Re: [PATCH] tcp: dont handle MTU reduction on LISTEN socket

[dormando <dormando@rydia.net>]

> From: Eric Dumazet <edumazet@google.com>
>
> When an ICMP ICMP_FRAG_NEEDED (or ICMPV6_PKT_TOOBIG) message finds a
> LISTEN socket, and this socket is currently owned by the user, we
> set TCP_MTU_REDUCED_DEFERRED flag in listener tsq_flags.
>
> This is bad because if we clone the parent before it had a chance to
> clear the flag, the child inherits the tsq_flags value, and next
> tcp_release_cb() on the child will decrement sk_refcnt.
>
> Result is that we might free a live TCP socket, as reported by
> Dormando.
>
> IPv4: Attempt to release TCP socket in state 1
>
> Fix this issue by testing sk_state against TCP_LISTEN early, so that we
> set TCP_MTU_REDUCED_DEFERRED on appropriate sockets (not a LISTEN one)
>
> This bug was introduced in commit 563d34d05786
> (tcp: dont drop MTU reduction indications)
>
> Reported-by: dormando <dormando@rydia.net>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> ---
>  net/ipv4/tcp_ipv4.c |   14 +++++++-------
>  net/ipv6/tcp_ipv6.c |    7 +++++++
>  2 files changed, 14 insertions(+), 7 deletions(-)
>
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index 4a8ec45..d09203c 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -274,13 +274,6 @@ static void tcp_v4_mtu_reduced(struct sock *sk)
>  	struct inet_sock *inet = inet_sk(sk);
>  	u32 mtu = tcp_sk(sk)->mtu_info;
>
> -	/* We are not interested in TCP_LISTEN and open_requests (SYN-ACKs
> -	 * send out by Linux are always <576bytes so they should go through
> -	 * unfragmented).
> -	 */
> -	if (sk->sk_state == TCP_LISTEN)
> -		return;
> -
>  	dst = inet_csk_update_pmtu(sk, mtu);
>  	if (!dst)
>  		return;
> @@ -408,6 +401,13 @@ void tcp_v4_err(struct sk_buff *icmp_skb, u32 info)
>  			goto out;
>
>  		if (code == ICMP_FRAG_NEEDED) { /* PMTU discovery (RFC1191) */
> +			/* We are not interested in TCP_LISTEN and open_requests
> +			 * (SYN-ACKs send out by Linux are always <576bytes so
> +			 * they should go through unfragmented).
> +			 */
> +			if (sk->sk_state == TCP_LISTEN)
> +				goto out;
> +
>  			tp->mtu_info = info;
>  			if (!sock_owned_by_user(sk)) {
>  				tcp_v4_mtu_reduced(sk);
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index 9b64600..f6d629f 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/net/ipv6/tcp_ipv6.c
> @@ -389,6 +389,13 @@ static void tcp_v6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
>  	}
>
>  	if (type == ICMPV6_PKT_TOOBIG) {
> +		/* We are not interested in TCP_LISTEN and open_requests
> +		 * (SYN-ACKs send out by Linux are always <576bytes so
> +		 * they should go through unfragmented).
> +		 */
> +		if (sk->sk_state == TCP_LISTEN)
> +			goto out;
> +
>  		tp->mtu_info = ntohl(info);
>  		if (!sock_owned_by_user(sk))
>  			tcp_v6_mtu_reduced(sk);
>
>
>

Thanks! We are all incredibly appreciative of your quick help with this.

I have 3.8.3 + this patch running on our machine for 20 hours so far. We
haven't had a kernel newer than 3.2 go for a full day before. I'm giving
it another peak cycle before calling it dead but it looks like you nailed
it.

I assume you no longer need the kernel disassembly?

thanks again,
-Dormando