purge.
A skb queued after that purge is therefore leaked.

Recheck ATM_VF_CLOSE under lock_sock() before queuing and drop the skb if
the socket is closing. ATM_VF_CLOSE is set by vcc_destroy_socket() under
the same lock, so the check is serialised against the purge.

Reaching this requires an attached signalling daemon (CAP_NET_ADMIN and
CAP_SYS_RAWIO), as only the daemon emits as_indicate.

Fixes: ae88a5d2f29b ("net: atm: fix crash due to unvalidated vcc pointer in sigd_send()")
Tested-by: Xiang Mei <xmei5@asu.edu>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Link: https://lore.kernel.org/all/aigrk5B3VzaWgKIF@Air.local/
---
 net/atm/signaling.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/atm/signaling.c b/net/atm/signaling.c
index 358fbe5e4d1d0..cb80b5a9d8452 100644
--- a/net/atm/signaling.c
+++ b/net/atm/signaling.c
@@ -150,6 +150,11 @@ static int sigd_send(struct atm_vcc *vcc, struct sk_buff *skb)
 		sk = sk_atm(vcc);
 		pr_debug("as_indicate!!!\n");
 		lock_sock(sk);
+		/* Don't queue onto a closing listener; the skb would leak. */
+		if (test_bit(ATM_VF_CLOSE, &vcc->flags)) {
+			dev_kfree_skb(skb);
+			goto as_indicate_complete;
+		}
 		if (sk_acceptq_is_full(sk)) {
 			sigd_enq(NULL, as_reject, vcc, NULL, NULL);
 			dev_kfree_skb(skb);
-- 
2.43.0