[PATCH v2] hwmon: (cros_ec) Handle temperature conversion overflows

The calculations converting between the different temperature units can
overflow on 32-bit systems, resulting in incorrect data.

Detect these overflows and handle them.

The code is written in a way that there is a single conversion function
and the compiler can recognize when overflows are impossible (on 64-bit
and in cros_ec_hwmon_temp_to_millicelsius()). If the compiler detects
this, it will optimize away the overflow checks.

Signed-off-by: Thomas Weißschuh <[email protected]>
---
Changes in v2:
- Drop already applied patch 1.
- Also handle overflow of u32 -> long.
- Clarify commit message wrt compiler optimizations.
- Use __always_inline over __flatten to allow the compiler to optimize
  away more unnecessary overflow checks.
- Link to v1: https://patch.msgid.link/[email protected]
---
 drivers/hwmon/cros_ec_hwmon.c | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/drivers/hwmon/cros_ec_hwmon.c b/drivers/hwmon/cros_ec_hwmon.c
index 1337b646e022..004a8180b665 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/cleanup.h>
 #include <linux/device.h>
 #include <linux/hwmon.h>
 #include <linux/math.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>
@@ -151,14 +153,28 @@ 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 __always_inline 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)
 {
-	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)
@@ -236,8 +252,13 @@ 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 (overflows_type(threshold, long))
+					*val = LONG_MAX;
+
+				if (cros_ec_hwmon_kelvin_to_millicelsius_overflow(threshold, val))
+					*val = LONG_MAX;
+			}
 		}
 	}
 

---
base-commit: 0109905cfdf4b64da014596f31c2ee5029ef0870
change-id: 20260630-cros_ec-hwmon-overflow-0381c8509df3

Best regards,
--  
Thomas Weißschuh <[email protected]>