netdev - Re: Possible race condition in conntracking

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Message-ID: <497F109A.7080502@trash.net>
Date:	Tue, 27 Jan 2009 14:48:10 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Tobias Klausmann <klausman@...warzvogel.de>
CC:	netdev@...r.kernel.org,
	Netfilter Development Mailinglist 
	<netfilter-devel@...r.kernel.org>
Subject: Re: Possible race condition in conntracking

Tobias Klausmann wrote:
> So the question remains what to do instead and how to do it. That
> probably is deep Netfilter mojo, so I could only speculate wildly.
> 
>> You should see the insert_failed conntrack counter show this
>> (/proc/net/stat/nf_conntrack).
> 
> We do, as I said in my first mail. Near as I can tell,
> nf_conntrack_confirm() is the only function that ever increases
> that counter, so it's definitely dropped there. As to how one
> could handle it differently, I have to defer to people with more
> Netfilter expertise. No point in "fixing" this by breaking other
> stuff.

Fixing this requires some rather intrusive changes. We need
to perform a lookup on the unconfirmed list when a conntrack
is not found in the hash and use the one we find there, if any.
The entries on that list are not reference counted and there
are a lot of assumptions in the code that an unconfirmed conntrack
is exclusively associated with a single packet. This needs to
be audited and fixed, but it looks quite hard.

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