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

From: Kees Cook <keescook@chromium.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: linux-hardening@vger.kernel.org,
	Russell King <linux@armlinux.org.uk>,
	linux-arm-kernel@lists.infradead.org,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	Bill Wendling <morbo@google.com>,
	Justin Stitt <justinstitt@google.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 38/82] arm: 3117/1: Refactor intentional wrap-around test
Date: Tue, 23 Jan 2024 14:41:59 -0800	[thread overview]
Message-ID: <202401231441.7A15A53419@keescook> (raw)
In-Reply-To: <Za-NRm5e_-Cfz1Nl@FVFF77S0Q05N>

On Tue, Jan 23, 2024 at 09:56:22AM +0000, Mark Rutland wrote:
> 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

Whoops, yes. I need to teach my prefix-guessing script to drop the ARM
patch tracker numbers...

-Kees

> 
> 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@armlinux.org.uk>
> > Cc: linux-arm-kernel@lists.infradead.org
> > Signed-off-by: Kees Cook <keescook@chromium.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
> > 
> > 

-- 
Kees Cook

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