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

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Date:	Tue, 06 Sep 2011 12:59:17 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"Yan, Zheng" <zheng.z.yan@...el.com>,
	"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 Tue, 2011-09-06 at 21:43 +0200, Eric Dumazet wrote:
> Le mardi 06 septembre 2011 à 12:33 -0700, Tim Chen a écrit :
> 
> > Yes, I think locking the sendmsg for the entire duration of
> > unix_stream_sendmsg makes a lot of sense.  It simplifies the logic a lot
> > more.  I'll try to cook something up in the next couple of days.
> 
> Thats not really possible, we cant hold a spinlock and call
> sock_alloc_send_skb() and/or memcpy_fromiovec(), wich might sleep.
> 
> You would need to prepare the full skb list, then :
> - stick the ref on the last skb of the list.
> 
> Transfert the whole skb list in other->sk_receive_queue in one go,
> instead of one after another.
> 
> Unfortunately, this would break streaming (big send(), and another
> thread doing the receive)
> 
> Listen, I am wondering why hackbench even triggers SCM code. This is
> really odd. We should not have a _single_ pid/cred ref/unref at all.
> 

Hackbench triggers the code because it has a bunch of threads sending
msgs on UNIX socket.
> 

Well, if the lock socket approach doesn't work, then my original patch
plus Yan Zheng's fix should still work.  I'll try to answer your
objections below:


> I was discussing of things after proposed patch, not current net-next.
> 
> This reads :
> 
> err = unix_scm_to_skb(siocb->scm, skb, !fds_sent, scm_ref);
> 
> So first skb is sent without ref taken, as mentioned in Changelog ?
> 

No. the first skb is sent *with* ref taken, as scm_ref is set to true for
first skb.

> 
> If second skb cannot be built, we exit this system call with an already
> queued skb. Receiver can then access to freed memory.
> 

No, we do have reference set.  For first skb, in unix_scm_to_skb.  For the 
second skb (which is the last skb), in scm_sent.  Should the second skb alloc failed,
we'll release the ref in scm_destroy.  Otherwise, the receiver will release
the references will consuming the skb.

Tim




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