[PATCH v5 04/10] iio: afe: rescale: reduce risk of integer overflow

From: Liam Beguin <liambeguin@gmail.com>
To: liambeguin@gmail.com, peda@axentia.se, jic23@kernel.org,
	lars@metafoo.de, pmeerw@pmeerw.net
Cc: linux-kernel@vger.kernel.org, linux-iio@vger.kernel.org,
	devicetree@vger.kernel.org, robh+dt@kernel.org
Subject: [PATCH v5 04/10] iio: afe: rescale: reduce risk of integer overflow
Date: Wed, 14 Jul 2021 23:12:09 -0400	[thread overview]
Message-ID: <20210715031215.1534938-5-liambeguin@gmail.com> (raw)
In-Reply-To: <20210715031215.1534938-1-liambeguin@gmail.com>

From: Liam Beguin <lvb@xiphos.com>

Reduce the risk of integer overflow by doing the scale calculation with
64bit integers and looking for a Greatest Common Divider for both parts
of the fractional value when required.

Signed-off-by: Liam Beguin <lvb@xiphos.com>
---
 drivers/iio/afe/iio-rescale.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
index 774eb3044edd..4c3cfd4d5181 100644
--- a/drivers/iio/afe/iio-rescale.c
+++ b/drivers/iio/afe/iio-rescale.c
@@ -39,7 +39,8 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
 			    int *val, int *val2, long mask)
 {
 	struct rescale *rescale = iio_priv(indio_dev);
-	unsigned long long tmp;
+	s64 tmp, tmp2;
+	u32 factor;
 	int ret;
 
 	switch (mask) {
@@ -67,8 +68,16 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
 		}
 		switch (ret) {
 		case IIO_VAL_FRACTIONAL:
-			*val *= rescale->numerator;
-			*val2 *= rescale->denominator;
+			tmp = (s64)*val * rescale->numerator;
+			tmp2 = (s64)*val2 * rescale->denominator;
+			if (check_mul_overflow(*val, rescale->numerator, (s32 *)&tmp) ||
+			check_mul_overflow(*val2, rescale->denominator, (s32 *)&tmp2)) {
+				factor = gcd(tmp, tmp2);
+				do_div(tmp, factor);
+				do_div(tmp2, factor);
+			}
+			*val = tmp;
+			*val2 = tmp2;
 			return ret;
 		case IIO_VAL_INT:
 			*val *= rescale->numerator;
-- 
2.30.1.489.g328c10930387

