Merge: CVE-2025-21669: vsock/virtio: discard packets if the transport changes

MR: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-9/-/merge_requests/6318

JIRA: https://issues.redhat.com/browse/RHEL-77228
CVE: CVE-2025-21669

```
vsock/virtio: discard packets if the transport changes

If the socket has been de-assigned or assigned to another transport,
we must discard any packets received because they are not expected
and would cause issues when we access vsk->transport.

A possible scenario is described by Hyunwoo Kim in the attached link,
where after a first connect() interrupted by a signal, and a second
connect() failed, we can find `vsk->transport` at NULL, leading to a
NULL pointer dereference.

Fixes: c0cfa2d ("vsock: add multi-transports support")
Cc: stable@vger.kernel.org
Reported-by: Hyunwoo Kim <v4bel@theori.io>
Reported-by: Wongi Lee <qwerty@theori.io>
Closes: https://lore.kernel.org/netdev/Z2LvdTTQR7dBmPb5@v4bel-B760M-AORUS-ELITE-AX/
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Hyunwoo Kim <v4bel@theori.io>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

(cherry picked from commit 2cb7c75)
```

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>

---

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Approved-by: Stefano Garzarella <sgarzare@redhat.com>
Approved-by: Luigi Leonardi <leonardi@redhat.com>
Approved-by: Stefan Hajnoczi <stefanha@redhat.com>
Approved-by: CKI KWF Bot <cki-ci-bot+kwf-gitlab-com@redhat.com>

Merged-by: Patrick Talbert <ptalbert@redhat.com>

Author: ptalbert

net/vmw_vsock/virtio_transport_common.c

@@ -1578,8 +1578,11 @@ void virtio_transport_recv_pkt(struct virtio_transport *t,
 
 	lock_sock(sk);
 
-	/* Check if sk has been closed before lock_sock */
-	if (sock_flag(sk, SOCK_DONE)) {
+	/* Check if sk has been closed or assigned to another transport before
+	 * lock_sock (note: listener sockets are not assigned to any transport)
+	 */
+	if (sock_flag(sk, SOCK_DONE) ||
+	    (sk->sk_state != TCP_LISTEN && vsk->transport != &t->transport)) {
 		(void)virtio_transport_reset_no_sock(t, skb);
 		release_sock(sk);
 		sock_put(sk);