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From:   Andrey Konovalov <andreyknvl@gmail.com>
Date:   Tue, 6 Apr 2021 00:43:22 +0200
Message-ID: <CA+fCnZc=+d036Ve7k-4OdtZ+epSd4QywRs-qc9JWV72Z8Er1yw@mail.gmail.com>
Subject: Re: [PATCH v2] kasan: fix kasan_byte_accessible() to be consistent
 with actual checks
To:     Peter Collingbourne <pcc@google.com>
Cc:     Marco Elver <elver@google.com>, Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        Evgenii Stepanov <eugenis@google.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Tue, Apr 6, 2021 at 12:07 AM Peter Collingbourne <pcc@google.com> wrote:
>
> We can sometimes end up with kasan_byte_accessible() being called
> on non-slab memory. For example ksize() and krealloc() may end up
> calling it on KFENCE allocated memory. In this case the memory will
> be tagged with KASAN_SHADOW_INIT, which a subsequent patch ("kasan:
> initialize shadow to TAG_INVALID for SW_TAGS") will set to the same
> value as KASAN_TAG_INVALID, causing kasan_byte_accessible() to fail
> when called on non-slab memory.
>
> This highlighted the fact that the check in kasan_byte_accessible()
> was inconsistent with checks as implemented for loads and stores
> (kasan_check_range() in SW tags mode and hardware-implemented
> checks in HW tags mode). kasan_check_range() does not have a
> check for KASAN_TAG_INVALID, and instead has a comparison against
> KASAN_SHADOW_START. In HW tags mode, we do not have either, but we
> do set TCR_EL1.TCMA which corresponds with the comparison against
> KASAN_TAG_KERNEL.
>
> Therefore, update kasan_byte_accessible() for both SW and HW tags
> modes to correspond with the respective checks on loads and stores.
>
> Link: https://linux-review.googlesource.com/id/Ic6d40803c57dcc6331bd97fbb9a60b0d38a65a36
> Signed-off-by: Peter Collingbourne <pcc@google.com>
> ---
>  mm/kasan/kasan.h   |  3 +--
>  mm/kasan/sw_tags.c | 10 +++++++---
>  2 files changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index 8c55634d6edd..e18e8da35255 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -368,8 +368,7 @@ static inline bool kasan_byte_accessible(const void *addr)
>         u8 ptr_tag = get_tag(addr);
>         u8 mem_tag = hw_get_mem_tag((void *)addr);
>
> -       return (mem_tag != KASAN_TAG_INVALID) &&
> -               (ptr_tag == KASAN_TAG_KERNEL || ptr_tag == mem_tag);
> +       return ptr_tag == KASAN_TAG_KERNEL || ptr_tag == mem_tag;
>  }
>
>  #else /* CONFIG_KASAN_HW_TAGS */
> diff --git a/mm/kasan/sw_tags.c b/mm/kasan/sw_tags.c
> index 94c2d33be333..00ae8913fc74 100644
> --- a/mm/kasan/sw_tags.c
> +++ b/mm/kasan/sw_tags.c
> @@ -121,10 +121,14 @@ bool kasan_check_range(unsigned long addr, size_t size, bool write,
>  bool kasan_byte_accessible(const void *addr)
>  {
>         u8 tag = get_tag(addr);
> -       u8 shadow_byte = READ_ONCE(*(u8 *)kasan_mem_to_shadow(kasan_reset_tag(addr)));
> +       void *untagged_addr = kasan_reset_tag(addr);
> +       u8 shadow_byte;
>
> -       return (shadow_byte != KASAN_TAG_INVALID) &&
> -               (tag == KASAN_TAG_KERNEL || tag == shadow_byte);
> +       if (untagged_addr < kasan_shadow_to_mem((void *)KASAN_SHADOW_START))
> +               return false;
> +
> +       shadow_byte = READ_ONCE(*(u8 *)kasan_mem_to_shadow(untagged_addr));
> +       return tag == KASAN_TAG_KERNEL || tag == shadow_byte;
>  }
>
>  #define DEFINE_HWASAN_LOAD_STORE(size)                                 \
> --
> 2.31.0.208.g409f899ff0-goog
>

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

Thank you, Peter!