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

[Peter Zijlstra]

On Tue, Jun 10, 2014 at 05:52:35AM -0700, Paul E. McKenney wrote:
> 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.

No such luck, this was regular kfree() from call_rcu(). And the callback
execution was typically delayed long enough to never 'see' the
use-after-free.

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