linux-kernel - Re: [PATCH] ipv4: avoid divide 0 error in tcp_incr

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Date:	Fri, 31 Oct 2014 09:24:42 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Chen Weilong <chenweilong@...wei.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ipv4: avoid divide 0 error in tcp_incr_quickack

cc-ing netdev

On Fri, Oct 31, 2014 at 7:50 AM, Chen Weilong <chenweilong@...wei.com> wrote:
> From: Weilong Chen <chenweilong@...wei.com>
>
> We got a problem like this:
>  [ffff8801c1a05570] machine_kexec at ffffffff81025039
>  [ffff8801c1a055d0] crash_kexec at ffffffff8109b253
>  [ffff8801c1a056a0] oops_end at ffffffff81442aed
>  [ffff8801c1a056d0] die at ffffffff81005603
>  [ffff8801c1a05700] do_trap at ffffffff81442448
>  [ffff8801c1a05760] do_divide_error at ffffffff81002c10
>  [ffff8801c1a05888] tcp_send_dupack at ffffffff81385e44
>  [ffff8801c1a058c8] tcp_validate_incoming at ffffffff813886b5
>  [ffff8801c1a05908] tcp_rcv_state_process at ffffffff8138d0b7
>  [ffff8801c1a05958] tcp_child_process at ffffffff81397255
>  [ffff8801c1a05988] tcp_v4_do_rcv at ffffffff81395a70
>  [ffff8801c1a059d8] tcp_v4_rcv at ffffffff81396fc8
>  [ffff8801c1a05a48] ip_local_deliver_finish at ffffffff813746e9
>  [ffff8801c1a05a78] ip_local_deliver at ffffffff81374a20
>  [ffff8801c1a05aa8] ip_rcv_finish at ffffffff81374389
>  [ffff8801c1a05ad8] ip_rcv at ffffffff81374c78
> There was a wrong ack packet coming during TCP handshake. The socket's state
> was TCP_SYN_RECV, its rcv_mss was not initialize yet. So
> tcp_send_dupack -> tcp_enter_quickack_mode got a divide 0 error.
> This patch add a state check before tcp_enter_quickack_mode.

ouch. Is it remote exploitable?

> Signed-off-by: Weilong Chen <chenweilong@...wei.com>
> ---
>  net/ipv4/tcp_input.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 4e4617e..9eb56dc 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -3986,7 +3986,8 @@ static void tcp_send_dupack(struct sock *sk, const struct sk_buff *skb)
>         if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
>             before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
>                 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
> -               tcp_enter_quickack_mode(sk);
> +               if (sk->sk_state != TCP_SYN_RECV)
> +                       tcp_enter_quickack_mode(sk);
>
>                 if (tcp_is_sack(tp) && sysctl_tcp_dsack) {
>                         u32 end_seq = TCP_SKB_CB(skb)->end_seq;
> --
> 1.7.12
>
>
> --
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