Re: [PATCH] Revert "kasan: drop skip_kasan_poison variable in free_pages_prepare"

[Andrey Konovalov <andreyknvl@gmail.com>]

On Fri, Feb 24, 2023 at 7:16 AM Peter Collingbourne <pcc@google.com> wrote:
>
> This reverts commit 487a32ec24be819e747af8c2ab0d5c515508086a.
>
> The should_skip_kasan_poison() function reads the PG_skip_kasan_poison
> flag from page->flags. However, this line of code in free_pages_prepare():
>
> page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
>
> clears most of page->flags, including PG_skip_kasan_poison, before calling
> should_skip_kasan_poison(), which meant that it would never return true
> as a result of the page flag being set. Therefore, fix the code to call
> should_skip_kasan_poison() before clearing the flags, as we were doing
> before the reverted patch.
>
> Signed-off-by: Peter Collingbourne <pcc@google.com>
> Fixes: 487a32ec24be ("kasan: drop skip_kasan_poison variable in free_pages_prepare")
> Cc: <stable@vger.kernel.org> # 6.1
> Link: https://linux-review.googlesource.com/id/Ic4f13affeebd20548758438bb9ed9ca40e312b79
> ---
>  mm/page_alloc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ac1fc986af44..7136c36c5d01 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1398,6 +1398,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
>                         unsigned int order, bool check_free, fpi_t fpi_flags)
>  {
>         int bad = 0;
> +       bool skip_kasan_poison = should_skip_kasan_poison(page, fpi_flags);
>         bool init = want_init_on_free();
>
>         VM_BUG_ON_PAGE(PageTail(page), page);
> @@ -1470,7 +1471,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
>          * With hardware tag-based KASAN, memory tags must be set before the
>          * page becomes unavailable via debug_pagealloc or arch_free_page.
>          */
> -       if (!should_skip_kasan_poison(page, fpi_flags)) {
> +       if (!skip_kasan_poison) {
>                 kasan_poison_pages(page, order, init);
>
>                 /* Memory is already initialized if KASAN did it internally. */
> --
> 2.39.2.637.g21b0678d19-goog
>

Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>

Thank you for fixing this, Peter!