Re: [PATCH] kasan: fix kasan_byte_accessible() to be consistent with actual checks

[Andrey Konovalov <andreyknvl@gmail.com>]

On Mon, Apr 5, 2021 at 11:43 PM 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 | 8 +++++---
>  2 files changed, 6 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..914225eeda99 100644
> --- a/mm/kasan/sw_tags.c
> +++ b/mm/kasan/sw_tags.c
> @@ -121,10 +121,12 @@ 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 = READ_ONCE(*(u8 *)kasan_mem_to_shadow(untagged_addr));

Hi Peter,

Let's move dereferencing shadow memory past the KASAN_SHADOW_START
check. Otherwise, in case the check is to fail, accessing shadow will
likely crash the kernel.

Thanks!

>
> -       return (shadow_byte != KASAN_TAG_INVALID) &&
> -               (tag == KASAN_TAG_KERNEL || tag == shadow_byte);
> +       return untagged_addr >=
> +                      kasan_shadow_to_mem((void *)KASAN_SHADOW_START) &&
> +              (tag == KASAN_TAG_KERNEL || tag == shadow_byte);
>  }
>
>  #define DEFINE_HWASAN_LOAD_STORE(size)                                 \
> --
> 2.31.0.208.g409f899ff0-goog
>