[PATCH] Input: usbtouchscreen - clamp NEXIO data_len/x_len to URB buffer size

[Greg Kroah-Hartman]

nexio_read_data() pulls data_len and x_len from a packed __be16 header
in the device's interrupt packet and then walks packet->data[0..x_len)
and packet->data[x_len..data_len) comparing each byte against a
threshold.

Both fields are 16-bit on the wire (max 65535).  The existing
adjustments shave at most 0x100 / 0x80 off, so the loop bound can still
reach roughly 0xfeff.  The URB transfer buffer for NEXIO is rept_size
(1024) bytes from usb_alloc_coherent(), with the first 7 occupied by the
packed header — so packet->data[] has 1017 valid bytes.  read_data()
callbacks are not given urb->actual_length, and nothing else bounds the
walk.

A device that lies about its length can get a ~64 KiB out-of-bounds read
past the coherent DMA allocation.  The first index whose byte exceeds
NEXIO_THRESHOLD lands in begin_x / begin_y and from there into the
reported touch coordinates, so adjacent kernel memory contents leak to
userspace as ABS_X / ABS_Y events.  Far enough out, the read can also
hit an unmapped page and fault.

Fix this all by clamping data_len to the buffer's data[] capacity and
x_len to data_len.

Cc: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
Fixes: 5197424cdccc ("Input: usbtouchscreen - add NEXIO (or iNexio) support")
Cc: stable <stable@xxxxxxxxxx>
Assisted-by: gkh_clanker_t1000
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/input/touchscreen/usbtouchscreen.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/input/touchscreen/usbtouchscreen.c b/drivers/input/touchscreen/usbtouchscreen.c
index 657555c8796c..866d4a7fbe42 100644
--- a/drivers/input/touchscreen/usbtouchscreen.c
+++ b/drivers/input/touchscreen/usbtouchscreen.c
@@ -1070,6 +1070,11 @@ static int nexio_read_data(struct usbtouch_usb *usbtouch, unsigned char *pkt)
 	if (x_len > 0xff)
 		x_len -= 0x80;
 
+	if (data_len > usbtouch->data_size - sizeof(*packet))
+		data_len = usbtouch->data_size - sizeof(*packet);
+	if (x_len > data_len)
+		x_len = data_len;
+
 	/* send ACK */
 	ret = usb_submit_urb(priv->ack, GFP_ATOMIC);
 	if (ret)
-- 
2.53.0