netdev - Re: [PATCH V2] ipv6: fix race condition regarding dst->expires and dst->from.

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Message-ID: <20130220163456.GB14501@hmsreliant.think-freely.org>
Date:	Wed, 20 Feb 2013 11:34:56 -0500
From:	Neil Horman <nhorman@...driver.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	YOSHIFUJI Hideaki <yoshfuji@...ux-ipv6.org>,
	Gao Feng <gaofeng@...fujitsu.com>, netdev@...r.kernel.org,
	davem@...emloft.net, "Steinar H. Gunderson" <sesse@...gle.com>
Subject: Re: [PATCH V2] ipv6: fix race condition regarding dst->expires and
 dst->from.

On Wed, Feb 20, 2013 at 08:12:40AM -0800, Eric Dumazet wrote:
> On Wed, 2013-02-20 at 19:29 +0900, YOSHIFUJI Hideaki wrote:
> > Eric Dumazet wrote:
> > | Some strange crashes happen in rt6_check_expired(), with access
> > | to random addresses.
> > |
> > | At first glance, it looks like the RTF_EXPIRES and
> > | stuff added in commit 1716a96101c49186b
> > | (ipv6: fix problem with expired dst cache)
> > | are racy : same dst could be manipulated at the same time
> > | on different cpus.
> > |
> > | At some point, our stack believes rt->dst.from contains a dst pointer,
> > | while its really a jiffie value (as rt->dst.expires shares the same area
> > | of memory)
> > |
> > | rt6_update_expires() should be fixed, or am I missing something ?
> > |
> > | CC Neil because of https://bugzilla.redhat.com/show_bug.cgi?id=892060
> > 
> > Because we do not have any locks for dst_entry, we cannot change
> > essential structure in the entry; e.g., we cannot change reference
> > to other entity.
> > 
> > To fix this issue, split 'from' and 'expires' field in dst_entry
> > out of union.  Once it is 'from' is assigned in the constructor,
> > keep the reference until the very last stage of the life time of
> > the object.
> > 
> > Of course, it is unsafe to change 'from', so make rt6_set_from simple
> > just for fresh entries.
> > 
> > Reported-by: Eric Dumazet <eric.dumazet@...il.com>
> > Reported-by: Neil Horman <nhorman@...driver.com>
> > CC: Gao Feng <gaofeng@...fujitsu.com>
> > Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@...ux-ipv6.org>
> > ---
> 
> This seems good to me, but I cant test it at this moment.
> 
> I CC Steinar as he reported one crash to me.
> 
> Thanks Yoshifuji !
> 
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>
> 
> Reported-by: Steinar H. Gunderson <sesse@...gle.com>
> 
I've also got requests in to test, and a recent fedora build running here:
http://koji.fedoraproject.org/koji/taskinfo?taskID=5036210

If anyone else wants to test it.  Although, looking at it, I think this is a
good fix:
Reviewed-by: Neil Horman <nhorman@...driver.com>

> 
> 
> 
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