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

[Michal Hocko]

On Thu 01-03-18 11:22:37, Mark Rutland wrote:
> 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().

true

> 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.

I would be careful about active_mm. At least {un}use_mm does some tricks
with it.

> > 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?

No, I was more thinking about future changes when we stop clearing
tsk->mm at exit. This is rather unlikely to be honest because that
requires more changes.
-- 
Michal Hocko
SUSE Labs