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

[Mark Rutland]

On Fri, Feb 16, 2018 at 05:17:57PM +0000, Mathieu Desnoyers wrote:
> ----- On Feb 16, 2018, at 11:53 AM, Mark Rutland mark.rutland at arm.com wrote:
> > 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 ?

That looks suspicious, but I don't think it can result in this
particular problem.

In find_lock_task_mm() we get the task lock, and check mm != NULL, which
means that mm->mm_count >= 1 (thanks to the implicit reference
context_switch()+finish_task_switch() manage). While we hold the task
lock, task->mm can't change beneath our feet, and hence that reference
can't be dropped by finish_task_switch().

Thus, immediately after the mmgrab(), we know mm->mm_count >= 2. That
shouldn't drop below 1 until the subsequent mmdrop(), even after we drop
the task lock, unless there's a misplaced mmdrop() elsewhere. Locally,
mmgrab() and mmdrop() are balanced.

However, if mm_users can be incremented behind our back, we might skip
updating the oom adjustments for other users of the mm.

Thanks,
Mark.