[PATCH] tipc: defer local Nagle backlog xmit from receive path

From: Bartosz Chronowski <[email protected]>

A local TIPC stream socket workload can trap a CPU in an endless receive
loop. The resulting soft lockup can make the system unavailable.

tipc_sk_push_backlog() can transmit delayed stream data while tipc_sk_rcv()
holds a destination socket's sk_lock.slock. Own-node transmission enters
tipc_sk_rcv() synchronously. A reply that reaches the ancestor socket
cannot acquire the still-held lock, and tipc_sk_rcv() retries without
consuming its input queue.

Pass the receive output queue to tipc_sk_push_backlog() from receive-side
callers. Queue own-node output there so the enclosing receive path sends it
after releasing the socket lock. Keep shutdown and remote-node transmission
on the existing direct path, preserving remote link congestion handling.

Fixes: c0bceb97db9e ("tipc: add smart nagle feature")
Assisted-by: Gemini:gemini-3.5-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=10a41dc44eef71aa9450
Link: https://syzkaller.appspot.com/ai_job?id=aa2dc129-33d3-42e7-bd0b-bf2198c9c9c2
Signed-off-by: Bartosz Chronowski <[email protected]>

---
diff --git a/net/tipc/socket.c b/net/tipc/socket.c
index e564341e0..33d744dc5 100644
--- a/net/tipc/socket.c
+++ b/net/tipc/socket.c
@@ -156,7 +156,8 @@ static int tipc_sk_insert(struct tipc_sock *tsk);
 static void tipc_sk_remove(struct tipc_sock *tsk);
 static int __tipc_sendstream(struct socket *sock, struct msghdr *m, size_t dsz);
 static int __tipc_sendmsg(struct socket *sock, struct msghdr *m, size_t dsz);
-static void tipc_sk_push_backlog(struct tipc_sock *tsk, bool nagle_ack);
+static void tipc_sk_push_backlog(struct tipc_sock *tsk, bool nagle_ack,
+				 struct sk_buff_head *xmitq);
 static int tipc_wait_for_connect(struct socket *sock, long *timeo_p);
 
 static const struct proto_ops packet_ops;
@@ -560,7 +561,7 @@ static void __tipc_shutdown(struct socket *sock, int error)
 					    !tsk_conn_cong(tsk)));
 
 	/* Push out delayed messages if in Nagle mode */
-	tipc_sk_push_backlog(tsk, false);
+	tipc_sk_push_backlog(tsk, false, NULL);
 	/* Remove pending SYN */
 	__skb_queue_purge(&sk->sk_write_queue);
 
@@ -1267,8 +1268,10 @@ void tipc_sk_mcast_rcv(struct net *net, struct sk_buff_head *arrvq,
 
 /* tipc_sk_push_backlog(): send accumulated buffers in socket write queue
  *                         when socket is in Nagle mode
+ * @xmitq: receive output queue, or NULL outside receive context
  */
-static void tipc_sk_push_backlog(struct tipc_sock *tsk, bool nagle_ack)
+static void tipc_sk_push_backlog(struct tipc_sock *tsk, bool nagle_ack,
+				 struct sk_buff_head *xmitq)
 {
 	struct sk_buff_head *txq = &tsk->sk.sk_write_queue;
 	struct sk_buff *skb = skb_peek_tail(txq);
@@ -1310,6 +1313,12 @@ static void tipc_sk_push_backlog(struct tipc_sock *tsk, bool nagle_ack)
 		tsk->pkt_cnt += skb_queue_len(txq);
 	tsk->snt_unacked += tsk->snd_backlog;
 	tsk->snd_backlog = 0;
+
+	if (xmitq && in_own_node(net, dnode)) {
+		skb_queue_splice_tail_init(txq, xmitq);
+		return;
+	}
+
 	rc = tipc_node_xmit(net, txq, dnode, tsk->portid);
 	if (rc == -ELINKCONG)
 		tsk->cong_link_cnt = 1;
@@ -1367,7 +1376,7 @@ static void tipc_sk_conn_proto_rcv(struct tipc_sock *tsk, struct sk_buff *skb,
 			goto exit;
 
 		was_cong = tsk_conn_cong(tsk);
-		tipc_sk_push_backlog(tsk, msg_nagle_ack(hdr));
+		tipc_sk_push_backlog(tsk, msg_nagle_ack(hdr), xmitq);
 		tsk->snt_unacked -= msg_conn_ack(hdr);
 		if (tsk->peer_caps & TIPC_BLOCK_FLOWCTL)
 			tsk->snd_win = msg_adv_win(hdr);
@@ -2165,7 +2174,7 @@ static void tipc_sk_proto_rcv(struct sock *sk,
 		smp_wmb();
 		tsk->cong_link_cnt--;
 		wakeup = true;
-		tipc_sk_push_backlog(tsk, false);
+		tipc_sk_push_backlog(tsk, false, xmitq);
 		break;
 	case GROUP_PROTOCOL:
 		tipc_group_proto_rcv(grp, &wakeup, hdr, inputq, xmitq);
@@ -2256,7 +2265,7 @@ static bool tipc_sk_filter_connect(struct tipc_sock *tsk, struct sk_buff *skb,
 		return false;
 	case TIPC_ESTABLISHED:
 		if (!skb_queue_empty(&sk->sk_write_queue))
-			tipc_sk_push_backlog(tsk, false);
+			tipc_sk_push_backlog(tsk, false, xmitq);
 		/* Accept only connection-based messages sent by peer */
 		if (likely(con_msg && !err && pport == oport &&
 			   pnode == onode)) {


base-commit: 1590cf0329716306e948a8fc29f1d3ee87d3989f
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