[PATCH 1/1] HID: wacom: Fix OOB write in wacom_hid_set_device_mode()

[Lee Jones <lee@kernel.org>]

wacom_hid_set_device_mode() currently assumes that the HID_DG_INPUTMODE
usage is always located in the first field (field[0]) of the feature report.
However, a device can specify HID_DG_INPUTMODE in a different field.

If HID_DG_INPUTMODE is in a field other than the first one and the first
field has a report_count smaller than the usage_index of HID_DG_INPUTMODE,
this leads to an out-of-bounds write to r->field[0]->value.

Fix this by storing the field index of HID_DG_INPUTMODE in 'struct
hid_data' during feature mapping.  In wacom_hid_set_device_mode(), use
this stored field index to access the correct field and add bounds
checks to ensure both the field index and the value index are within
valid ranges before writing.

Cc: stable@vger.kernel.org
Fixes: 5ae6e89f7409 ("HID: wacom: implement the finger part of the HID generic handling")
Signed-off-by: Lee Jones <lee@kernel.org>
---
 drivers/hid/wacom_sys.c | 13 ++++++++++---
 drivers/hid/wacom_wac.h |  1 +
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/hid/wacom_sys.c b/drivers/hid/wacom_sys.c
index 1b1112772777..a6c5281afa06 100644
--- a/drivers/hid/wacom_sys.c
+++ b/drivers/hid/wacom_sys.c
@@ -341,6 +341,7 @@ static void wacom_feature_mapping(struct hid_device *hdev,
 
 		hid_data->inputmode = field->report->id;
 		hid_data->inputmode_index = usage->usage_index;
+		hid_data->inputmode_field_index = field->index;
 		break;
 
 	case HID_UP_DIGITIZER:
@@ -556,9 +557,14 @@ static int wacom_hid_set_device_mode(struct hid_device *hdev)
 
 	re = &(hdev->report_enum[HID_FEATURE_REPORT]);
 	r = re->report_id_hash[hid_data->inputmode];
-	if (r) {
-		r->field[0]->value[hid_data->inputmode_index] = 2;
-		hid_hw_request(hdev, r, HID_REQ_SET_REPORT);
+	if (r && hid_data->inputmode_field_index >= 0 &&
+	    hid_data->inputmode_field_index < r->maxfield) {
+		struct hid_field *field = r->field[hid_data->inputmode_field_index];
+
+		if (field && hid_data->inputmode_index < field->report_count) {
+			field->value[hid_data->inputmode_index] = 2;
+			hid_hw_request(hdev, r, HID_REQ_SET_REPORT);
+		}
 	}
 	return 0;
 }
@@ -2819,6 +2825,7 @@ static int wacom_probe(struct hid_device *hdev,
 		return error;
 
 	wacom_wac->hid_data.inputmode = -1;
+	wacom_wac->hid_data.inputmode_field_index = -1;
 	wacom_wac->mode_report = -1;
 
 	if (hid_is_usb(hdev)) {
diff --git a/drivers/hid/wacom_wac.h b/drivers/hid/wacom_wac.h
index c8803d5c6a71..b2e74d7ab3c4 100644
--- a/drivers/hid/wacom_wac.h
+++ b/drivers/hid/wacom_wac.h
@@ -298,6 +298,7 @@ struct wacom_shared {
 struct hid_data {
 	__s16 inputmode;	/* InputMode HID feature, -1 if non-existent */
 	__s16 inputmode_index;	/* InputMode HID feature index in the report */
+	__s16 inputmode_field_index; /* InputMode HID feature field index in the report */
 	bool sense_state;
 	bool inrange_state;
 	bool invert_state;
-- 
2.54.0.563.g4f69b47b94-goog