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

[Mathieu Desnoyers]

----- On Feb 16, 2018, at 11:53 AM, Mark Rutland mark.rutland at arm.com wrote:

> Hi,
> 
> On Thu, Feb 15, 2018 at 10:08:56PM +0000, Mathieu Desnoyers wrote:
>> My current theory: do_exit() gets preempted after having set current->mm
>> to NULL, and after having issued mmput(), which brings the mm_count down
>> to 0.
>>
>> Unfortunately, if the scheduler switches from a userspace thread
>> to a kernel thread, context_switch() loads prev->active_mm which still
>> points to the now-freed mm, mmgrab the mm, and eventually does mmdrop
>> in finish_task_switch().
> 
> For this to happen, we need to get to the mmput() in exit_mm() with:
> 
>  mm->mm_count == 1
>  mm->mm_users == 1
>  mm == active_mm
> 
> ... but AFAICT, this cannot happen.
> 
> If there's no context_switch between clearing current->mm and the
> mmput(), then mm->mm_count >= 2, thanks to the prior mmgrab() and the
> active_mm reference (in mm_count) that context_switch+finish_task_switch
> manage.
> 
> If there is a context_switch between the two, then AFAICT, either:
> 
> a) The task re-inherits its old mm as active_mm, and mm_count >= 2. In
>   context_switch we mmgrab() the active_mm to inherit it, and in
>   finish_task_switch() we drop the oldmm, balancing the mmgrab() with
>   an mmput().
> 
>   e.g we go task -> kernel_task -> task
> 
> b) At some point, another user task is scheduled, and we switch to its
>   mm. We don't mmgrab() the active_mm, but we mmdrop() the oldmm, which
>   means mm_count >= 1. Since we witched to a new mm, if we switch back
>   to the first task, it cannot have its own mm as active_mm.
> 
>   e.g. we go task -> other_task -> task
> 
> I suspect we have a bogus mmdrop or mmput elsewhere, and do_exit() and
> finish_task_switch() aren't to blame.

Currently reviewing: fs/proc/base.c: __set_oom_adj()

        /*
         * Make sure we will check other processes sharing the mm if this is
         * not vfrok which wants its own oom_score_adj.
         * pin the mm so it doesn't go away and get reused after task_unlock
         */
        if (!task->vfork_done) {
                struct task_struct *p = find_lock_task_mm(task);

                if (p) {
                        if (atomic_read(&p->mm->mm_users) > 1) {
                                mm = p->mm;
                                mmgrab(mm);
                        }
                        task_unlock(p);
                }
        }

Considering that mmput() done by exit_mm() is done outside of the
task_lock critical section, I wonder how the mm_users load is
synchronized ?

Thanks,

Mathieu

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