Re: [PATCH] tcp: don't update snd_nxt, when a socket is switched from repair mode

[David Miller <davem@davemloft.net>]

From: Andrey Vagin <avagin@openvz.org>
Date: Tue, 19 Nov 2013 22:10:06 +0400

> 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: [<ffffffff815aaaaf>] tcp_rearm_rto+0xcf/0x150
> ...
> [  146.301158] Call Trace:
> [  146.301158]  [<ffffffff815ab7f0>] 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.
> 
> Signed-off-by: Andrey Vagin <avagin@openvz.org>

Applied and queued up for -stable, thank you.