netdev - Re: race in skb_splice

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Message-ID: <20080527110144.GA15141@2ka.mipt.ru>
Date:	Tue, 27 May 2008 15:01:44 +0400
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Octavian Purdila <opurdila@...acom.com>
Cc:	netdev@...r.kernel.org
Subject: Re: race in skb_splice_bits?

On Tue, May 27, 2008 at 03:25:23AM +0300, Octavian Purdila (opurdila@...acom.com) wrote:
> 
> Hi,
> 
> The following socket lock dropping in skb_splice_bits seems to open a race 
> condition which causes an invalid kernel access:
> 
> >        if (spd.nr_pages) {
> >                int ret;
> >
> >                /*                                                                                                                            
> >                 * Drop the socket lock, otherwise we have reverse                                                                            
> >                 * locking dependencies between sk_lock and i_mutex                                                                           
> >                 * here as compared to sendfile(). We enter here                                                                              
> >                 * with the socket lock held, and splice_to_pipe() will                                                                       
> >                 * grab the pipe inode lock. For sendfile() emulation,                                                                        
> >                 * we call into ->sendpage() with the i_mutex lock held                                                                       
> >                 * and networking will grab the socket lock.                                                                                  
> >                 */

What about sock_hold() here?
It will prevent from socket freeing and read/write to it will
immediately return with error if socket was closed by another thread.

> >                release_sock(__skb->sk);                                                                                                    
> >                ret = splice_to_pipe(pipe, &spd);
> >                lock_sock(__skb->sk);

And appropriate sock_put(sk);

-- 
	Evgeniy Polyakov
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