From dbde497966804e63a38fdedc1e3815e77097efc2 Mon Sep 17 00:00:00 2001 From: Andrey Vagin Date: Tue, 19 Nov 2013 22:10:06 +0400 Subject: [PATCH] tcp: don't update snd_nxt, when a socket is switched from repair mode snd_nxt must be updated synchronously with sk_send_head. Otherwise tp->packets_out may be updated incorrectly, what may bring a kernel panic. Here is a kernel panic from my host. [ 103.043194] BUG: unable to handle kernel NULL pointer dereference at 0000000000000048 [ 103.044025] IP: [] tcp_rearm_rto+0xcf/0x150 ... [ 146.301158] Call Trace: [ 146.301158] [] tcp_ack+0xcc0/0x12c0 Before this panic a tcp socket was restored. This socket had sent and unsent data in the write queue. Sent data was restored in repair mode, then the socket was switched from reapair mode and unsent data was restored. After that the socket was switched back into repair mode. In that moment we had a socket where write queue looks like this: snd_una snd_nxt write_seq |_________|________| | sk_send_head After a second switching from repair mode the state of socket was changed: snd_una snd_nxt, write_seq |_________ ________| | sk_send_head This state is inconsistent, because snd_nxt and sk_send_head are not synchronized. Bellow you can find a call trace, how packets_out can be incremented twice for one skb, if snd_nxt and sk_send_head are not synchronized. In this case packets_out will be always positive, even when sk_write_queue is empty. tcp_write_wakeup skb = tcp_send_head(sk); tcp_fragment if (!before(tp->snd_nxt, TCP_SKB_CB(buff)->end_seq)) tcp_adjust_pcount(sk, skb, diff); tcp_event_new_data_sent tp->packets_out += tcp_skb_pcount(skb); I think update of snd_nxt isn't required, when a socket is switched from repair mode. Because it's initialized in tcp_connect_init. Then when a write queue is restored, snd_nxt is incremented in tcp_event_new_data_sent, so it's always is in consistent state. I have checked, that the bug is not reproduced with this patch and all tests about restoring tcp connections work fine. Cc: Pavel Emelyanov Cc: Eric Dumazet Cc: "David S. Miller" Cc: Alexey Kuznetsov Cc: James Morris Cc: Hideaki YOSHIFUJI Cc: Patrick McHardy Signed-off-by: Andrey Vagin Acked-by: Pavel Emelyanov Acked-by: Eric Dumazet Signed-off-by: David S. Miller --- net/ipv4/tcp_output.c | 1 - 1 file changed, 1 deletion(-) diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index c5231d9b06d7..7820f3a7dd70 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -3097,7 +3097,6 @@ void tcp_send_window_probe(struct sock *sk) { if (sk->sk_state == TCP_ESTABLISHED) { tcp_sk(sk)->snd_wl1 = tcp_sk(sk)->rcv_nxt - 1; - tcp_sk(sk)->snd_nxt = tcp_sk(sk)->write_seq; tcp_xmit_probe_skb(sk, 0); } } -- 2.20.1