Re: [PATCH v4] x86/suspend: fix false positive KASAN warning on suspend/resume

[Josh Poimboeuf]

On Fri, Dec 02, 2016 at 10:09:03PM +0100, Pavel Machek wrote:
> On Fri 2016-12-02 11:42:21, Josh Poimboeuf wrote:
> > Resuming from a suspend operation is showing a KASAN false positive
> > warning:
> > 
> > 
> > Reported-by: Scott Bauer <scott.bauer@xxxxxxxxx>
> > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> 
> Acked-by: Pavel Machek <pavel@xxxxxx>
> 
> > diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
> > index 0e9505f..b2a0cff 100644
> > --- a/mm/kasan/kasan.c
> > +++ b/mm/kasan/kasan.c
> > @@ -80,7 +80,14 @@ void kasan_unpoison_task_stack(struct task_struct *task)
> >  /* Unpoison the stack for the current task beyond a watermark sp value. */
> >  asmlinkage void kasan_unpoison_task_stack_below(const void *watermark)
> >  {
> > -	__kasan_unpoison_stack(current, watermark);
> > +	/*
> > +	 * Calculate the task stack base address.  Avoid using 'current'
> > +	 * because this function is called by early resume code which hasn't
> > +	 * yet set up the percpu register (%gs).
> > +	 */
> > +	void *base = (void *)((unsigned long)watermark & ~(THREAD_SIZE - 1));
> > +
> > +	kasan_unpoison_shadow(base, watermark - base);
> >  }
> >  
> 
> I know you modified this code to be arch-independend... but is it
> really? I guess it is portable enough across architectures that run
> kasan today..

Yes, it's arch-independent as far as I know.

All the implementations of alloc_thread_stack_node() in kernel/fork.c
create THREAD_SIZE sized/aligned stacks.

ia64 has its own implementation of alloc_thread_stack_node(), which also
has a THREAD_SIZE sized/aligned stack, with task_struct stored at the
beginning.

For those architectures for which stack grows up, they would need to
call a different helper which unpoisons the stack above the watermark,
but that was also the case before my patch.

-- 
Josh