linux-kernel - Re: [PATCH v7 3/3] mm/slub: extend redzone check to extra allocated kmalloc space than requested

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Date:   Fri, 11 Nov 2022 09:12:19 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com
Subject: Re: [PATCH v7 3/3] mm/slub: extend redzone check to extra allocated
 kmalloc space than requested

On 11/11/22 07:46, Feng Tang wrote:
> On Thu, Nov 10, 2022 at 04:48:35PM +0100, Vlastimil Babka wrote:
>> On 10/21/22 05:24, Feng Tang wrote:
>> > kmalloc will round up the request size to a fixed size (mostly power
>> > of 2), so there could be a extra space than what is requested, whose
>> > size is the actual buffer size minus original request size.
>> > 
>> > To better detect out of bound access or abuse of this space, add
>> > redzone sanity check for it.
>> > 
>> > In current kernel, some kmalloc user already knows the existence of
>> > the space and utilizes it after calling 'ksize()' to know the real
>> > size of the allocated buffer. So we skip the sanity check for objects
>> > which have been called with ksize(), as treating them as legitimate
>> > users.
>> 
>> Hm so once Kees's effort is finished and all ksize() users behave correctly,
>> we can drop all that skip_orig_size_check() code, right?
> 
> Yes, will update the commit log.
> 
>> > In some cases, the free pointer could be saved inside the latter
>> > part of object data area, which may overlap the redzone part(for
>> > small sizes of kmalloc objects). As suggested by Hyeonggon Yoo,
>> > force the free pointer to be in meta data area when kmalloc redzone
>> > debug is enabled, to make all kmalloc objects covered by redzone
>> > check.
>> > 
>> > Suggested-by: Vlastimil Babka <vbabka@...e.cz>
>> > Signed-off-by: Feng Tang <feng.tang@...el.com>
>> > Acked-by: Hyeonggon Yoo <42.hyeyoo@...il.com>
>> 
>> Looks fine, but a suggestion below:
>> 
> [...]
>> > @@ -966,13 +982,27 @@ static __printf(3, 4) void slab_err(struct kmem_cache *s, struct slab *slab,
>> >  static void init_object(struct kmem_cache *s, void *object, u8 val)
>> >  {
>> >  	u8 *p = kasan_reset_tag(object);
>> > +	unsigned int orig_size = s->object_size;
>> >  
>> > -	if (s->flags & SLAB_RED_ZONE)
>> > +	if (s->flags & SLAB_RED_ZONE) {
>> >  		memset(p - s->red_left_pad, val, s->red_left_pad);
>> >  
>> > +		if (slub_debug_orig_size(s) && val == SLUB_RED_ACTIVE) {
>> > +			orig_size = get_orig_size(s, object);
>> > +
>> > +			/*
>> > +			 * Redzone the extra allocated space by kmalloc
>> > +			 * than requested.
>> > +			 */
>> > +			if (orig_size < s->object_size)
>> > +				memset(p + orig_size, val,
>> > +				       s->object_size - orig_size);
>> 
>> Wondering if we can remove this if - memset and instead below:
>> 
>> > +		}
>> > +	}
>> > +
>> >  	if (s->flags & __OBJECT_POISON) {
>> > -		memset(p, POISON_FREE, s->object_size - 1);
>> > -		p[s->object_size - 1] = POISON_END;
>> > +		memset(p, POISON_FREE, orig_size - 1);
>> > +		p[orig_size - 1] = POISON_END;
>> >  	}
>> >  
>> >  	if (s->flags & SLAB_RED_ZONE)
>> 
>> This continues by:
>>     memset(p + s->object_size, val, s->inuse - s->object_size);
>> Instead we could do this, no?
>>     memset(p + orig_size, val, s->inuse - orig_size);
> 
> Yep, the code is much simpler and cleaner! thanks
>  
> I also change the name from 'orig_size' to 'poison_size', as below:
> 
> Thanks,
> Feng

Thanks! Now merged all to slab/for-6.2/kmalloc_redzone and for-next