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

[Mateusz Guzik]

On Thu, Feb 20, 2014 at 01:14:40PM -0500, 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".
> 

Well, extent of damage caused by non-panicking BUG_ON (if any) depends
on the state when kref_get was executed.

However, since this condition is already a sign of big trouble (and a
potential exploitation attempt), I think a WARN_ON_ONCE is not
sufficient.

That said, can you elaborate on your concers? You just don't like that
comment, don't want that BUG_ON (want a panic instead) or maybe you
don't like the change at all (or something else)?

Thanks,
-- 
Mateusz Guzik
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