[PATCH v4] Input: elan_i2c - prevent division by zero and arithmetic underflow

[Ranjan Kumar <kumarranja@chromium.org>]

The Elan I2C touchpad driver queries the device for its physical
dimensions and trace counts to calculate the device resolution and width.
However, if the device firmware or device tree provides invalid zero
values for x_traces or y_traces, it results in a fatal division-by-zero
exception leading to a kernel panic during device probe.

Add checks to ensure these parameters are non-zero before performing
the division. If invalid trace values are detected, fall back to a safe
default of 1.

Additionally, prevent an arithmetic underflow in the touch reporting
logic. Previously, if the calculated or fallback width was smaller than
ETP_FWIDTH_REDUCE (90), the subtraction would underflow, resulting in a
massive unsigned integer being reported to userspace. Clamp the adjusted
width to a minimum of 0 to safely handle small physical dimensions and
fallback scenarios.

Completing the probe with safe fallback values ensures the sysfs nodes
are created, keeping the firmware update path intact so a recovery
firmware can be flashed to the device.

Fixes: 6696777c6506 ("Input: add driver for Elan I2C/SMbus touchpad")
Fixes: e3a9a1290688 ("Input: elan_i2c - do not query the info if they are provided")
Signed-off-by: Ranjan Kumar <kumarranja@chromium.org>
---
Changes in v4:
 - Reverted probe fallback width back to 1.
 - Added check in elan_report_contact() to clamp adjusted widths to a 
   minimum of 0, fixing the root cause of the arithmetic underflow.

Changes in v3:
 - Changed fallback width from 1 to ETP_FWIDTH_REDUCE to prevent underflow.

Changes in v2:
 - Added check for invalid trace values of 0 to prevent division by zero.

 drivers/input/mouse/elan_i2c_core.c | 36 ++++++++++++++++++++++++-----
 1 file changed, 30 insertions(+), 6 deletions(-)

diff --git a/drivers/input/mouse/elan_i2c_core.c b/drivers/input/mouse/elan_i2c_core.c
index 5cba02a156ce..edb0a28d25aa 100644
--- a/drivers/input/mouse/elan_i2c_core.c
+++ b/drivers/input/mouse/elan_i2c_core.c
@@ -428,8 +428,17 @@ static int elan_query_device_parameters(struct elan_tp_data *data)
 		if (error)
 			return error;
 	}
-	data->width_x = data->max_x / x_traces;
-	data->width_y = data->max_y / y_traces;
+
+	if (!x_traces || !y_traces) {
+		dev_warn(&client->dev,
+			 "invalid trace numbers: x=%u, y=%u\n",
+			 x_traces, y_traces);
+		data->width_x = 1;
+		data->width_y = 1;
+	} else {
+		data->width_x = data->max_x / x_traces;
+		data->width_y = data->max_y / y_traces;
+	}
 
 	if (device_property_read_u32(&client->dev,
 				     "touchscreen-x-mm", &x_mm) ||
@@ -443,8 +452,16 @@ static int elan_query_device_parameters(struct elan_tp_data *data)
 		data->x_res = elan_convert_resolution(hw_x_res, data->pattern);
 		data->y_res = elan_convert_resolution(hw_y_res, data->pattern);
 	} else {
-		data->x_res = (data->max_x + 1) / x_mm;
-		data->y_res = (data->max_y + 1) / y_mm;
+		if (unlikely(x_mm == 0 || y_mm == 0)) {
+			dev_warn(&client->dev,
+				 "invalid physical dimensions: x_mm=%u, y_mm=%u\n",
+				 x_mm, y_mm);
+			data->x_res = 1;
+			data->y_res = 1;
+		} else {
+			data->x_res = (data->max_x + 1) / x_mm;
+			data->y_res = (data->max_y + 1) / y_mm;
+		}
 	}
 
 	if (device_property_read_bool(&client->dev, "elan,clickpad"))
@@ -956,6 +973,7 @@ static void elan_report_contact(struct elan_tp_data *data, int contact_num,
 
 		if (data->report_features & ETP_FEATURE_REPORT_MK) {
 			unsigned int mk_x, mk_y, area_x, area_y;
+			int adj_width_x, adj_width_y;
 			u8 mk_data = high_precision ?
 				packet[ETP_MK_DATA_OFFSET + contact_num] :
 				finger_data[3];
@@ -967,8 +985,14 @@ static void elan_report_contact(struct elan_tp_data *data, int contact_num,
 			 * To avoid treating large finger as palm, let's reduce
 			 * the width x and y per trace.
 			 */
-			area_x = mk_x * (data->width_x - ETP_FWIDTH_REDUCE);
-			area_y = mk_y * (data->width_y - ETP_FWIDTH_REDUCE);
+
+			adj_width_x = data->width_x > ETP_FWIDTH_REDUCE ?
+					data->width_x - ETP_FWIDTH_REDUCE : 0;
+			adj_width_y = data->width_y > ETP_FWIDTH_REDUCE ?
+					data->width_y - ETP_FWIDTH_REDUCE : 0;
+
+			area_x = mk_x * adj_width_x;
+			area_y = mk_y * adj_width_y;
 
 			input_report_abs(input, ABS_TOOL_WIDTH, mk_x);
 			input_report_abs(input, ABS_MT_TOUCH_MAJOR,
-- 
2.54.0.1099.g489fc7bff1-goog