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

[Michal Hocko]

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));

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