Re: [PATCH] mm/kasan: Fix false positive invalid-free reports with CONFIG_KASAN_SW_TAGS=y

[Andrey Konovalov]

On Mon, Aug 19, 2019 at 7:26 PM Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote:
>
> The code like this:
>
>         ptr = kmalloc(size, GFP_KERNEL);
>         page = virt_to_page(ptr);
>         offset = offset_in_page(ptr);
>         kfree(page_address(page) + offset);
>
> may produce false-positive invalid-free reports on the kernel with
> CONFIG_KASAN_SW_TAGS=y.
>
> In the example above we loose the original tag assigned to 'ptr',
> so kfree() gets the pointer with 0xFF tag. In kfree() we check that
> 0xFF tag is different from the tag in shadow hence print false report.
>
> Instead of just comparing tags, do the following:
>  1) Check that shadow doesn't contain KASAN_TAG_INVALID. Otherwise it's
>     double-free and it doesn't matter what tag the pointer have.
>
>  2) If pointer tag is different from 0xFF, make sure that tag in the shadow
>     is the same as in the pointer.
>
> Fixes: 7f94ffbc4c6a ("kasan: add hooks implementation for tag-based mode")
> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> Reported-by: Walter Wu <walter-zh.wu@xxxxxxxxxxxx>
> Reported-by: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>

Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>

> ---
>  mm/kasan/common.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 895dc5e2b3d5..3b8cde0cb5b2 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -406,8 +406,14 @@ static inline bool shadow_invalid(u8 tag, s8 shadow_byte)
>         if (IS_ENABLED(CONFIG_KASAN_GENERIC))
>                 return shadow_byte < 0 ||
>                         shadow_byte >= KASAN_SHADOW_SCALE_SIZE;
> -       else
> -               return tag != (u8)shadow_byte;
> +
> +       /* else CONFIG_KASAN_SW_TAGS: */
> +       if ((u8)shadow_byte == KASAN_TAG_INVALID)
> +               return true;
> +       if ((tag != KASAN_TAG_KERNEL) && (tag != (u8)shadow_byte))
> +               return true;
> +
> +       return false;
>  }
>
>  static bool __kasan_slab_free(struct kmem_cache *cache, void *object,
> --
> 2.21.0
>