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

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

[Kees Cook]

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@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>
Cc: "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
Cc: Mahesh Salgaonkar <mahesh@linux.ibm.com>
Cc: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Cc: dingsenjie <dingsenjie@yulong.com>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
Cc: Naveen N. Rao <naveen.n.rao@linux.ibm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 arch/powerpc/platforms/powernv/opal-prd.c | 2 +-
 arch/powerpc/xmon/xmon.c                  | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/platforms/powernv/opal-prd.c b/arch/powerpc/platforms/powernv/opal-prd.c
index b66b06efcef1..eaf95dc82925 100644
--- a/arch/powerpc/platforms/powernv/opal-prd.c
+++ b/arch/powerpc/platforms/powernv/opal-prd.c
@@ -51,7 +51,7 @@ static bool opal_prd_range_is_valid(uint64_t addr, uint64_t size)
 	struct device_node *parent, *node;
 	bool found;
 
-	if (addr + size < addr)
+	if (add_would_overflow(addr, size))
 		return false;
 
 	parent = of_find_node_by_path("/reserved-memory");
diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index b3b94cd37713..b91fdda49434 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c
@@ -3252,7 +3252,7 @@ memzcan(void)
 		} else if (!ok && ook)
 			printf("%.8lx\n", a - mskip);
 		ook = ok;
-		if (a + mskip < a)
+		if (add_would_overflow(a, mskip))
 			break;
 	}
 	if (ook)
-- 
2.34.1

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

[Michael Ellerman]

Kees Cook <keescook@chromium.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@ellerman.id.au>
> Cc: Nicholas Piggin <npiggin@gmail.com>
> Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> Cc: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>
> Cc: "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
> Cc: Mahesh Salgaonkar <mahesh@linux.ibm.com>
> Cc: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
> Cc: dingsenjie <dingsenjie@yulong.com>
> Cc: linuxppc-dev@lists.ozlabs.org
> Cc: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
> Cc: Naveen N. Rao <naveen.n.rao@linux.ibm.com>
> Signed-off-by: Kees Cook <keescook@chromium.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@ellerman.id.au> (powerpc)

cheers