[PATCH] tcp: fix tcp_rtx_queue_tail in case of empty retransmit queue

[Tim Froidcoeur]

Commit 8c3088f895a0 ("tcp: be more careful in tcp_fragment()")
triggers following stack trace:

[25244.848046] kernel BUG at ./include/linux/skbuff.h:1406!
[25244.859335] RIP: 0010:skb_queue_prev+0x9/0xc
[25244.888167] Call Trace:
[25244.889182]  <IRQ>
[25244.890001]  tcp_fragment+0x9c/0x2cf
[25244.891295]  tcp_write_xmit+0x68f/0x988
[25244.892732]  __tcp_push_pending_frames+0x3b/0xa0
[25244.894347]  tcp_data_snd_check+0x2a/0xc8
[25244.895775]  tcp_rcv_established+0x2a8/0x30d
[25244.897282]  tcp_v4_do_rcv+0xb2/0x158
[25244.898666]  tcp_v4_rcv+0x692/0x956
[25244.899959]  ip_local_deliver_finish+0xeb/0x169
[25244.901547]  __netif_receive_skb_core+0x51c/0x582
[25244.903193]  ? inet_gro_receive+0x239/0x247
[25244.904756]  netif_receive_skb_internal+0xab/0xc6
[25244.906395]  napi_gro_receive+0x8a/0xc0
[25244.907760]  receive_buf+0x9a1/0x9cd
[25244.909160]  ? load_balance+0x17a/0x7b7
[25244.910536]  ? vring_unmap_one+0x18/0x61
[25244.911932]  ? detach_buf+0x60/0xfa
[25244.913234]  virtnet_poll+0x128/0x1e1
[25244.914607]  net_rx_action+0x12a/0x2b1
[25244.915953]  __do_softirq+0x11c/0x26b
[25244.917269]  ? handle_irq_event+0x44/0x56
[25244.918695]  irq_exit+0x61/0xa0
[25244.919947]  do_IRQ+0x9d/0xbb
[25244.921065]  common_interrupt+0x85/0x85
[25244.922479]  </IRQ>

tcp_rtx_queue_tail() (called by tcp_fragment()) can call
tcp_write_queue_prev() on the first packet in the queue, which will trigger
the BUG in tcp_write_queue_prev(), because there is no previous packet.

This happens when the retransmit queue is empty, for example in case of a
zero window.

Patch is needed for 4.4, 4.9 and 4.14 stable branches.

Fixes: 8c3088f895a0 ("tcp: be more careful in tcp_fragment()")
Signed-off-by: Tim Froidcoeur <tim.froidcoeur@xxxxxxxxxxxx>
Signed-off-by: Matthieu Baerts <matthieu.baerts@xxxxxxxxxxxx>
---
 include/net/tcp.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 9de2c8cdcc51..1e70ca75c8bf 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1705,6 +1705,10 @@ static inline struct sk_buff *tcp_rtx_queue_tail(const struct sock *sk)
 {
 	struct sk_buff *skb = tcp_send_head(sk);
 
+	/* empty retransmit queue, for example due to zero window */
+	if (skb == tcp_write_queue_head(sk))
+		return NULL;
+
 	return skb ? tcp_write_queue_prev(sk, skb) : tcp_write_queue_tail(sk);
 }
 
-- 
2.23.0


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