[PATCH v2] kasan, mm: fix crash with HW_TAGS and DEBUG_PAGEALLOC

[Andrew Morton]

On Sat,  6 Mar 2021 00:36:33 +0100 Andrey Konovalov <andreyknvl at google.com> wrote:

> Currently, kasan_free_nondeferred_pages()->kasan_free_pages() is called
> after debug_pagealloc_unmap_pages(). This causes a crash when
> debug_pagealloc is enabled, as HW_TAGS KASAN can't set tags on an
> unmapped page.
> 
> This patch puts kasan_free_nondeferred_pages() before
> debug_pagealloc_unmap_pages() and arch_free_page(), which can also make
> the page unavailable.
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1304,6 +1304,12 @@ static __always_inline bool free_pages_prepare(struct page *page,
>  
>  	kernel_poison_pages(page, 1 << order);
>  
> +	/*
> +	 * With hardware tag-based KASAN, memory tags must be set before the
> +	 * page becomes unavailable via debug_pagealloc or arch_free_page.
> +	 */
> +	kasan_free_nondeferred_pages(page, order, fpi_flags);
> +
>  	/*
>  	 * arch_free_page() can make the page's contents inaccessible.  s390
>  	 * does this.  So nothing which can access the page's contents should
> @@ -1313,8 +1319,6 @@ static __always_inline bool free_pages_prepare(struct page *page,
>  
>  	debug_pagealloc_unmap_pages(page, 1 << order);
>  
> -	kasan_free_nondeferred_pages(page, order, fpi_flags);
> -
>  	return true;
>  }

kasan_free_nondeferred_pages() has only two args in current mainline.

I fixed that in the obvious manner...