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

----- On Feb 14, 2018, at 11:51 AM, Mark Rutland mark.rutland at arm.com wrote:

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

quoted

Hi Mark,

Hi Will,

quoted

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

Thanks for taking a look!

quoted

On Wed, Feb 14, 2018 at 12:02:54PM +0000, Mark Rutland wrote:
The interesting thing here is on the exit path:

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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.

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 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?

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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...

[...]

quoted

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? ;)

The compiler barrier would not change anything, because task_lock()
already implies a compiler barrier (provided by the arch spin lock
inline asm memory clobber). So compiler-wise, it cannot move the
mmgrab(mm) after the store "current->mm = NULL".

However, given the scenario involves multiples CPUs (one doing exit_mm(),
the other doing context switch), the actual order of perceived load/store
can be shuffled. And AFAIU nothing prevents the CPU from ordering the
atomic_inc() done by mmgrab(mm) _after_ the store to current->mm.

I wonder if we should not simply add a smp_mb__after_atomic() into
mmgrab() instead ? I see that e.g. futex.c does:

static inline void futex_get_mm(union futex_key *key)
{
        mmgrab(key->private.mm);
        /*
         * Ensure futex_get_mm() implies a full barrier such that
         * get_futex_key() implies a full barrier. This is relied upon
         * as smp_mb(); (B), see the ordering comment above.
         */
        smp_mb__after_atomic();
}

It could prevent nasty subtle bugs in other mmgrab() users.

Thoughts ?

Thanks,

Mathieu

Thanks,
Mark.

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