Re: [PATCH v3 3/3] kasan: allow freed user page poisoning to be disabled with HW tags

[Andrey Konovalov <andreyknvl@gmail.com>]

+Jann

On Wed, May 12, 2021 at 11:09 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,
> make it possible to configure the kernel to disable freed user page
> poisoning when using HW tags via the new kasan.skip_user_poison_on_free
> command line option.

So the potential scenario that would be undetectable with
kasan.skip_user_poison_on_free enabled is: 1) kernel allocates a user
page and maps it for userspace, 2) the page gets freed in the kernel,
3) kernel accesses the page leading to a use-after-free. Is this
correct?

If bugs involving use-after-free accesses on user pages is something
that is extremely rare, perhaps we could just change the default and
avoid adding a command line switch.

Jann, maybe you have an idea of how common something like this is or
have other inputs?

Peter, is the plan to have this poisoning disabled in production? Is
there an estimate on slow this is?



> Signed-off-by: Peter Collingbourne <pcc@google.com>
> Link: https://linux-review.googlesource.com/id/I716846e2de8ef179f44e835770df7e6307be96c9
> ---
>  include/linux/gfp.h            | 13 ++++++++++---
>  include/linux/page-flags.h     |  9 +++++++++
>  include/trace/events/mmflags.h |  9 ++++++++-
>  mm/kasan/hw_tags.c             | 10 ++++++++++
>  mm/page_alloc.c                | 12 +++++++-----
>  5 files changed, 44 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 68ba237365dc..9a77e5660b07 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, and only if a command line option is set.
>   */
>  #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))
>
>  /**
> @@ -320,7 +326,8 @@ struct vm_area_struct;
>  #define GFP_NOWAIT     (__GFP_KSWAPD_RECLAIM)
>  #define GFP_NOIO       (__GFP_RECLAIM)
>  #define GFP_NOFS       (__GFP_RECLAIM | __GFP_IO)
> -#define GFP_USER       (__GFP_RECLAIM | __GFP_IO | __GFP_FS | __GFP_HARDWALL)
> +#define GFP_USER       (__GFP_RECLAIM | __GFP_IO | __GFP_FS | \
> +                        __GFP_HARDWALL | __GFP_SKIP_KASAN_POISON)
>  #define GFP_DMA                __GFP_DMA
>  #define GFP_DMA32      __GFP_DMA32
>  #define GFP_HIGHUSER   (GFP_USER | __GFP_HIGHMEM)
> 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 34362c8d0955..954d5c2f7683 100644
> --- a/mm/kasan/hw_tags.c
> +++ b/mm/kasan/hw_tags.c
> @@ -238,10 +238,20 @@ struct kasan_track *kasan_get_free_track(struct kmem_cache *cache,
>         return &alloc_meta->free_track[0];
>  }
>
> +static bool skip_user_poison_on_free;
> +static int __init skip_user_poison_on_free_param(char *buf)
> +{
> +       return kstrtobool(buf, &skip_user_poison_on_free);
> +}
> +early_param("kasan.skip_user_poison_on_free", skip_user_poison_on_free_param);
> +
>  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 (skip_user_poison_on_free && (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 24e6f668ef73..2c3ac15ddd54 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);
>
> --
> 2.31.1.607.g51e8a6a459-goog
>