[PATCH v2] HID: wacom: validate battery range before scaling

[Aldo Ariel Panzardo]

Battery events divide by the difference between the logical maximum
and minimum without validating the range.  A malformed HID descriptor
with equal limits can therefore trigger a divide-by-zero exception.

Move percentage conversion to a helper that widens the arithmetic,
rejects non-positive ranges, and accounts for a nonzero logical
minimum.  Clamp the result to [0, 100] instead of rejecting
out-of-range values, so that real hardware whose ADC occasionally
reports slightly beyond its declared limits still registers its
battery and updates capacity normally.

Fixes: 37d160193834 ("HID: wacom: generic: Scale battery capacity measurements to percentages")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Aldo Ariel Panzardo <qwe.aldo@xxxxxxxxx>
---

Changes in v2:
  - Clamp the computed percentage to [0, 100] instead of returning
    false, so that slightly out-of-range ADC values do not prevent
    battery registration (WACOM_QUIRK_BATTERY) or silently drop
    updates.  Reported by Sashiko AI review.

 drivers/hid/wacom_wac.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/drivers/hid/wacom_wac.c b/drivers/hid/wacom_wac.c
index da1f0ea85..XXXXXXX 100644
--- a/drivers/hid/wacom_wac.c
+++ b/drivers/hid/wacom_wac.c
@@ -7,6 +7,7 @@
 #include "wacom.h"
 #include <linux/input/mt.h>
 #include <linux/jiffies.h>
+#include <linux/math64.h>

 /* resolution for penabled devices */
 #define WACOM_PL_RES		20
@@ -1937,6 +1938,22 @@ static void wacom_wac_battery_usage_mapping(struct hid_device *hdev,
 	return;
 }

+static __s32 wacom_wac_battery_percentage(struct hid_field *field, __s32 value)
+{
+	s64 range = (s64)field->logical_maximum - field->logical_minimum;
+	s64 result;
+
+	if (range <= 0)
+		return 0;
+
+	result = div64_s64(((s64)value - field->logical_minimum) * 100,
+			   range);
+
+	return clamp_val(result, 0, 100);
+}
+
 static void wacom_wac_battery_event(struct hid_device *hdev, struct hid_field *field,
 		struct hid_usage *usage, __s32 value)
 {
@@ -1950,7 +1967,8 @@ static void wacom_wac_battery_event(struct hid_device *hdev, struct hid_field *f
 			wacom_wac->hid_data.bat_status = POWER_SUPPLY_STATUS_UNKNOWN;
 		}
 		else {
-			value = value * 100 / (field->logical_maximum - field->logical_minimum);
+			value = wacom_wac_battery_percentage(field, value);
 			wacom_wac->hid_data.battery_capacity = value;
 			wacom_wac->hid_data.bat_connected = 1;
 			wacom_wac->hid_data.bat_status = WACOM_POWER_SUPPLY_STATUS_AUTO;
@@ -1958,7 +1976,8 @@ static void wacom_wac_battery_event(struct hid_device *hdev, struct hid_field *f
 		wacom_wac->features.quirks |= WACOM_QUIRK_BATTERY;
 		break;
 	case WACOM_HID_WD_BATTERY_LEVEL:
-		value = value * 100 / (field->logical_maximum - field->logical_minimum);
+		value = wacom_wac_battery_percentage(field, value);
 		wacom_wac->hid_data.battery_capacity = value;
 		wacom_wac->hid_data.bat_connected = 1;
 		wacom_wac->hid_data.bat_status = WACOM_POWER_SUPPLY_STATUS_AUTO;
--
2.43.0