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

[Andrey Konovalov]

On Fri, Feb 26, 2021 at 2:25 AM 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().
>
> Besides fixing the crash, this also makes the annotation order consistent
> with debug_pagealloc_map_pages() preceding kasan_alloc_pages().
>
> Signed-off-by: Andrey Konovalov <andreyknvl at google.com>
> ---
>  mm/page_alloc.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c89e7b107514..54bc237fd319 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1311,10 +1311,14 @@ static __always_inline bool free_pages_prepare(struct page *page,
>          */
>         arch_free_page(page, order);
>
> -       debug_pagealloc_unmap_pages(page, 1 << order);
> -
> +       /*
> +        * With hardware tag-based KASAN, memory tags must be set
> +        * before unmapping the page with debug_pagealloc.
> +        */
>         kasan_free_nondeferred_pages(page, order, fpi_flags);

Looking at this again, I think we need to move kasan_() callback above
arch_free_page(), as that can also make the page unavailable. I'll
send v2.