[PATCH] Input: touchwin - reset the packet index on every complete packet

[Bryam Vargas via B4 Relay]

From: Bryam Vargas <hexlabsecurity@xxxxxxxxx>

tw_interrupt() accumulates each non-zero serial byte into a fixed
three-byte buffer with a running index that is only reset once a full
packet has been received *and* the device's two Y bytes agree:

	tw->data[tw->idx++] = data;
	if (tw->idx == TW_LENGTH && tw->data[1] == tw->data[2]) {
		...
		tw->idx = 0;
	}

The reset is gated on tw->data[1] == tw->data[2], a value the device
controls.  A malicious, malfunctioning or counterfeit Touchwindow
peripheral can stream non-zero bytes whose 2nd and 3rd bytes differ: the
index reaches TW_LENGTH without the equality holding, is never reset, and
keeps growing, so tw->data[tw->idx++] walks off the end of the three-byte
array and the rest of the heap-allocated struct tw, one attacker-chosen
byte at a time -- an unbounded, device-driven heap out-of-bounds write.

Reset the index on every completed packet and report an event only when
the two Y bytes match, like the other serio touchscreen drivers do.

Fixes: 11ea3173d5f2 ("Input: add driver for Touchwin serial touchscreens")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Bryam Vargas <hexlabsecurity@xxxxxxxxx>
---
Reachable on every serial byte once the device is bound as SERIO_TOUCHWIN
(inputattach --touchwin): serport_ldisc_receive -> serio_interrupt ->
tw_interrupt, with no length validation on the path.  The peripheral need
only avoid zero bytes (any data != 0) and send a 2nd byte different from
the 3rd to defeat the reset.

struct tw is a 64-byte kmalloc-64 object; the unbounded store crosses the
allocation once the running index reaches 40.

Verified with a faithful in-kernel KASAN litmus (the verbatim struct tw
and the tw_interrupt accumulation), CONFIG_KASAN=y on x86_64:

  Arm A, 64 non-zero bytes with byte[1] != byte[2]:
    BUG: KASAN: slab-out-of-bounds in ... Write of size 1
    located 0 bytes to the right of the allocated 64-byte region
    ... cache kmalloc-64 of size 64
  Arm B, with this patch (index reset every packet): clean
  Arm C, benign device (byte[1] == byte[2]): clean

  AddressSanitizer (x86_64 and i386): heap-buffer-overflow WRITE, both ABIs.

Reproducer and full logs available on request.
---
 drivers/input/touchscreen/touchwin.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/drivers/input/touchscreen/touchwin.c b/drivers/input/touchscreen/touchwin.c
index 099fd88e65d8..3ed33378dfcd 100644
--- a/drivers/input/touchscreen/touchwin.c
+++ b/drivers/input/touchscreen/touchwin.c
@@ -63,12 +63,15 @@ static irqreturn_t tw_interrupt(struct serio *serio,
 	if (data) {		/* touch */
 		tw->touched = 1;
 		tw->data[tw->idx++] = data;
-		/* verify length and that the two Y's are the same */
-		if (tw->idx == TW_LENGTH && tw->data[1] == tw->data[2]) {
-			input_report_abs(dev, ABS_X, tw->data[0]);
-			input_report_abs(dev, ABS_Y, tw->data[1]);
-			input_report_key(dev, BTN_TOUCH, 1);
-			input_sync(dev);
+		/* a full packet ends the accumulation, valid or not */
+		if (tw->idx == TW_LENGTH) {
+			/* report only if the two Y's are the same */
+			if (tw->data[1] == tw->data[2]) {
+				input_report_abs(dev, ABS_X, tw->data[0]);
+				input_report_abs(dev, ABS_Y, tw->data[1]);
+				input_report_key(dev, BTN_TOUCH, 1);
+				input_sync(dev);
+			}
 			tw->idx = 0;
 		}
 	} else if (tw->touched) {	/* untouch */

---
base-commit: 8e65320d91cdc3b241d4b94855c88459b91abf66
change-id: 20260613-b4-disp-69921bfd-051063dda4b7

Best regards,
-- 
Bryam Vargas <hexlabsecurity@xxxxxxxxx>