Re: [PATCH] mm/slub.c: Fix wrong address during slab padding restoration

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 22 Jan 2018 12:32:58 +0530 Balasubramani Vivekanandan <balasubramani_vivekanandan@mentor.com> wrote:

> From: Balasubramani Vivekanandan <balasubramani_vivekanandan@mentor.com>
> 
> Start address calculated for slab padding restoration was wrong.
> Wrong address would point to some section before padding and
> could cause corruption
> 
> ...
>
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -838,6 +838,7 @@ static int slab_pad_check(struct kmem_cache *s, struct page *page)
>  	u8 *start;
>  	u8 *fault;
>  	u8 *end;
> +	u8 *pad;
>  	int length;
>  	int remainder;
>  
> @@ -851,8 +852,9 @@ static int slab_pad_check(struct kmem_cache *s, struct page *page)
>  	if (!remainder)
>  		return 1;
>  
> +	pad = end - remainder;
>  	metadata_access_enable();
> -	fault = memchr_inv(end - remainder, POISON_INUSE, remainder);
> +	fault = memchr_inv(pad, POISON_INUSE, remainder);
>  	metadata_access_disable();
>  	if (!fault)
>  		return 1;
> @@ -860,9 +862,9 @@ static int slab_pad_check(struct kmem_cache *s, struct page *page)
>  		end--;
>  
>  	slab_err(s, page, "Padding overwritten. 0x%p-0x%p", fault, end - 1);
> -	print_section(KERN_ERR, "Padding ", end - remainder, remainder);
> +	print_section(KERN_ERR, "Padding ", pad, remainder);
>  
> -	restore_bytes(s, "slab padding", POISON_INUSE, end - remainder, end);
> +	restore_bytes(s, "slab padding", POISON_INUSE, fault, end);
>  	return 0;
>  }

I don't see why it matters?  The current code will overwrite
POISON_INUSE bytes with POISON_INUSE, won't it?

That's a bit strange but not incorrect?

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