netdev - Re: [PATCH net] tcp: fix tcp_mtu_probe() vs highest

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Date:   Wed, 01 Nov 2017 21:20:17 +0900 (KST)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     ycheng@...gle.com, alexei.starovoitov@...il.com,
        oleksandr@...alenko.name, guro@...com, netdev@...r.kernel.org,
        ncardwell@...gle.com, brakmo@...com
Subject: Re: [PATCH net] tcp: fix tcp_mtu_probe() vs highest_sack

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Mon, 30 Oct 2017 23:08:20 -0700

> From: Eric Dumazet <edumazet@...gle.com>
> 
> 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 <edumazet@...gle.com>
> Reported-by: Alexei Starovoitov <alexei.starovoitov@...il.com>
> Reported-by: Roman Gushchin <guro@...com>
> Reported-by: Oleksandr Natalenko <oleksandr@...alenko.name>

Applied and queued up for -stable.