Subject: Re: [Oops] unix_dgram_connect locking problem?

From: Frederik Deweerdt <[email-masked]>
Date: Mon, 14 May 2007 20:49:08 +0200

Hi David,

On Mon, May 14, 2007 at 03:20:54AM -0700, David Miller wrote:
> From: Frederik Deweerdt <[email-masked]>
> Date: Fri, 11 May 2007 17:00:14 +0200
> 
> > I think that not unix_state_rlock'ing "other" in
> > unix_dgram_connect may cause it to become NULL while passing it to
> > selinux_socket_unix_may_send. With the following patch applied, I've
> > seen no oops so far (1-2 hours as opposed to a few minutes before applying
> > the patch). Any thoughts?
> 
> Thanks for this report and patch, similar code in UNIX stream connect
> has the following comment:
> 
> 	/* Latch our state.
> 
> 	   It is tricky place. We need to grab write lock and cannot
> 	   drop lock on peer. It is dangerous because deadlock is
> 	   possible. Connect to self case and simultaneous
> 	   attempt to connect are eliminated by checking socket
> 	   state. other is TCP_LISTEN, if sk is TCP_LISTEN we
> 	   check this before attempt to grab lock.
> 
> 	   Well, and we have to recheck the state after socket locked.
> 	 */
>  ...
> 	unix_state_wlock_nested(sk);
> 
> So I think we need to be careful about deadlocks wrt. holding
> both wlock on sk and rlock on other at the same time in
> the dgram case too.
Thanks for your answer. I'll write and test a patch taking this into
account. I'm thinking to check if sk == other, in which case we don't
need to rlock. This is sufficient to avoid the deadlock case, isn't
it?. If yes, I wonder why the stream case has to resort to the sk_state?

Thanks,
Frederik