Re: [PATCH] kasan: remove PG_skip_kasan_poison flag

[Andrey Konovalov <andreyknvl@gmail.com>]

On Fri, Feb 24, 2023 at 7:51 AM Peter Collingbourne <pcc@google.com> wrote:
>
> Code inspection reveals that PG_skip_kasan_poison is redundant with
> kasantag, because the former is intended to be set iff the latter is
> the match-all tag. It can also be observed that it's basically pointless
> to poison pages which have kasantag=0, because any pages with this tag
> would have been pointed to by pointers with match-all tags, so poisoning
> the pages would have little to no effect in terms of bug detection.
> Therefore, change the condition in should_skip_kasan_poison() to check
> kasantag instead, and remove PG_skip_kasan_poison.

This seems reasonable.

> Signed-off-by: Peter Collingbourne <pcc@google.com>
> Link: https://linux-review.googlesource.com/id/I57f825f2eaeaf7e8389d6cf4597c8a5821359838
> ---
> I sent this independently of
> https://lore.kernel.org/all/20230224061550.177541-1-pcc@google.com/
> because I initially thought that the patches were independent.
> But moments after sending it, I realized that this patch depends on
> that one, because without that patch, this patch will end up disabling
> page poisoning altogether! But it's too late to turn them into a series
> now; I'll do that for v2.
>
>  include/linux/page-flags.h     |  9 ---------
>  include/trace/events/mmflags.h |  9 +--------
>  mm/page_alloc.c                | 28 ++++++++--------------------
>  3 files changed, 9 insertions(+), 37 deletions(-)
>
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index a7e3a3405520..74f81a52e7e1 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -135,9 +135,6 @@ enum pageflags {
>  #ifdef CONFIG_ARCH_USES_PG_ARCH_X
>         PG_arch_2,
>         PG_arch_3,
> -#endif
> -#ifdef CONFIG_KASAN_HW_TAGS
> -       PG_skip_kasan_poison,
>  #endif
>         __NR_PAGEFLAGS,
>
> @@ -594,12 +591,6 @@ TESTCLEARFLAG(Young, young, PF_ANY)
>  PAGEFLAG(Idle, idle, PF_ANY)
>  #endif
>
> -#ifdef CONFIG_KASAN_HW_TAGS
> -PAGEFLAG(SkipKASanPoison, skip_kasan_poison, PF_HEAD)
> -#else
> -PAGEFLAG_FALSE(SkipKASanPoison, skip_kasan_poison)
> -#endif
> -
>  /*
>   * PageReported() is used to track reported free pages within the Buddy
>   * allocator. We can use the non-atomic version of the test and set
> diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> index 9db52bc4ce19..c448694fc7e9 100644
> --- a/include/trace/events/mmflags.h
> +++ b/include/trace/events/mmflags.h
> @@ -96,12 +96,6 @@
>  #define IF_HAVE_PG_ARCH_X(flag,string)
>  #endif
>
> -#ifdef CONFIG_KASAN_HW_TAGS
> -#define IF_HAVE_PG_SKIP_KASAN_POISON(flag,string) ,{1UL << flag, string}
> -#else
> -#define IF_HAVE_PG_SKIP_KASAN_POISON(flag,string)
> -#endif
> -
>  #define __def_pageflag_names                                           \
>         {1UL << PG_locked,              "locked"        },              \
>         {1UL << PG_waiters,             "waiters"       },              \
> @@ -130,8 +124,7 @@ IF_HAVE_PG_HWPOISON(PG_hwpoison,    "hwpoison"      )               \
>  IF_HAVE_PG_IDLE(PG_young,              "young"         )               \
>  IF_HAVE_PG_IDLE(PG_idle,               "idle"          )               \
>  IF_HAVE_PG_ARCH_X(PG_arch_2,           "arch_2"        )               \
> -IF_HAVE_PG_ARCH_X(PG_arch_3,           "arch_3"        )               \
> -IF_HAVE_PG_SKIP_KASAN_POISON(PG_skip_kasan_poison, "skip_kasan_poison")
> +IF_HAVE_PG_ARCH_X(PG_arch_3,           "arch_3"        )
>
>  #define show_page_flags(flags)                                         \
>         (flags) ? __print_flags(flags, "|",                             \
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 7136c36c5d01..2509b8bde8d5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1380,7 +1380,7 @@ static inline bool should_skip_kasan_poison(struct page *page, fpi_t fpi_flags)
>         return deferred_pages_enabled() ||
>                (!IS_ENABLED(CONFIG_KASAN_GENERIC) &&
>                 (fpi_flags & FPI_SKIP_KASAN_POISON)) ||
> -              PageSkipKASanPoison(page);
> +              page_kasan_tag(page) == 0xff;

Please also update the comment above should_skip_kasan_poison.

I think we can drop #3 and #4 from that comment and instead add a more
generic #3: "Page tags have not been assigned, as unpoisoning has been
skipped".

>  }
>
>  static void kernel_init_pages(struct page *page, int numpages)
> @@ -2511,22 +2511,13 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
>                 /* Take note that memory was initialized by the loop above. */
>                 init = false;
>         }
> -       if (!should_skip_kasan_unpoison(gfp_flags)) {
> -               /* Try unpoisoning (or setting tags) and initializing memory. */
> -               if (kasan_unpoison_pages(page, order, init)) {
> -                       /* Take note that memory was initialized by KASAN. */
> -                       if (kasan_has_integrated_init())
> -                               init = false;
> -                       /* Take note that memory tags were set by KASAN. */
> -                       reset_tags = false;
> -               } else {
> -                       /*
> -                        * KASAN decided to exclude this allocation from being
> -                        * (un)poisoned due to sampling. Make KASAN skip
> -                        * poisoning when the allocation is freed.
> -                        */
> -                       SetPageSkipKASanPoison(page);
> -               }
> +       if (!should_skip_kasan_unpoison(gfp_flags) &&
> +           kasan_unpoison_pages(page, order, init)) {
> +               /* Take note that memory was initialized by KASAN. */
> +               if (kasan_has_integrated_init())
> +                       init = false;
> +               /* Take note that memory tags were set by KASAN. */
> +               reset_tags = false;
>         }
>         /*
>          * If memory tags have not been set by KASAN, reset the page tags to
> @@ -2539,9 +2530,6 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
>         /* If memory is still not initialized, initialize it now. */
>         if (init)
>                 kernel_init_pages(page, 1 << order);
> -       /* Propagate __GFP_SKIP_KASAN_POISON to page flags. */
> -       if (kasan_hw_tags_enabled() && (gfp_flags & __GFP_SKIP_KASAN_POISON))
> -               SetPageSkipKASanPoison(page);

With this removed, __GFP_SKIP_KASAN_POISON is no longer used and can
be removed too.


>
>         set_page_owner(page, order, gfp_flags);
>         page_table_check_alloc(page, order);
> --
> 2.39.2.637.g21b0678d19-goog
>