HID: wacom: fix OOB read in wacom_wac_pen_serial_enforce()

The 'wacom_wac_pen_serial_enforce()' function may calculate and pass an
invalid offset to hid_field_extract(), resulting in memory reads at
incorrect addresses -- possibly beyond the end of the report.  If a
field in the HID descriptor lists more usages than its Report Count
actually reserves space for, the function's inner 'j' will walk past
the end of the field:

	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);

A descriptor 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>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>

1 files 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;