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References: <20240122235208.work.748-kees@kernel.org> <20240123002814.1396804-55-keescook@chromium.org>
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From: Andrey Konovalov <andreyknvl@gmail.com>
Date: Thu, 25 Jan 2024 23:35:30 +0100
Message-ID: <CA+fCnZdiLCDTcOmc7x_eHV9oNQhNq_sU1_w+rafoBA1FVEbwrA@mail.gmail.com>
Subject: Re: [PATCH 55/82] kasan: Refactor intentional wrap-around test
To: Kees Cook <keescook@chromium.org>
Cc: linux-hardening@vger.kernel.org, Andrey Ryabinin <ryabinin.a.a@gmail.com>, 
	Alexander Potapenko <glider@google.com>, Dmitry Vyukov <dvyukov@google.com>, 
	Vincenzo Frascino <vincenzo.frascino@arm.com>, Andrew Morton <akpm@linux-foundation.org>, 
	kasan-dev@googlegroups.com, linux-mm@kvack.org, 
	"Gustavo A. R. Silva" <gustavoars@kernel.org>, Bill Wendling <morbo@google.com>, 
	Justin Stitt <justinstitt@google.com>, linux-kernel@vger.kernel.org
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On Tue, Jan 23, 2024 at 1:29=E2=80=AFAM Kees Cook <keescook@chromium.org> w=
rote:
>
> 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/68df3755e383e6fecf2354a67b08f92f185365=
94 [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@gmail.com>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Andrey Konovalov <andreyknvl@gmail.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: kasan-dev@googlegroups.com
> Cc: linux-mm@kvack.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  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 =3D=3D 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, b=
ool write,
>         if (unlikely(size =3D=3D 0))
>                 return true;
>
> -       if (unlikely(addr + size < addr))
> +       if (unlikely(add_would_overflow(addr, size)))
>                 return !kasan_report(addr, size, write, ret_ip);
>
>         tag =3D get_tag((const void *)addr);
> --
> 2.34.1
>

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

Thanks!