[PATCH net] tcp: fix NULL pointer dereference when restarting a Fast Open listener

[Hyunwoo Kim <imv4bel@gmail.com>]

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) and then listen() on the same
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 subsequent reqsk_queue_alloc() also initializes rskq_rst_head,
rskq_rst_tail, and qlen in that order without holding the lock. If a
request_sock is added between the initialization of rskq_rst_head and
rskq_rst_tail, 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. If
reqsk_fastopen_remove() has already passed the TCP_LISTEN check,
inet_csk_listen_stop() waits until the list insertion completes.

Also initialize rskq_rst_head, rskq_rst_tail, and qlen under the same
lock in reqsk_queue_alloc(). Drop the references to the detached
request_sock objects after releasing the lock.

Fixes: 0536fcc039a8 ("tcp: prepare fastopen code for upcoming listener changes")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
---
 net/ipv4/inet_connection_sock.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index e56beec438773..5590fc7efb88d 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -1333,9 +1333,11 @@ static int inet_ulp_can_listen(const struct sock *sk)
 
 static void reqsk_queue_alloc(struct request_sock_queue *queue)
 {
+	spin_lock_bh(&queue->fastopenq.lock);
 	queue->fastopenq.rskq_rst_head = NULL;
 	queue->fastopenq.rskq_rst_tail = NULL;
 	queue->fastopenq.qlen = 0;
+	spin_unlock_bh(&queue->fastopenq.lock);
 
 	queue->rskq_accept_head = NULL;
 }
@@ -1533,17 +1535,15 @@ 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