[PATCH v3] HID: wacom: fix OOB read in wacom_wac_pen_serial_enforce()

[Wei Jie LAW <98lawweijie@gmail.com>]

From: Wei Jie Law <98lawweijie@gmail.com>

wacom_wac_pen_serial_enforce() iterates over a field's usages (up to
field->maxusage) but indexes the report bits by j * report_size, the
position of value slot j -- and only the field->report_count value
slots reserved by the Report Count exist in the report:

	for (i = 0; i < report->maxfield; i++) {
		for (j = 0; j < report->field[i]->maxusage; j++) {
			...
			value = hid_field_extract(hdev, raw_data + 1,
						  offset + j * size, size);

hid_add_field() sizes the usage array with max(usage_index,
report_count), so a report descriptor can declare far more usages
than its Report Count reserves room for in the report.  One listing
12288 usages against Report Count 1 has the loop extract the usage
at index 12287 from bit offset 98296 -- about 12 KB past a 2-byte
received report.  The value is stored in wacom_wac->serial[0] and
can reach userspace as an MSC_SERIAL event, making this an
information disclosure.

Clamp the loop to field->report_count, the number of value slots the
report holds. Value slots past the last declared usage are still
scanned; they reuse that usage (HID 1.11, 6.2.2.8).

Verified on v6.12.105 with a UHID reproducer: a 2-byte report from
such a descriptor trips KASAN before the patch and not after it.

Fixes: 83417206427b ("HID: wacom: Queue events with missing type/serial data for later processing")
Suggested-by: Jason Gerecke <killertofu@gmail.com>
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5
Assisted-by: GLM:glm-5.3
Signed-off-by: Wei Jie Law <98lawweijie@gmail.com>
---
Changes in v3, per Jason Gerecke's review:

 - reworded the explanation: the usage array is sized for maxusage;
   the descriptor just declares more usages than its Report Count
   reserves room for in the report, and the loop trusted maxusage
   over report_count

 - bound inner loop to field->report_count

The reproducer is available on request.

Compile-tested on 6ba2c27cb9aa (x86_64, wacom_sys.o); the runtime
verification used the identical wacom_sys.c built as a module for
v6.12.105.

v2: https://lore.kernel.org/linux-input/20260825103104.12090-1-98lawweijie@gmail.com/
v1: https://lore.kernel.org/linux-input/20260822120926.153849-1-98lawweijie@gmail.com/

 drivers/hid/wacom_sys.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/hid/wacom_sys.c b/drivers/hid/wacom_sys.c
index 0eafa483b7f7..40770affdbde 100644
--- a/drivers/hid/wacom_sys.c
+++ b/drivers/hid/wacom_sys.c
@@ -113,8 +113,9 @@ static int wacom_wac_pen_serial_enforce(struct hid_device *hdev,
 
 	/* Queue events which have invalid tool type or serial number */
 	for (i = 0; i < report->maxfield; i++) {
-		for (j = 0; j < report->field[i]->maxusage; j++) {
-			struct hid_field *field = report->field[i];
+		struct hid_field *field = report->field[i];
+
+		for (j = 0; j < field->report_count; j++) {
 			struct hid_usage *usage = &field->usage[j];
 			unsigned int equivalent_usage = wacom_equivalent_usage(usage->hid);
 			unsigned int offset;
-- 
2.43.0