Re: [RFC PATCH] Fix: membarrier: racy access to p->mm in membarrier_global_expedited()

[Mathieu Desnoyers]

----- On Jan 28, 2019, at 3:46 PM, paulmck paulmck@xxxxxxxxxxxxx wrote:

> On Mon, Jan 28, 2019 at 12:27:03PM -0800, Linus Torvalds wrote:
>> On Mon, Jan 28, 2019 at 10:27 AM Mathieu Desnoyers
>> <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>> >
>> > Jann Horn identified a racy access to p->mm in the global expedited
>> > command of the membarrier system call.
>> >
>> > The suggested fix is to hold the task_lock() around the accesses to
>> > p->mm and to the mm_struct membarrier_state field to guarantee the
>> > existence of the mm_struct.
>> 
>> Hmm. I think this is right. You shouldn't access another threads mm
>> pointer without proper locking.
>> 
>> That said, we *could* make the mm_cachep be SLAB_TYPESAFE_BY_RCU,
>> which would allow speculatively reading data off the mm pointer under
>> RCU. It might not be the *right* mm if somebody just did an exit, but
>> for things like this it shouldn't matter.
> 
> That sounds much simpler and more effective than the contention-reduction
> approach that I suggested.  ;-)

I'd be tempted to stick to the locking approach for a fix, and implement
Linus' type-safe mm_cachep idea if anyone complains about the overhead
of membarrier GLOBAL_EXPEDITED (and submit for a future merge window).

I tested the KASAN splat reproducer from Jann locally, and confirmed that
my patch fixes the issue it reproduces.

Please let me know if the task_lock() approach is OK as a fix for now.

I'm also awaiting a Tested-by from Jann before submitting this for real.

Thanks,

Mathieu

> 
>							Thanx, Paul
> 
>> But if this is the only case that might care, it sounds like just
>> doing the proper locking is the right approach.
>> 
>>            Linus

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com