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

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

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@gmail.com>
Fixes: 5197424cdccc ("Input: usbtouchscreen - add NEXIO (or iNexio) support")
Cc: stable <stable@kernel.org>
Assisted-by: gkh_clanker_t1000
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://patch.msgid.link/2026042026-chlorine-epidermis-fd6d@gregkh
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>