netdev - Re: [PATCH -next v2] unix stream: Fix use-after-free crashes

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Date:	Tue, 06 Sep 2011 09:25:26 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	"Yan, Zheng" <zheng.z.yan@...el.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"sfr@...b.auug.org.au" <sfr@...b.auug.org.au>,
	"jirislaby@...il.com" <jirislaby@...il.com>,
	"sedat.dilek@...il.com" <sedat.dilek@...il.com>, alex.shi@...el.com
Subject: Re: [PATCH -next v2] unix stream: Fix use-after-free crashes

On Sun, 2011-09-04 at 13:44 +0800, Yan, Zheng wrote:
> Commit 0856a30409 (Scm: Remove unnecessary pid & credential references
> in Unix socket's send and receive path) introduced a use-after-free bug.
> It passes the scm reference to the first skb. Skb(s) afterwards may
> reference freed data structure because the first skb can be destructed
> by the receiver at anytime. The fix is by passing the scm reference to
> the very last skb.
> 
> Signed-off-by: Zheng Yan <zheng.z.yan@...el.com>
> Reported-by: Jiri Slaby <jirislaby@...il.com>
> ---

Thanks for finding this bug in my original patch.  I've missed the case
where receiving side could have released the all the references to the
credential before the send side is using the credential again for
subsequent skbs in the stream, thus causing the problem we saw.  Getting
an extra reference for pid/credentials at the beginning of the stream
and not getting reference for the last skb is the right approach.

Thanks also to Sedat, Valdis and Jiri for their extensive testing to
discover the bug and testing the subsequent fixes. 

Acked-by: Tim Chen <tim.c.chen@...ux.intel.com>

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