Re: [PATCH v5 4/4] kasan: disable freed user page poisoning with HW tags

[Andrey Konovalov <andreyknvl@gmail.com>]

On Tue, Jun 1, 2021 at 10:51 PM Peter Collingbourne <pcc@google.com> wrote:
>
> Poisoning freed pages protects against kernel use-after-free. The
> likelihood of such a bug involving kernel pages is significantly higher
> than that for user pages. At the same time, poisoning freed pages can
> impose a significant performance cost, which cannot always be justified
> for user pages given the lower probability of finding a bug. Therefore,
> disable freed user page poisoning when using HW tags. We identify
> "user" pages via the flag set GFP_HIGHUSER_MOVABLE, which indicates
> a strong likelihood of not being directly accessible to the kernel.
>
> Signed-off-by: Peter Collingbourne <pcc@google.com>
> Link: https://linux-review.googlesource.com/id/I716846e2de8ef179f44e835770df7e6307be96c9
> ---
> v4:
> - move flag to GFP_HIGHUSER_MOVABLE
> - remove command line flag
>
>  include/linux/gfp.h            | 13 ++++++++++---
>  include/linux/page-flags.h     |  9 +++++++++
>  include/trace/events/mmflags.h |  9 ++++++++-
>  mm/kasan/hw_tags.c             |  3 +++
>  mm/page_alloc.c                | 12 +++++++-----
>  5 files changed, 37 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 68ba237365dc..e6102dfa4faa 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -54,8 +54,9 @@ struct vm_area_struct;
>  #define ___GFP_THISNODE                0x200000u
>  #define ___GFP_ACCOUNT         0x400000u
>  #define ___GFP_ZEROTAGS                0x800000u
> +#define ___GFP_SKIP_KASAN_POISON       0x1000000u
>  #ifdef CONFIG_LOCKDEP
> -#define ___GFP_NOLOCKDEP       0x1000000u
> +#define ___GFP_NOLOCKDEP       0x2000000u
>  #else
>  #define ___GFP_NOLOCKDEP       0
>  #endif
> @@ -233,17 +234,22 @@ struct vm_area_struct;
>   *
>   * %__GFP_ZEROTAGS returns a page with zeroed memory tags on success, if
>   * __GFP_ZERO is set.
> + *
> + * %__GFP_SKIP_KASAN_POISON returns a page which does not need to be poisoned
> + * on deallocation. Typically used for userspace pages. Currently only has an
> + * effect in HW tags mode.
>   */
>  #define __GFP_NOWARN   ((__force gfp_t)___GFP_NOWARN)
>  #define __GFP_COMP     ((__force gfp_t)___GFP_COMP)
>  #define __GFP_ZERO     ((__force gfp_t)___GFP_ZERO)
>  #define __GFP_ZEROTAGS ((__force gfp_t)___GFP_ZEROTAGS)
> +#define __GFP_SKIP_KASAN_POISON        ((__force gfp_t)___GFP_SKIP_KASAN_POISON)
>
>  /* Disable lockdep for GFP context tracking */
>  #define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)
>
>  /* Room for N __GFP_FOO bits */
> -#define __GFP_BITS_SHIFT (24 + IS_ENABLED(CONFIG_LOCKDEP))
> +#define __GFP_BITS_SHIFT (25 + IS_ENABLED(CONFIG_LOCKDEP))
>  #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
>
>  /**
> @@ -324,7 +330,8 @@ struct vm_area_struct;
>  #define GFP_DMA                __GFP_DMA
>  #define GFP_DMA32      __GFP_DMA32
>  #define GFP_HIGHUSER   (GFP_USER | __GFP_HIGHMEM)
> -#define GFP_HIGHUSER_MOVABLE   (GFP_HIGHUSER | __GFP_MOVABLE)
> +#define GFP_HIGHUSER_MOVABLE   (GFP_HIGHUSER | __GFP_MOVABLE | \
> +                        __GFP_SKIP_KASAN_POISON)
>  #define GFP_TRANSHUGE_LIGHT    ((GFP_HIGHUSER_MOVABLE | __GFP_COMP | \
>                          __GFP_NOMEMALLOC | __GFP_NOWARN) & ~__GFP_RECLAIM)
>  #define GFP_TRANSHUGE  (GFP_TRANSHUGE_LIGHT | __GFP_DIRECT_RECLAIM)
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 04a34c08e0a6..40e2c5000585 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -137,6 +137,9 @@ enum pageflags {
>  #endif
>  #ifdef CONFIG_64BIT
>         PG_arch_2,
> +#endif
> +#ifdef CONFIG_KASAN_HW_TAGS
> +       PG_skip_kasan_poison,
>  #endif
>         __NR_PAGEFLAGS,
>
> @@ -443,6 +446,12 @@ 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)
> +#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 629c7a0eaff2..390270e00a1d 100644
> --- a/include/trace/events/mmflags.h
> +++ b/include/trace/events/mmflags.h
> @@ -85,6 +85,12 @@
>  #define IF_HAVE_PG_ARCH_2(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"       },              \
> @@ -112,7 +118,8 @@ IF_HAVE_PG_UNCACHED(PG_uncached,    "uncached"      )               \
>  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_2(PG_arch_2,           "arch_2"        )
> +IF_HAVE_PG_ARCH_2(PG_arch_2,           "arch_2"        )               \
> +IF_HAVE_PG_SKIP_KASAN_POISON(PG_skip_kasan_poison, "skip_kasan_poison")
>
>  #define show_page_flags(flags)                                         \
>         (flags) ? __print_flags(flags, "|",                             \
> diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
> index 41fd5326ee0a..ed5e5b833d61 100644
> --- a/mm/kasan/hw_tags.c
> +++ b/mm/kasan/hw_tags.c
> @@ -246,6 +246,9 @@ void kasan_alloc_pages(struct page *page, unsigned int order, gfp_t flags)
>          */
>         bool init = !want_init_on_free() && want_init_on_alloc(flags);
>
> +       if (flags & __GFP_SKIP_KASAN_POISON)
> +               SetPageSkipKASanPoison(page);
> +
>         if (flags & __GFP_ZEROTAGS) {
>                 int i;
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 13937e793fda..5ad76e540a22 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -394,11 +394,12 @@ static DEFINE_STATIC_KEY_TRUE(deferred_pages);
>   * on-demand allocation and then freed again before the deferred pages
>   * initialization is done, but this is not likely to happen.
>   */
> -static inline bool should_skip_kasan_poison(fpi_t fpi_flags)
> +static inline bool should_skip_kasan_poison(struct page *page, fpi_t fpi_flags)
>  {
>         return static_branch_unlikely(&deferred_pages) ||
>                (!IS_ENABLED(CONFIG_KASAN_GENERIC) &&
> -               (fpi_flags & FPI_SKIP_KASAN_POISON));
> +               (fpi_flags & FPI_SKIP_KASAN_POISON)) ||
> +              PageSkipKASanPoison(page);
>  }
>
>  /* Returns true if the struct page for the pfn is uninitialised */
> @@ -449,10 +450,11 @@ defer_init(int nid, unsigned long pfn, unsigned long end_pfn)
>         return false;
>  }
>  #else
> -static inline bool should_skip_kasan_poison(fpi_t fpi_flags)
> +static inline bool should_skip_kasan_poison(struct page *page, fpi_t fpi_flags)
>  {
>         return (!IS_ENABLED(CONFIG_KASAN_GENERIC) &&
> -               (fpi_flags & FPI_SKIP_KASAN_POISON));
> +               (fpi_flags & FPI_SKIP_KASAN_POISON)) ||
> +              PageSkipKASanPoison(page);
>  }
>
>  static inline bool early_page_uninitialised(unsigned long pfn)
> @@ -1244,7 +1246,7 @@ 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(fpi_flags);
> +       bool skip_kasan_poison = should_skip_kasan_poison(page, fpi_flags);
>
>         VM_BUG_ON_PAGE(PageTail(page), page);
>
> --
> 2.32.0.rc1.229.g3e70b5a671-goog
>

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