linux-hardening - Re: [PATCH 60/82] powerpc: Refactor intentional wrap-around test

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Message-ID: <87zfw66y3i.fsf@mail.lhotse>
Date: Mon, 12 Feb 2024 16:38:41 +1100
From: Michael Ellerman <mpe@...erman.id.au>
To: Kees Cook <keescook@...omium.org>, linux-hardening@...r.kernel.org
Cc: Kees Cook <keescook@...omium.org>, Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>, "Aneesh Kumar K.V"
 <aneesh.kumar@...nel.org>, "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Mahesh Salgaonkar <mahesh@...ux.ibm.com>, Vasant Hegde
 <hegdevasant@...ux.vnet.ibm.com>, dingsenjie <dingsenjie@...ong.com>,
 linuxppc-dev@...ts.ozlabs.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 60/82] powerpc: Refactor intentional wrap-around test

Kees Cook <keescook@...omium.org> writes:
> 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: Michael Ellerman <mpe@...erman.id.au>
> Cc: Nicholas Piggin <npiggin@...il.com>
> Cc: Christophe Leroy <christophe.leroy@...roup.eu>
> Cc: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>
> Cc: "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>
> Cc: Mahesh Salgaonkar <mahesh@...ux.ibm.com>
> Cc: Vasant Hegde <hegdevasant@...ux.vnet.ibm.com>
> Cc: dingsenjie <dingsenjie@...ong.com>
> Cc: linuxppc-dev@...ts.ozlabs.org
> Cc: Aneesh Kumar K.V <aneesh.kumar@...nel.org>
> Cc: Naveen N. Rao <naveen.n.rao@...ux.ibm.com>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  arch/powerpc/platforms/powernv/opal-prd.c | 2 +-
>  arch/powerpc/xmon/xmon.c                  | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)

Acked-by: Michael Ellerman <mpe@...erman.id.au> (powerpc)

cheers