linux-kernel - Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand unprotected when accessed by /proc)

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Date:	Tue, 10 Jun 2014 05:52:35 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Clark Williams <williams@...hat.com>
Subject: Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand
 unprotected when accessed by /proc)

On Tue, Jun 10, 2014 at 10:37:26AM +0200, Peter Zijlstra wrote:
> On Mon, Jun 09, 2014 at 09:26:13AM -0700, Paul E. McKenney wrote:
> > That would indeed be a bad thing, as it could potentially lead to
> > use-after-free bugs.  Though one could argue that any code that resulted
> > in use-after-free would be quite aggressive.  But still...
> 
> Let me hijack this thread for yet another issue... So I had an RCU
> related use-after-free the other day, and while Sasha was able to
> trigger it quite easily, I had a multi-day struggle to reproduce.
> 
> Once I figured out what the exact problem was it was also clear to me
> why it was so hard for me to reproduce.
> 
> So normally its easier to trigger races on bigger machines, more cpus,
> more concurrency, more races, all good.
> 
> _However_ with RCU the grace period machinery is slower the bigger the
> machine, so bigger machine, slower grace period, slower RCU free, less
> likely to hit use-after-free.
> 
> So I was thinking, and I know you all will go kick me for this because
> the very last thing we need is what I'm about to propose: more RCU
> flavours :-).
> 
> How about an rcu_read_unlock() reference counted RCU variant that's
> ultra aggressive in doing the callbacks in order to better trigger such
> issues?

If you are using synchronize_rcu() for the update side, then I suggest
rcutorture.gp_exp=1 to force use expediting throughout.

							Thanx, Paul

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