tcp: fix NULL pointer dereference when restarting a Fast Open listener

When a valid RST is received for a Fast Open child accepted before the three-way handshake completes, reqsk_fastopen_remove() adds the request_sock to the listener's RST list if the listener is in TCP_LISTEN.

An application can call shutdown(SHUT_RD) on a listener socket. inet_csk_listen_stop() checks rskq_rst_head without holding fastopenq.lock. If rskq_rst_head is NULL, it does not acquire the lock and therefore does not synchronize with an insertion by reqsk_fastopen_remove() after that function has passed the TCP_LISTEN check.

The more likely outcome of this race, even without a subsequent listen(), is a leak. If reqsk_fastopen_remove() inserts a request_sock after inet_csk_listen_stop() observes an empty list and returns, the request_sock is stranded in rskq_rst_head and keeps a reference on the listener. The socket is already in TCP_CLOSE, so a subsequent close() does not call inet_csk_listen_stop() again.

If listen() is called again on the same socket before the delayed list insertion, the insertion can overlap the queue initialization in reqsk_queue_alloc(). If it occurs after rskq_rst_head is set to NULL but before rskq_rst_tail is set to NULL, rskq_rst_head points to the request_sock while rskq_rst_tail is NULL.

When a valid RST is later received for another accepted Fast Open child, rskq_rst_head is not NULL, so reqsk_fastopen_remove() attempts to write the request_sock pointer to rskq_rst_tail->dl_next while rskq_rst_tail is NULL. KASAN reports:

  BUG: KASAN: null-ptr-deref in reqsk_fastopen_remove+0x147/0x210
  Write of size 8 at addr 0000000000000088 by task swapper/3/0
  ...
  Call Trace:
   <IRQ>
   ...
   reqsk_fastopen_remove+0x147/0x210
   tcp_check_req+0xa70/0x1060
   ...
   tcp_rcv_state_process+0x1cb/0x2ae0
   ...
   tcp_v4_do_rcv+0x1fe/0x740
   tcp_v4_rcv+0x1744/0x2070
   ...
   ip_protocol_deliver_rcu+0x5c/0x280
   ip_local_deliver_finish+0x171/0x330
   ip_local_deliver+0x107/0x370
   ...
   </IRQ>

To widen the race window, the issue was reproduced after adding mdelay(3000) immediately after reqsk_fastopen_remove() checks that the listener is in TCP_LISTEN and immediately after reqsk_queue_alloc() sets rskq_rst_head to NULL.

Make inet_csk_listen_stop() acquire fastopenq.lock regardless of the value of rskq_rst_head and detach the list while holding the lock. With this synchronization, reqsk_fastopen_remove() either inserts the request_sock before inet_csk_listen_stop() acquires the lock, in which case inet_csk_listen_stop() reclaims it, or sees that the listener is not in TCP_LISTEN and does not insert anything.

---
Changes in v2:
- Drop the reqsk_queue_alloc() locking hunk and keep only the inet_csk_listen_stop() change. (Eric)
- Document the more likely request_sock leak and the listener reference it holds. (Eric)
- v1: https://lore.kernel.org/all/avGpsTb2hsGIjSRk@v4bel/

---
 net/ipv4/inet_connection_sock.c | 20 +++++++++-----------
 1 file changed, 9 insertions(+), 11 deletions(-)

index e56beec438773..b9f3efb671017 100644
@@ -1533,17 +1533,15 @@ void inet_csk_listen_stop(struct sock *sk)
 		if (!has_current_bpf_ctx())
 			cond_resched();
 	}
-	if (queue->fastopenq.rskq_rst_head) {
-		/* Free all the reqs queued in rskq_rst_head. */
-		spin_lock_bh(&queue->fastopenq.lock);
-		req = queue->fastopenq.rskq_rst_head;
-		queue->fastopenq.rskq_rst_head = NULL;
-		spin_unlock_bh(&queue->fastopenq.lock);
-		while (req != NULL) {
-			next = req->dl_next;
-			reqsk_put(req);
-			req = next;
-		}
+	/* Free all the reqs queued in rskq_rst_head. */
+	spin_lock_bh(&queue->fastopenq.lock);
+	req = queue->fastopenq.rskq_rst_head;
+	queue->fastopenq.rskq_rst_head = NULL;
+	spin_unlock_bh(&queue->fastopenq.lock);
+	while (req != NULL) {
+		next = req->dl_next;
+		reqsk_put(req);
+		req = next;
 	}
 	WARN_ON_ONCE(sk->sk_ack_backlog);
 }

2.43.0