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

[Neal Cardwell <ncardwell@google.com>]

On Tue, Oct 31, 2017 at 2:08 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> From: Eric Dumazet <edumazet@google.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@google.com>
> Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Reported-by: Roman Gushchin <guro@fb.com>
> Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>
> ---
>  include/net/tcp.h     |    6 +++---
>  net/ipv4/tcp_output.c |    3 ++-
>  2 files changed, 5 insertions(+), 4 deletions(-)

Acked-by: Neal Cardwell <ncardwell@google.com>

Nice catch, indeed. Thanks, Eric!

neal