On Tue, 2013-11-19 at 22:10 +0400, Andrey Vagin wrote:
> 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.
>
> Cc: Pavel Emelyanov <[email protected]>
> Cc: Eric Dumazet <[email protected]>
> Cc: "David S. Miller" <[email protected]>
> Cc: Alexey Kuznetsov <[email protected]>
> Cc: James Morris <[email protected]>
> Cc: Hideaki YOSHIFUJI <[email protected]>
> Cc: Patrick McHardy <[email protected]>
> Signed-off-by: Andrey Vagin <[email protected]>
> ---
> net/ipv4/tcp_output.c | 1 -
> 1 file changed, 1 deletion(-)
Fixes: c0e88ff0f256 ("tcp: Repair socket queues")
Acked-by: Eric Dumazet <[email protected]>
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