Re: [PATCH 2/3] kasan: avoid re-poisoning tag-based kmalloc redzones

[Dev Jain <dev.jain@arm.com>]

> 
> I got the problem now. The problem is that krealloc() does not know what was the
> old allocation size.
> 
> So only talking about HW tags, can't we simply do the following. We can use
> HW tags integrated init to initialize only the part we want to unpoison.
> 
> 
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index b7d05c2a6d93d..c424be3cd02bb 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -474,7 +474,7 @@ void * __must_check __kasan_krealloc(const void *object, size_t size, gfp_t flag
>  	 * Part of it might already have been unpoisoned, but it's unknown
>  	 * how big that part is.
>  	 */
> -	kasan_unpoison(object, size, false);
> +	kasan_unpoison(object, size, want_init_on_alloc(flags));
> 
>  	slab = virt_to_slab(object);
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 0baa906f39ab8..62ee59fc389d8 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -6686,7 +6686,7 @@ __do_krealloc(const void *p, size_t new_size, unsigned long align, gfp_t flags,
>  		goto alloc_new;
> 
>  	/* Zero out spare memory. */
> -	if (want_init_on_alloc(flags)) {
> +	if (want_init_on_alloc(flags) && !kasan_has_integrated_init()) {
>  		kasan_disable_current();
>  		if (orig_size && orig_size < new_size)
>  			memset(kasan_reset_tag(p) + orig_size, 0, new_size - orig_size);


Okay, this would overwrite the data which we still need.


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