linux-hardening - [PATCH 79/82] staging: vme_user: Refactor intentional wrap-around test

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Date: Mon, 22 Jan 2024 16:27:54 -0800
From: Kees Cook <keescook@...omium.org>
To: linux-hardening@...r.kernel.org
Cc: Kees Cook <keescook@...omium.org>,
	Martyn Welch <martyn@...chs.me.uk>,
	Manohar Vanga <manohar.vanga@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Soumya Negi <soumya.negi97@...il.com>,
	Alexon Oliveira <alexondunkan@...il.com>,
	linux-staging@...ts.linux.dev,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 79/82] staging: vme_user: Refactor intentional wrap-around test

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: Martyn Welch <martyn@...chs.me.uk>
Cc: Manohar Vanga <manohar.vanga@...il.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Soumya Negi <soumya.negi97@...il.com>
Cc: Alexon Oliveira <alexondunkan@...il.com>
Cc: linux-staging@...ts.linux.dev
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 drivers/staging/vme_user/vme.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/staging/vme_user/vme.c b/drivers/staging/vme_user/vme.c
index e9461a7a7ab8..a0acf2a295cd 100644
--- a/drivers/staging/vme_user/vme.c
+++ b/drivers/staging/vme_user/vme.c
@@ -165,7 +165,7 @@ int vme_check_window(struct vme_bridge *bridge, u32 aspace,
 {
 	int retval = 0;

-	if (vme_base + size < size)
+	if (add_would_overflow(size, vme_base))
 		return -EINVAL;

 	switch (aspace) {
-- 
2.34.1