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

On Thu, Feb 15, 2018 at 02:22:39PM +0000, Will Deacon wrote:

Instead, we've come up with a more plausible sequence that can in theory
happen on a single CPU:

<task foo calls exit()>

do_exit
	exit_mm

If this is the last task of the process, we would expect:

  mm_count == 1
  mm_users == 1

at this point.

		mmgrab(mm);			// foo's mm has count +1
		BUG_ON(mm != current->active_mm);
		task_lock(current);
		current->mm = NULL;
		task_unlock(current);

So the whole active_mm is basically the last 'real' mm, and its purpose
is to avoid switch_mm() between user tasks and kernel tasks.

A kernel task has !->mm. We do this by incrementing mm_count when
switching from user to kernel task and decrementing when switching from
kernel to user.

What exit_mm() does is change a user task into a 'kernel' task. So it
should increment mm_count to mirror the context switch. I suspect this
is what the mmgrab() in exit_mm() is for.

<irq and ctxsw to kthread>

context_switch(prev=foo, next=kthread)
	mm = next->mm;
	oldmm = prev->active_mm;

	if (!mm) {				// True for kthread
		next->active_mm = oldmm;
		mmgrab(oldmm);			// foo's mm has count +2
	}

	if (!prev->mm) {			// True for foo
		rq->prev_mm = oldmm;
	}

	finish_task_switch
		mm = rq->prev_mm;
		if (mm) {			// True (foo's mm)
			mmdrop(mm);		// foo's mm has count +1
		}

	[...]

<ctxsw to task bar>

context_switch(prev=kthread, next=bar)
	mm = next->mm;
	oldmm = prev->active_mm;		// foo's mm!

	if (!prev->mm) {			// True for kthread
		rq->prev_mm = oldmm;
	}

	finish_task_switch
		mm = rq->prev_mm;
		if (mm) {			// True (foo's mm)
			mmdrop(mm);		// foo's mm has count +0

The context switch into the next user task will then decrement. At this
point foo no longer has a reference to its mm, except on the stack.

		}

	[...]

<ctxsw back to task foo>

context_switch(prev=bar, next=foo)
	mm = next->mm;
	oldmm = prev->active_mm;

	if (!mm) {				// True for foo
		next->active_mm = oldmm;	// This is bar's mm
		mmgrab(oldmm);			// bar's mm has count +1
	}


	[return back to exit_mm]

Enter mm_users, this counts the number of tasks associated with the mm.
We start with 1 in mm_init(), and when it drops to 0, we decrement
mm_count. Since we also start with mm_count == 1, this would appear
consistent.

  mmput() // --mm_users == 0, which then results in:

mmdrop(mm);					// foo's mm has count -1

In the above case, that's the very last reference to the mm, and since
we started out with mm_count == 1, this -1 makes 0 and we do the actual
free.

At this point, we've got an imbalanced count on the mm and could free it
prematurely as seen in the KASAN log.

I'm not sure I see premature. At this point mm_users==0, mm_count==0 and
we freed mm and there is no further use of the on-stack mm pointer and
foo no longer has a pointer to it in either ->mm or ->active_mm. It's
well and proper dead.

A subsequent context-switch away from foo would therefore result in a
use-after-free.

At the above point, foo no longer has a reference to mm, we cleared ->mm
early, and the context switch to bar cleared ->active_mm. The switch
back into foo then results with foo->active_mm == bar->mm, which is
fine.