Re: [PATCH] kref: oops on zero or negative refcount

[Rafael Aquini <aquini@redhat.com>]

On Thu, Feb 20, 2014 at 01:17:44PM -0500, Rik van Riel wrote:
> On 02/20/2014 01:14 PM, Dave Jones wrote:
> > On Thu, Feb 20, 2014 at 06:44:59PM +0100, Mateusz Guzik wrote:
> >  > In use after free situations, it is possible for one thread to write to
> >  > memory that has just been reallocated to a new user. This could open up
> >  > potential security issues.
> >  > 
> >  > diff --git a/include/linux/kref.h b/include/linux/kref.h
> >  > index 484604d..c3f8a0a 100644
> >  > --- a/include/linux/kref.h
> >  > +++ b/include/linux/kref.h
> >  > @@ -43,8 +43,10 @@ static inline void kref_get(struct kref *kref)
> >  >  	/* If refcount was 0 before incrementing then we have a race
> >  >  	 * condition when this kref is freeing by some other thread right now.
> >  >  	 * In this case one should use kref_get_unless_zero()
> >  > +	 *
> >  > +	 * Terminate the current thread to stop potential security exploits.
> >  >  	 */
> >  > -	WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
> >  > +	BUG_ON(atomic_inc_return(&kref->refcount) < 2);
> > 
> > This isn't "terminating the thread", this is "lock up the box".
> 
> Only if kref_get holds a lock while encountering a refcount
> underflow, right?
>

Yes, and in a quick glance through the tree it seems we have several
codesites where we can find such condition likely to happen,
unfortunately.