Re: [PATCH v4] HID: wacom: fix OOB read in wacom_wac_pen_serial_enforce()

[Jiri Kosina]

On Wed, 9 Sep 2026, Wei Jie LAW wrote:

> 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@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: Claude:claude-opus-5
> Assisted-by: GLM:glm-5.3
> Signed-off-by: Wei Jie Law <98lawweijie@xxxxxxxxx>
> Reviewed-by: Jason Gerecke <jason.gerecke@xxxxxxxxx>

Applied, thanks.

-- 
Jiri Kosina
SUSE Labs