netdev - Re: Small problem with tcp

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Date:	Wed, 15 Sep 2010 22:41:47 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Marshall <tdm.code@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Small problem with tcp_poll and RST

Le mercredi 15 septembre 2010 à 13:17 -0700, Tom Marshall a écrit :
> The code in tcp_poll seems to suffer from a race condition which can
> result in POLLIN but not POLLOUT for an outbound socket connection to
> a closed peer.
> 
> This can happen if, for example, the RST comes in immediately after
> checking sk->sk_err.  It is a small window of opportunity and so it
> only happens rarely.
> 
> Note this code has remained pretty much unchanged in 2.6.x for years,
> and the problem readily reproduces on a wide variety of systems (RHEL
> 5.x, Ubuntu 10.04, etc.)
> 
> I suppose it is arguable whether this is a bug or whether it deserves
> to be fixed, but it did cause an issue with some (admittedly broken)
> userspace code at my company.
> 
> I do not fully understand the intricacies of the interactions between
> the TCP state machine and the tcp_poll function (which runs unlocked).
>  However, I did find that the below appears to fix the issue.  Since
> the overhead is minimal when the socket state does not change, it
> should have very little performance impact.
> 
>         unsigned char oldstate;
> again:
>         oldstate = sk->sk_state;
> 
>         /* body of tcp_poll */
> 
>         if (sk->sk_state != oldstate)
>                 goto again;
> 

Hi Tom

Unfortunately this might shorten the race window.

Are you saying an application is blocked on a poll(events=POLLOUT) ?

If yes, we have a bug elsewhere (in RST processing)



--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html