Re: [PATCH 55/82] kasan: Refactor intentional wrap-around test

[Andrey Konovalov]

On Tue, Jan 23, 2024 at 1:29 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> In an effort to separate intentional arithmetic wrap-around from
> unexpected wrap-around, we need to refactor places that depend on this
> kind of math. One of the most common code patterns of this is:
>
>         VAR + value < VAR
>
> Notably, this is considered "undefined behavior" for signed and pointer
> types, which the kernel works around by using the -fno-strict-overflow
> option in the build[1] (which used to just be -fwrapv). Regardless, we
> want to get the kernel source to the position where we can meaningfully
> instrument arithmetic wrap-around conditions and catch them when they
> are unexpected, regardless of whether they are signed[2], unsigned[3],
> or pointer[4] types.
>
> Refactor open-coded wrap-around addition test to use add_would_overflow().
> This paves the way to enabling the wrap-around sanitizers in the future.
>
> Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
> Link: https://github.com/KSPP/linux/issues/26 [2]
> Link: https://github.com/KSPP/linux/issues/27 [3]
> Link: https://github.com/KSPP/linux/issues/344 [4]
> Cc: Andrey Ryabinin <ryabinin.a.a@xxxxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: kasan-dev@xxxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  mm/kasan/generic.c | 2 +-
>  mm/kasan/sw_tags.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
> index df6627f62402..f9bc29ae09bd 100644
> --- a/mm/kasan/generic.c
> +++ b/mm/kasan/generic.c
> @@ -171,7 +171,7 @@ static __always_inline bool check_region_inline(const void *addr,
>         if (unlikely(size == 0))
>                 return true;
>
> -       if (unlikely(addr + size < addr))
> +       if (unlikely(add_would_overflow(addr, size)))
>                 return !kasan_report(addr, size, write, ret_ip);
>
>         if (unlikely(!addr_has_metadata(addr)))
> diff --git a/mm/kasan/sw_tags.c b/mm/kasan/sw_tags.c
> index 220b5d4c6876..79a3bbd66c32 100644
> --- a/mm/kasan/sw_tags.c
> +++ b/mm/kasan/sw_tags.c
> @@ -80,7 +80,7 @@ bool kasan_check_range(const void *addr, size_t size, bool write,
>         if (unlikely(size == 0))
>                 return true;
>
> -       if (unlikely(addr + size < addr))
> +       if (unlikely(add_would_overflow(addr, size)))
>                 return !kasan_report(addr, size, write, ret_ip);
>
>         tag = get_tag((const void *)addr);
> --
> 2.34.1
>

Acked-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>

Thanks!