Re: KASAN: use-after-free Read in get_mem_cgroup_from_mm

[zhong jiang]

On 2019/3/16 5:39, Andrea Arcangeli wrote:
> On Fri, Mar 08, 2019 at 03:10:08PM +0800, zhong jiang wrote:
>> I can reproduce the issue in arm64 qemu machine.  The issue will leave after applying the
>> patch.
>>
>> Tested-by: zhong jiang <zhongjiang@xxxxxxxxxx>
> Thanks a lot for the quick testing!
>
>> Meanwhile,  I just has a little doubt whether it is necessary to use RCU to free the task struct or not.
>> I think that mm->owner alway be NULL after failing to create to process. Because we call mm_clear_owner.
> I wish it was enough, but the problem is that the other CPU may be in
> the middle of get_mem_cgroup_from_mm() while this runs, and it would
> dereference mm->owner while it is been freed without the call_rcu
> affter we clear mm->owner. What prevents this race is the
As you had said, It would dereference mm->owner after we clear mm->owner.

But after we clear mm->owner,  mm->owner should be NULL.  Is it right?

And mem_cgroup_from_task will check the parameter. 
you mean that it is possible after checking the parameter to  clear the owner .
and the NULL pointer will trigger. :-(

Thanks,
zhong jiang
> rcu_read_lock() in get_mem_cgroup_from_mm() and the corresponding
> call_rcu to free the task struct in the fork failure path (again only
> if CONFIG_MEMCG=y is defined). Considering you can reproduce this tiny
> race on arm64 qemu (perhaps tcg JIT timing variantions helps?), you
> might also in theory be able to still reproduce the race condition if
> you remove the call_rcu from delayed_free_task and you replace it with
> free_task.
>
> .
>