netdev - Re: Bug in net/ipv6/ip6_fib.c:fib6_dump

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Message-ID: <4FE37783.9000409@akamai.com>
Date:	Thu, 21 Jun 2012 14:35:31 -0500
From:	Josh Hunt <johunt@...mai.com>
To:	"davem@...emloft.net" <davem@...emloft.net>,
	"kaber@...sh.net" <kaber@...sh.net>
CC:	Debabrata Banerjee <dbavatar@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"yoshfuji@...ux-ipv6.org" <yoshfuji@...ux-ipv6.org>,
	"jmorris@...ei.org" <jmorris@...ei.org>,
	"pekkas@...core.fi" <pekkas@...core.fi>,
	"kuznet@....inr.ac.ru" <kuznet@....inr.ac.ru>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"eric.dumazet@...il.com" <eric.dumazet@...il.com>
Subject: Re: Bug in net/ipv6/ip6_fib.c:fib6_dump_table()

On 06/12/2012 12:22 PM, Debabrata Banerjee wrote:
> Looks like commit 2bec5a369ee79576a3eea2c23863325089785a2c "ipv6: fib:
> fix crash when changing large fib while dumping" is the culprit. The
> result of this code is that if there is a tree addition while a dump
> has suspended because the netlink skb is full, it will simply go back
> to the top of the tree and you end up with duplicate/triplicate/etc
> routes. It looks like the code attempts to count nodes, but it's a
> linear count and the data structure is a tree so that's a big problem.
> The net result is potentially DOSable, since if route table updates
> happen often enough in proportion to table size, a dump will attempt
> to return an infinite amount of routes (observed). So this commit
> should be reverted. However I am interested in the problem that commit
> tried to solve, if anyone has more information on that. My assumption
> is the fib tree gets corrupted and eventually it crashes in
> fib6_dump_table(), which I assume can still happen.
> 
> I can easily demonstrate the bug by adding cloned/cache routes while I
> check the results of fib6_dump_table:
> 
> root@...2-25-43-12.deploy.akamaitechnologies.com:~# ip -6 -o route
> show table cache |tee tmp | wc -l; sort tmp | uniq -u | wc -l
> 593
> 189
> root@...2-25-43-12.deploy.akamaitechnologies.com:~# ip -6 -o route
> show table cache |tee tmp | wc -l; sort tmp | uniq -u | wc -l
> 884
> 16
> root@...2-25-43-12.deploy.akamaitechnologies.com:~# ip -6 -o route
> show table cache |tee tmp | wc -l; sort tmp | uniq -u | wc -l
> 888
> 78
> root@...2-25-43-12.deploy.akamaitechnologies.com:~# ip -6 -o route
> show table cache |tee tmp | wc -l; sort tmp | uniq -u | wc -l
> 507
> 507
> root@...2-25-43-12.deploy.akamaitechnologies.com:~# ip -6 -o route
> show table cache |tee tmp | wc -l; sort tmp | uniq -u | wc -l
> 533
> 533
> root@...2-25-43-12.deploy.akamaitechnologies.com:~# ip -6 -o route
> show table cache |tee tmp | wc -l; sort tmp | uniq -u | wc -l
> 571
> 571
> 
> Thanks,
> Debabrata

Ping?

Can anyone provide details of the crash which was intended to be fixed
by 2bec5a369ee79576a3eea2c23863325089785a2c? With this patch in and
doing concurrent adds/deletes and dumping the table via netlink causes
duplicate entries to be reported. Reverting this patch causes those
problems to go away. We can provide a more detailed test if that is
needed, but so far our testing has been unable to reproduce the crash
mentioned in the above commit with it reverted.

Thanks
Josh
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