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

Paul E. McKenney
2014-06-09T16:26:19Z

On Sun, Jun 08, 2014 at 03:07:18PM +0200, Oleg Nesterov wrote:
> On 06/06, Paul E. McKenney wrote:
> >
> > On Tue, Jun 03, 2014 at 10:01:25PM +0200, Oleg Nesterov wrote:
> > >
> > > I'll try to recheck rt_mutex_unlock() tomorrow. _Perhaps_ rcu_read_unlock()
> > > should be shifted from lock_task_sighand() to unlock_task_sighand() to
> > > ensure that rt_mutex_unlock() does nothihg with this memory after it
> > > makes another lock/unlock possible.
> > >
> > > But if we need this (currently I do not think so), this doesn't depend on
> > > SLAB_DESTROY_BY_RCU. And, at first glance, in this case rcu_read_unlock_special()
> > > might be wrong too.
> >
> > OK, I will bite... What did I mess up in rcu_read_unlock_special()?
> >
> > This function does not report leaving the RCU read-side critical section
> > until after its call to rt_mutex_unlock() has returned, so any RCU
> > read-side critical sections in rt_mutex_unlock() will be respected.
>
> Sorry for confusion.
>
> I only meant that afaics rcu_read_unlock_special() equally depends on the
> fact that rt_mutex_unlock() does nothing with "struct rt_mutex" after it
> makes another rt_mutex_lock() + rt_mutex_unlock() possible, otherwise this
> code is wrong (and unlock_task_sighand() would be wrong too).
>
> Just to simplify the discussion... suppose we add "atomic_t nr_slow_unlock"
> into "struct rt_mutex" and change rt_mutex_slowunlock() to do
> atomic_inc(&lock->nr_slow_unlock) after it drops ->wait_lock. Of course this
> would be ugly, just for illustration.

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...

> In this case atomic_inc() above can write to rcu_boost()'s stack after this
> functions returns to the caller. And unlock_task_sighand() would be wrong
> too, atomic_inc() could write to the memory which was already returned to
> system because "unlock" path runs outside of rcu-protected section.
>
> But it seems to me that currently we are safe, rt_mutex_unlock() doesn't do
> something like this, a concurrent rt_mutex_lock() must always take wait_lock
> too.
>
>
> And while this is off-topic and I can be easily wrong, it seems that the
> normal "struct mutex" is not safe in this respect. If nothing else, once
> __mutex_unlock_common_slowpath()->__mutex_slowpath_needs_to_unlock() sets
> lock->count = 1, a concurent mutex_lock() can take and then release this
> mutex before __mutex_unlock_common_slowpath() takes ->wait_lock.
>
> So _perhaps_ we should not rely on this property of rt_mutex "too much".

Well, I could easily move the rt_mutex from rcu_boost()'s stack to the
rcu_node structure, if that would help. That said, I still have my
use-after-free concern above.

Thanx, Paul