Messages in this thread | | | Date | Mon, 9 Jun 2014 09:26:13 -0700 | From | "Paul E. McKenney" <> | Subject | Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand unprotected when accessed by /proc) |
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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
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