USB: serial: belkin_sa: validate interrupt status length

USB: serial: belkin_sa: validate interrupt status length

The Belkin interrupt callback treats interrupt data as a four-byte
status report and reads LSR/MSR fields at offsets 2 and 3. The
interrupt-in buffer length is derived from endpoint wMaxPacketSize, and
short interrupt transfers may complete successfully with a smaller
actual_length.

Check the completed interrupt packet length before parsing status
fields so short interrupt endpoints and short successful packets are
ignored instead of causing out-of-bounds or stale status-byte reads.

KASAN report as below:

BUG: KASAN: slab-out-of-bounds in belkin_sa_read_int_callback()
Read of size 1
Call trace:
  belkin_sa_read_int_callback() (drivers/usb/serial/belkin_sa.c:202)
  __usb_hcd_giveback_urb() (drivers/usb/core/hcd.c:1630)
  dummy_timer() (?:?)

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Assisted-by: Codex:gpt-5.5
Signed-off-by: Zhang Cen <rollkingzzc@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>