Re: [PATCH] Detect early free of a live mm

[Mark Rutland]

On Thu, Mar 01, 2018 at 10:35:22AM +0100, Michal Hocko wrote:
> On Wed 28-02-18 12:18:10, Mark Rutland wrote:
> > Ugh, I messed up Peter's email when sending this out. please
> > s/infraded/infradead/ if replying to the first mail.
> > 
> > Sorry about that.
> > 
> > Mark.
> > 
> > On Wed, Feb 28, 2018 at 12:14:58PM +0000, Mark Rutland wrote:
> > > KASAN splats indicate that in some cases we free a live mm, then
> > > continue to access it, with potentially disastrous results. This is
> > > likely due to a mismatched mmdrop() somewhere in the kernel, but so far
> > > the culprit remains elusive.
> > > 
> > > Let's have __mmdrop() verify that the mm isn't live for the current
> > > task, similar to the existing check for init_mm. This way, we can catch
> > > this class of issue earlier, and without requiring KASAN.
> 
> Wouldn't it be better to check the mm_users count instead?
> {VM_}BUG_ON(atomic_read(&mm->mm_users));

Perhaps, but that won't catch a mismatched mmput(), which could
decrement mm_users (and mm_count) to zero early, before current->mm is
cleared in exit_mm().

Locally, I'm testing with:

  BUG_ON(mm == current->mm);
  BUG_ON(mm == current->active_mm);
  BUG_ON(refcount_read(&mm->mm_users) != 0);
  BUG_ON(refcount_read(&mm->mm_count) != 0);

... so as to also catch an early free from another thread.

> Relying on current->mm resetting works currently but this is quite a
> subtle dependency.

Do you mean because it only catches an early free in a thread using that
mm, or is there another subtlety you had in mind?

Thanks,
Mark.