netdev - Re: [PATCH 5/5] xfrm: Fix unlink race when policies are deleted.

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Message-ID: <20140227065353.GM32371@secunet.com>
Date:	Thu, 27 Feb 2014 07:53:53 +0100
From:	Steffen Klassert <steffen.klassert@...unet.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
CC:	David Miller <davem@...emloft.net>, <netdev@...r.kernel.org>
Subject: Re: [PATCH 5/5] xfrm: Fix unlink race when policies are deleted.

On Thu, Feb 27, 2014 at 02:30:15PM +0800, Herbert Xu wrote:
> On Thu, Feb 27, 2014 at 02:27:12PM +0800, Herbert Xu wrote:
> > On Thu, Feb 27, 2014 at 07:16:39AM +0100, Steffen Klassert wrote:
> > > When a policy is unlinked from the lists in thread context,
> > > the xfrm timer can fire before we can mark this policy as dead.
> > > So reinitialize the bydst hlist, then hlist_unhashed() will
> > > notice that this policy is not linked and will avoid a
> > > doulble unlink of that policy.
> > > 
> > > Reported-by: Xianpeng Zhao <673321875@...com>
> > > Signed-off-by: Steffen Klassert <steffen.klassert@...unet.com>
> > 
> > If this race condition is real then I don't see how this fix
> > can be correct since there aren't any barriers between the check
> > and the unlink.
> 
> Looking at the code it seems that we've already taken the policy
> lock so how can this race occur?

We take the policy lock in xfrm_policy_delete() before we
unlink the policy. But we release it before we mark the
policy as dead with xfrm_policy_kill(). If the timer fires
in the meantime it can be unlinked from the timer first
and then again from xfrm_policy_delete().

This issue was originally reported here:
https://bugzilla.kernel.org/show_bug.cgi?id=70471
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