linux-hardening - Re: [PATCH 35/82] ACPI: custom_method: Refactor intentional wrap-around test

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Date: Wed, 24 Jan 2024 12:16:08 -0800
From: Kees Cook <keescook@...omium.org>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: linux-hardening@...r.kernel.org, Len Brown <lenb@...nel.org>,
	linux-acpi@...r.kernel.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 35/82] ACPI: custom_method: Refactor intentional
 wrap-around test

On Wed, Jan 24, 2024 at 08:52:48PM +0100, Rafael J. Wysocki wrote:
> On Tue, Jan 23, 2024 at 2:03 AM Kees Cook <keescook@...omium.org> 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: "Rafael J. Wysocki" <rafael@...nel.org>
> > Cc: Len Brown <lenb@...nel.org>
> > Cc: linux-acpi@...r.kernel.org
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> >  drivers/acpi/custom_method.c | 2 +-
> 
> I may attempt to drop custom_method.c in this cycle, is there a
> problem if I take this into my tree for now?

The helper doesn't exist in tree yet, but it may be a bit before these
refactors land, so if custom_method vanishes before then, that's great!
:)

-Kees

-- 
Kees Cook