[PATCH 2/2] hwmon: (cros_ec) Handle temperature conversion overflows

The calculations converting between the different temperature units can
overflow, resulting in incorrect data.

Detect these overflows and report them.

Signed-off-by: Thomas Weißschuh <[email protected]>
---
 drivers/hwmon/cros_ec_hwmon.c | 30 ++++++++++++++++++++++++------
 1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/drivers/hwmon/cros_ec_hwmon.c b/drivers/hwmon/cros_ec_hwmon.c
index 44291799dd8e..4b402142b46f 100644
--- a/drivers/hwmon/cros_ec_hwmon.c
+++ b/drivers/hwmon/cros_ec_hwmon.c
@@ -5,11 +5,13 @@
  *  Copyright (C) 2024 Thomas Weißschuh <[email protected]>
  */
 
+#include <linux/build_bug.h>
 #include <linux/device.h>
 #include <linux/hwmon.h>
 #include <linux/math.h>
 #include <linux/mod_devicetable.h>
 #include <linux/module.h>
+#include <linux/overflow.h>
 #include <linux/platform_device.h>
 #include <linux/platform_data/cros_ec_commands.h>
 #include <linux/platform_data/cros_ec_proto.h>
@@ -150,14 +152,28 @@ static bool cros_ec_hwmon_is_error_temp(u8 temp)
 /* This differs slightly from the variant in units.h to avoid rounding inconsistencies. */
 #define CROS_EC_HWMON_ABSOLUTE_ZERO_MILLICELSIUS (-273000)
 
-static long cros_ec_hwmon_kelvin_to_millicelsius(long t)
+static bool cros_ec_hwmon_kelvin_to_millicelsius_overflow(long t, long *ret)
 {
-	return t * MILLIDEGREE_PER_DEGREE + CROS_EC_HWMON_ABSOLUTE_ZERO_MILLICELSIUS;
+	if (check_mul_overflow(t, MILLIDEGREE_PER_DEGREE, ret))
+		return true;
+
+	if (check_add_overflow(*ret, CROS_EC_HWMON_ABSOLUTE_ZERO_MILLICELSIUS, ret))
+		return true;
+
+	return false;
 }
 
-static long cros_ec_hwmon_temp_to_millicelsius(u8 temp)
+static long __flatten cros_ec_hwmon_temp_to_millicelsius(u8 temp)
 {
-	return cros_ec_hwmon_kelvin_to_millicelsius((((long)temp) + EC_TEMP_SENSOR_OFFSET));
+	long ret;
+
+	if (check_add_overflow(temp, EC_TEMP_SENSOR_OFFSET, &ret))
+		BUILD_BUG();
+
+	if (cros_ec_hwmon_kelvin_to_millicelsius_overflow(ret, &ret))
+		BUILD_BUG();
+
+	return ret;
 }
 
 static bool cros_ec_hwmon_attr_is_temp_threshold(u32 attr)
@@ -235,8 +251,10 @@ static int cros_ec_hwmon_read(struct device *dev, enum hwmon_sensor_types type,
 			ret = cros_ec_hwmon_read_temp_threshold(priv->cros_ec, channel,
 								cros_ec_hwmon_attr_to_thres(attr),
 								&threshold);
-			if (ret == 0)
-				*val = cros_ec_hwmon_kelvin_to_millicelsius(threshold);
+			if (ret == 0) {
+				if (cros_ec_hwmon_kelvin_to_millicelsius_overflow(threshold, val))
+					*val = LONG_MAX;
+			}
 		}
 	}
 

-- 
2.55.0