Re: [PATCH v4 3/3] mm/slub: Actually fix freelist pointer vs redzoning

[Kees Cook <keescook@chromium.org>]

On Tue, Jun 08, 2021 at 01:56:33PM -0700, Andrew Morton wrote:
> On Tue,  8 Jun 2021 11:39:55 -0700 Kees Cook <keescook@chromium.org> wrote:
> 
> > It turns out that SLUB redzoning ("slub_debug=Z") checks from
> > s->object_size rather than from s->inuse (which is normally bumped
> > to make room for the freelist pointer), so a cache created with an
> > object size less than 24 would have the freelist pointer written beyond
> > s->object_size, causing the redzone to be corrupted by the freelist
> > pointer. This was very visible with "slub_debug=ZF":
> > 
> > BUG test (Tainted: G    B            ): Right Redzone overwritten
> > -----------------------------------------------------------------------------
> > 
> > INFO: 0xffff957ead1c05de-0xffff957ead1c05df @offset=1502. First byte 0x1a instead of 0xbb
> > INFO: Slab 0xffffef3950b47000 objects=170 used=170 fp=0x0000000000000000 flags=0x8000000000000200
> > INFO: Object 0xffff957ead1c05d8 @offset=1496 fp=0xffff957ead1c0620
> > 
> > Redzone  (____ptrval____): bb bb bb bb bb bb bb bb               ........
> > Object   (____ptrval____): 00 00 00 00 00 f6 f4 a5               ........
> > Redzone  (____ptrval____): 40 1d e8 1a aa                        @....
> > Padding  (____ptrval____): 00 00 00 00 00 00 00 00               ........
> > 
> > Adjust the offset to stay within s->object_size.
> > 
> > (Note that no caches of in this size range are known to exist in the
> > kernel currently.)
> 
> We already have
> https://lkml.kernel.org/r/6746FEEA-FD69-4792-8DDA-C78F5FE7DA02@psu.edu.
> Is this patch better?

Yes, I believe so, since it reduces code and corrects the size checking
more directly (and more clearly demonstrates the redzone calculation
problem in the commit log).

-Kees

> 
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -3689,7 +3689,6 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
> >  {
> >  	slab_flags_t flags = s->flags;
> >  	unsigned int size = s->object_size;
> > -	unsigned int freepointer_area;
> >  	unsigned int order;
> >  
> >  	/*
> > @@ -3698,13 +3697,6 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
> >  	 * the possible location of the free pointer.
> >  	 */
> >  	size = ALIGN(size, sizeof(void *));
> > -	/*
> > -	 * This is the area of the object where a freepointer can be
> > -	 * safely written. If redzoning adds more to the inuse size, we
> > -	 * can't use that portion for writing the freepointer, so
> > -	 * s->offset must be limited within this for the general case.
> > -	 */
> > -	freepointer_area = size;
> >  
> >  #ifdef CONFIG_SLUB_DEBUG
> >  	/*
> > @@ -3730,7 +3722,7 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
> >  
> >  	/*
> >  	 * With that we have determined the number of bytes in actual use
> > -	 * by the object. This is the potential offset to the free pointer.
> > +	 * by the object and redzoning.
> >  	 */
> >  	s->inuse = size;
> >  
> > @@ -3753,13 +3745,13 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
> >  		 */
> >  		s->offset = size;
> >  		size += sizeof(void *);
> > -	} else if (freepointer_area > sizeof(void *)) {
> > +	} else {
> >  		/*
> >  		 * Store freelist pointer near middle of object to keep
> >  		 * it away from the edges of the object to avoid small
> >  		 * sized over/underflows from neighboring allocations.
> >  		 */
> > -		s->offset = ALIGN(freepointer_area / 2, sizeof(void *));
> > +		s->offset = ALIGN_DOWN(s->object_size / 2, sizeof(void *));
> >  	}
> >  
> >  #ifdef CONFIG_SLUB_DEBUG
> > -- 
> > 2.25.1

-- 
Kees Cook