Subject: Re: [PATCH net] tcp: fix tcp_mtu_probe() vs highest_sack

From: Oleksandr Natalenko <[email-masked]>
Date: Fri, 03 Nov 2017 19:22:00 +0100

Hi.

Thanks for the fix.

However, tcp_fastretrans_alert() warning case still remains open even with 
this patch. Do I understand correctly that these are 2 different issues?

Currently, I use latest 4.13 stable kernel + this patch and still get:

WARNING: CPU: 1 PID: 736 at net/ipv4/tcp_input.c:2826 tcp_fastretrans_alert
+0x7c8/0x990

Any idea on this?

On úterý 31. října 2017 7:08:20 CET Eric Dumazet wrote:
> From: Eric Dumazet <[email-masked]>
> 
> Based on SNMP values provided by Roman, Yuchung made the observation
> that some crashes in tcp_sacktag_walk() might be caused by MTU probing.
> 
> Looking at tcp_mtu_probe(), I found that when a new skb was placed
> in front of the write queue, we were not updating tcp highest sack.
> 
> If one skb is freed because all its content was copied to the new skb
> (for MTU probing), then tp->highest_sack could point to a now freed skb.
> 
> Bad things would then happen, including infinite loops.
> 
> This patch renames tcp_highest_sack_combine() and uses it
> from tcp_mtu_probe() to fix the bug.
> 
> Note that I also removed one test against tp->sacked_out,
> since we want to replace tp->highest_sack regardless of whatever
> condition, since keeping a stale pointer to freed skb is a recipe
> for disaster.
> 
> Fixes: a47e5a988a57 ("[TCP]: Convert highest_sack to sk_buff to allow direct
> access") Signed-off-by: Eric Dumazet <[email-masked]>
> Reported-by: Alexei Starovoitov <[email-masked]>
> Reported-by: Roman Gushchin <[email-masked]>
> Reported-by: Oleksandr Natalenko <[email-masked]>
> ---
>  include/net/tcp.h     |    6 +++---
>  net/ipv4/tcp_output.c |    3 ++-
>  2 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index
> 33599d17522d6a19b9d9a316cc1579cd5e71ee32..e6d0002a1b0bc5f28c331a760823c8dc9
> 2f8fe24 100644 --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -1771,12 +1771,12 @@ static inline void tcp_highest_sack_reset(struct
> sock *sk) tcp_sk(sk)->highest_sack = tcp_write_queue_head(sk);
>  }
> 
> -/* Called when old skb is about to be deleted (to be combined with new skb)
> */ -static inline void tcp_highest_sack_combine(struct sock *sk,
> +/* Called when old skb is about to be deleted and replaced by new skb */
> +static inline void tcp_highest_sack_replace(struct sock *sk,
>  					    struct sk_buff *old,
>  					    struct sk_buff *new)
>  {
> -	if (tcp_sk(sk)->sacked_out && (old == tcp_sk(sk)->highest_sack))
> +	if (old == tcp_highest_sack(sk))
>  		tcp_sk(sk)->highest_sack = new;
>  }
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index
> ae60dd3faed0adc71731bc686f878afd4c628d32..823003eef3a21a5cc5c27e0be9f46159a
> fa060df 100644 --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2062,6 +2062,7 @@ static int tcp_mtu_probe(struct sock *sk)
>  	nskb->ip_summed = skb->ip_summed;
> 
>  	tcp_insert_write_queue_before(nskb, skb, sk);
> +	tcp_highest_sack_replace(sk, skb, nskb);
> 
>  	len = 0;
>  	tcp_for_write_queue_from_safe(skb, next, sk) {
> @@ -2665,7 +2666,7 @@ static bool tcp_collapse_retrans(struct sock *sk,
> struct sk_buff *skb) else if (!skb_shift(skb, next_skb, next_skb_size))
>  			return false;
>  	}
> -	tcp_highest_sack_combine(sk, next_skb, skb);
> +	tcp_highest_sack_replace(sk, next_skb, skb);
> 
>  	tcp_unlink_write_queue(next_skb, sk);