Re: arm64/v4.16-rc1: KASAN: use-after-free Read in finish_task_switch

[Mathieu Desnoyers]

----- On Feb 15, 2018, at 6:49 AM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote:

> On Wed, Feb 14, 2018 at 06:53:44PM +0000, Mathieu Desnoyers wrote:
>> However, given the scenario involves multiples CPUs (one doing exit_mm(),
>> the other doing context switch), the actual order of perceived load/store
>> can be shuffled. And AFAIU nothing prevents the CPU from ordering the
>> atomic_inc() done by mmgrab(mm) _after_ the store to current->mm.
>> 
>> I wonder if we should not simply add a smp_mb__after_atomic() into
>> mmgrab() instead ? I see that e.g. futex.c does:
> 
> Don't think so, the futex case is really rather special and I suspect
> this one is too. I would much rather have explicit comments rather than
> implicit works by magic.
> 
> As per the rationale used for refcount_t, increments should be
> unordered, because you ACQUIRE your object _before_ you can do the
> increment.
> 
> The futex thing is simply abusing a bunch of implied barriers and
> patching up the holes in paths that didn't already imply a barrier in
> order to avoid having to add explicit barriers (which had measurable
> performance issues).
> 
> And here we have explicit ordering outside of the reference counting
> too, we want to ensure the reference is incremented before we modify
> a second object.
> 
> This ordering is not at all related to acquiring the reference, so
> bunding it seems odd.

I understand your point. Will's added barrier and comment is fine.

Thanks,

Mathieu



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