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From: Kees Cook <keescook@chromium.org>
To: linux-hardening@vger.kernel.org
Cc: Kees Cook <keescook@chromium.org>,
	Jeremy Kerr <jk@ozlabs.org>,
	Joel Stanley <joel@jms.id.au>,
	Alistar Popple <alistair@popple.id.au>,
	Eddie James <eajames@linux.ibm.com>,
	linux-fsi@lists.ozlabs.org,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	Bill Wendling <morbo@google.com>,
	Justin Stitt <justinstitt@google.com>,
	linux-kernel@vger.kernel.org
Subject: [PATCH 21/82] drivers/fsi: Refactor intentional wrap-around calculation
Date: Mon, 22 Jan 2024 16:26:56 -0800
Message-Id: <20240123002814.1396804-21-keescook@chromium.org>
In-Reply-To: <20240122235208.work.748-kees@kernel.org>
References: <20240122235208.work.748-kees@kernel.org>
Precedence: bulk
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit

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 unsigned wrap-around addition test to use
check_add_overflow(), retaining the result for later usage (which removes
the redundant open-coded addition). 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: Jeremy Kerr <jk@ozlabs.org>
Cc: Joel Stanley <joel@jms.id.au>
Cc: Alistar Popple <alistair@popple.id.au>
Cc: Eddie James <eajames@linux.ibm.com>
Cc: linux-fsi@lists.ozlabs.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 drivers/fsi/fsi-core.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/fsi/fsi-core.c b/drivers/fsi/fsi-core.c
index 097d5a780264..46b24d0aadc6 100644
--- a/drivers/fsi/fsi-core.c
+++ b/drivers/fsi/fsi-core.c
@@ -381,10 +381,12 @@ EXPORT_SYMBOL_GPL(fsi_slave_write);
 int fsi_slave_claim_range(struct fsi_slave *slave,
 			  uint32_t addr, uint32_t size)
 {
-	if (addr + size < addr)
+	uint32_t sum;
+
+	if (check_add_overflow(addr, size, &sum))
 		return -EINVAL;
 
-	if (addr + size > slave->size)
+	if (sum > slave->size)
 		return -EINVAL;
 
 	/* todo: check for overlapping claims */
-- 
2.34.1