Input: touchwin - reset the packet index on every complete packet

commit 478cdd7 upstream.

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: 11ea317 ("Input: add driver for Touchwin serial touchscreens")
Cc: stable@vger.kernel.org
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Link: https://patch.msgid.link/20260613-b4-disp-69921bfd-v1-1-82c036899959@proton.me
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: bryamzxz

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 */