[PATCH v4] HID: wacom: fix OOB read in wacom_wac_pen_serial_enforce()
From: Wei Jie LAW
Date: Tue Sep 08 2026 - 21:19:59 EST
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>
---
Changes in v4, per Jason Gerecke's review of v3:
- reworded the opening paragraph using Jason's suggested text;
- picked up Jason's Reviewed-by
- no functional change: the diff is byte-for-byte the v3 diff
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.
v3: https://lore.kernel.org/linux-input/20260828033323.82958-1-98lawweijie@xxxxxxxxx/
v2: https://lore.kernel.org/linux-input/20260825103104.12090-1-98lawweijie@xxxxxxxxx/
v1: https://lore.kernel.org/linux-input/20260822120926.153849-1-98lawweijie@xxxxxxxxx/
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