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Date:   Tue, 10 Dec 2019 23:34:19 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Christoph Paasch <cpaasch@...le.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Jason Baron <jbaron@...mai.com>
Subject: [PATCH net] tcp: do not send empty skb from tcp_write_xmit()

Backport of commit fdfc5c8594c2 ("tcp: remove empty skb from
write queue in error cases") in linux-4.14 stable triggered
various bugs. One of them has been fixed in commit ba2ddb43f270
("tcp: Don't dequeue SYN/FIN-segments from write-queue"), but
we still have crashes in some occasions.

Root-cause is that when tcp_sendmsg() has allocated a fresh
skb and could not append a fragment before being blocked
in sk_stream_wait_memory(), tcp_write_xmit() might be called
and decide to send this fresh and empty skb.

Sending an empty packet is not only silly, it might have caused
many issues we had in the past with tp->packets_out being
out of sync.

Fixes: c65f7f00c587 ("[TCP]: Simplify SKB data portion allocation with NETIF_F_SG.")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Christoph Paasch <cpaasch@...le.com>
Cc: Neal Cardwell <ncardwell@...gle.com>
Cc: Jason Baron <jbaron@...mai.com>
---
 net/ipv4/tcp_output.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index b184f03d743715ef4b2d166ceae651529be77953..57f434a8e41ffd6bc584cb4d9e87703491a378c1 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2438,6 +2438,14 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 		if (tcp_small_queue_check(sk, skb, 0))
 			break;
 
+		/* Argh, we hit an empty skb(), presumably a thread
+		 * is sleeping in sendmsg()/sk_stream_wait_memory().
+		 * We do not want to send a pure-ack packet and have
+		 * a strange looking rtx queue with empty packet(s).
+		 */
+		if (TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq)
+			break;
+
 		if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp)))
 			break;
 
-- 
2.24.0.525.g8f36a354ae-goog

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