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
>               mmgrab(mm);                     // foo's mm has count +1
>               BUG_ON(mm != current->active_mm);
>               task_lock(current);
>               current->mm = NULL;
>               task_unlock(current);
> <irq and ctxsw to kthread>


> mmdrop(mm);                                   // foo's mm has count -1
> At this point, we've got an imbalanced count on the mm and could free it
> prematurely as seen in the KASAN log. A subsequent context-switch away
> from foo would therefore result in a use-after-free.

Peter already dismissed an algorithm issue here but I thought I'd give
model checking a go (it only deals with mm_users/mm_count; also added a
heavily simplified exec_mmap() in the loop):


As expected, it didn't show any problems (though it does not take memory
ordering into account).

Now, there are lots of other mmget/mmput and mmgrab/mmdrop throughout
the kernel and finding an imbalanced call needs more work.


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