Re: [RFC/PATCH RESEND -next 14/21] mm: slub: kasan: disable kasan when touching unaccessible memory

[Joonsoo Kim]

On Wed, Jul 09, 2014 at 03:30:08PM +0400, Andrey Ryabinin wrote:
> Some code in slub could validly touch memory marked by kasan as unaccessible.
> Even though slub.c doesn't instrumented, functions called in it are instrumented,
> so to avoid false positive reports such places are protected by
> kasan_disable_local()/kasan_enable_local() calls.
> 
> Signed-off-by: Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx>
> ---
>  mm/slub.c | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 6ddedf9..c8dbea7 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -560,8 +560,10 @@ static void print_tracking(struct kmem_cache *s, void *object)
>  	if (!(s->flags & SLAB_STORE_USER))
>  		return;
>  
> +	kasan_disable_local();
>  	print_track("Allocated", get_track(s, object, TRACK_ALLOC));
>  	print_track("Freed", get_track(s, object, TRACK_FREE));
> +	kasan_enable_local();

I don't think that this is needed since print_track() doesn't call
external function with object pointer. print_track() call pr_err(), but,
before calling, it retrieve t->addrs[i] so memory access only occurs
in slub.c.

>  }
>  
>  static void print_page_info(struct page *page)
> @@ -604,6 +606,8 @@ static void print_trailer(struct kmem_cache *s, struct page *page, u8 *p)
>  	unsigned int off;	/* Offset of last byte */
>  	u8 *addr = page_address(page);
>  
> +	kasan_disable_local();
> +
>  	print_tracking(s, p);
>  
>  	print_page_info(page);
> @@ -632,6 +636,8 @@ static void print_trailer(struct kmem_cache *s, struct page *page, u8 *p)
>  		/* Beginning of the filler is the free pointer */
>  		print_section("Padding ", p + off, s->size - off);
>  
> +	kasan_enable_local();
> +
>  	dump_stack();
>  }

And, I recommend that you put this hook on right place.
At a glance, the problematic function is print_section() which have
external function call, print_hex_dump(), with object pointer.
If you disable kasan in print_section, all the below thing won't be
needed, I guess.

Thanks.

>  
> @@ -1012,6 +1018,8 @@ static noinline int alloc_debug_processing(struct kmem_cache *s,
>  					struct page *page,
>  					void *object, unsigned long addr)
>  {
> +
> +	kasan_disable_local();
>  	if (!check_slab(s, page))
>  		goto bad;
>  
> @@ -1028,6 +1036,7 @@ static noinline int alloc_debug_processing(struct kmem_cache *s,
>  		set_track(s, object, TRACK_ALLOC, addr);
>  	trace(s, page, object, 1);
>  	init_object(s, object, SLUB_RED_ACTIVE);
> +	kasan_enable_local();
>  	return 1;
>  
>  bad:
> @@ -1041,6 +1050,7 @@ bad:
>  		page->inuse = page->objects;
>  		page->freelist = NULL;
>  	}
> +	kasan_enable_local();
>  	return 0;
>  }
>  
> @@ -1052,6 +1062,7 @@ static noinline struct kmem_cache_node *free_debug_processing(
>  
>  	spin_lock_irqsave(&n->list_lock, *flags);
>  	slab_lock(page);
> +	kasan_disable_local();
>  
>  	if (!check_slab(s, page))
>  		goto fail;
> @@ -1088,6 +1099,7 @@ static noinline struct kmem_cache_node *free_debug_processing(
>  	trace(s, page, object, 0);
>  	init_object(s, object, SLUB_RED_INACTIVE);
>  out:
> +	kasan_enable_local();
>  	slab_unlock(page);
>  	/*
>  	 * Keep node_lock to preserve integrity
> @@ -1096,6 +1108,7 @@ out:
>  	return n;
>  
>  fail:
> +	kasan_enable_local();
>  	slab_unlock(page);
>  	spin_unlock_irqrestore(&n->list_lock, *flags);
>  	slab_fix(s, "Object at 0x%p not freed", object);
> @@ -1371,8 +1384,11 @@ static void setup_object(struct kmem_cache *s, struct page *page,
>  				void *object)
>  {
>  	setup_object_debug(s, page, object);
> -	if (unlikely(s->ctor))
> +	if (unlikely(s->ctor)) {
> +		kasan_disable_local();
>  		s->ctor(object);
> +		kasan_enable_local();
> +	}
>  }
>  static struct page *new_slab(struct kmem_cache *s, gfp_t flags, int node)
> @@ -1425,11 +1441,12 @@ static void __free_slab(struct kmem_cache *s, struct page *page)
>  
>  	if (kmem_cache_debug(s)) {
>  		void *p;
> -
> +		kasan_disable_local();
>  		slab_pad_check(s, page);
>  		for_each_object(p, s, page_address(page),
>  						page->objects)
>  			check_object(s, page, p, SLUB_RED_INACTIVE);
> +		kasan_enable_local();
>  	}
>  
>  	kmemcheck_free_shadow(page, compound_order(page));
> -- 
> 1.8.5.5
> 
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