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

[Mark Rutland]

On Wed, Feb 14, 2018 at 03:07:41PM +0000, Will Deacon wrote:
> Hi Mark,

Hi Will,

> Cheers for the report. These things tend to be a pain to debug, but I've had
> a go.

Thanks for taking a look!

> On Wed, Feb 14, 2018 at 12:02:54PM +0000, Mark Rutland wrote:
> The interesting thing here is on the exit path:
> 
> > Freed by task 10882:
> >  save_stack mm/kasan/kasan.c:447 [inline]
> >  set_track mm/kasan/kasan.c:459 [inline]
> >  __kasan_slab_free+0x114/0x220 mm/kasan/kasan.c:520
> >  kasan_slab_free+0x10/0x18 mm/kasan/kasan.c:527
> >  slab_free_hook mm/slub.c:1393 [inline]
> >  slab_free_freelist_hook mm/slub.c:1414 [inline]
> >  slab_free mm/slub.c:2968 [inline]
> >  kmem_cache_free+0x88/0x270 mm/slub.c:2990
> >  __mmdrop+0x164/0x248 kernel/fork.c:604
> 
> ^^ This should never run, because there's an mmgrab() about 8 lines above
> the mmput() in exit_mm.
> 
> >  mmdrop+0x50/0x60 kernel/fork.c:615
> >  __mmput kernel/fork.c:981 [inline]
> >  mmput+0x270/0x338 kernel/fork.c:992
> >  exit_mm kernel/exit.c:544 [inline]
> 
> Looking at exit_mm:
> 
>         mmgrab(mm);
>         BUG_ON(mm != current->active_mm);
>         /* more a memory barrier than a real lock */
>         task_lock(current);
>         current->mm = NULL;
>         up_read(&mm->mmap_sem);
>         enter_lazy_tlb(mm, current);
>         task_unlock(current);
>         mm_update_next_owner(mm);
>         mmput(mm);
> 
> Then the comment already rings some alarm bells: our spin_lock (as used
> by task_lock) has ACQUIRE semantics, so the mmgrab (which is unordered
> due to being an atomic_inc) can be reordered with respect to the assignment
> of NULL to current->mm.
> 
> If the exit()ing task had recently migrated from another CPU, then that
> CPU could concurrently run context_switch() and take this path:
> 
> 	if (!prev->mm) {
> 		prev->active_mm = NULL;
> 		rq->prev_mm = oldmm;
> 	}

IIUC, on the prior context_switch, next->mm == NULL, so we set
next->active_mm to prev->mm.

Then, in this context_switch we set oldmm = prev->active_mm (where prev
is next from the prior context switch).

... right?

> which then means finish_task_switch will call mmdrop():
> 
> 	struct mm_struct *mm = rq->prev_mm;
> 	[...]
> 	if (mm) {
> 		membarrier_mm_sync_core_before_usermode(mm);
> 		mmdrop(mm);
> 	}

... then here we use what was prev->active_mm in the most recent context
switch.

So AFAICT, we're never concurrently accessing a task_struct::mm field
here, only prev::{mm,active_mm} while prev is current...

[...]

> diff --git a/kernel/exit.c b/kernel/exit.c
> index 995453d9fb55..f91e8d56b03f 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -534,8 +534,9 @@ static void exit_mm(void)
>         }
>         mmgrab(mm);
>         BUG_ON(mm != current->active_mm);
> -       /* more a memory barrier than a real lock */
>         task_lock(current);
> +       /* Ensure we've grabbed the mm before setting current->mm to NULL */
> +       smp_mb__after_spin_lock();
>         current->mm = NULL;

... and thus I don't follow why we would need to order these with
anything more than a compiler barrier (if we're preemptible here).

What have I completely misunderstood? ;)

Thanks,
Mark.