linux-kernel - Re: [PATCH 38/82] arm: 3117/1: Refactor intentional wrap-around test

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Date: Tue, 23 Jan 2024 09:56:22 +0000
From: Mark Rutland <mark.rutland@....com>
To: Kees Cook <keescook@...omium.org>
Cc: linux-hardening@...r.kernel.org, Russell King <linux@...linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 38/82] arm: 3117/1: Refactor intentional wrap-around test

The commit title is odd here; '3117/1' is the patch tracker name for the last
patch. The title should probably be:

	arm: nwfpe: Refactor intentional wrap-around test

Mark.

On Mon, Jan 22, 2024 at 04:27:13PM -0800, Kees Cook 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: Russell King <linux@...linux.org.uk>
> Cc: linux-arm-kernel@...ts.infradead.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  arch/arm/nwfpe/softfloat.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm/nwfpe/softfloat.c b/arch/arm/nwfpe/softfloat.c
> index ffa6b438786b..0635b1eda1d3 100644
> --- a/arch/arm/nwfpe/softfloat.c
> +++ b/arch/arm/nwfpe/softfloat.c
> @@ -603,7 +603,7 @@ static floatx80
>      roundBits = zSig0 & roundMask;
>      if ( 0x7FFD <= (bits32) ( zExp - 1 ) ) {
>          if (    ( 0x7FFE < zExp )
> -             || ( ( zExp == 0x7FFE ) && ( zSig0 + roundIncrement < zSig0 ) )
> +             || ( ( zExp == 0x7FFE ) && (add_would_overflow(zSig0, roundIncrement)) )
>             ) {
>              goto overflow;
>          }
> -- 
> 2.34.1
> 
>