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

[zhong jiang]

On 2019/3/17 3:42, Andrea Arcangeli wrote:
> On Sat, Mar 16, 2019 at 05:38:54PM +0800, zhong jiang wrote:
>> 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. :-(
> Dereference mm->owner didn't mean reading the value of the mm->owner
> pointer, it really means to dereference the value of the pointer. It's
> like below:
>
> get_mem_cgroup_from_mm()		failing fork()
> ----					---
> task = mm->owner
> 					mm->owner = NULL;
> 					free(mm->owner)
> *task /* use after free */
>
> We didn't set mm->owner to NULL before, so the window for the race was
> larger, but setting mm->owner to NULL only hides the problem and it
> can still happen (albeit with a smaller window).
>
> If get_mem_cgroup_from_mm() can see at any time mm->owner not NULL,
> then the free of the task struct must be delayed until after
> rcu_read_unlock has returned in get_mem_cgroup_from_mm(). This is
> the standard RCU model, the freeing must be delayed until after the
> next quiescent point.

Thank you for your explaination patiently.  The patch should go to upstream too.  I think you
should send a formal patch to the mainline.  Maybe other people suffer from
the issue.  :-)

Thanks,
zhong jiang
> BTW, both mm_update_next_owner() and mm_clear_owner() should have used
> WRITE_ONCE when they write to mm->owner, I can update that too but
> it's just to not to make assumptions that gcc does the right thing
> (and we still rely on gcc to do the right thing in other places) so
> that is just an orthogonal cleanup.
>
> Thanks,
> Andrea
>
> .
>