net/9p: Fix buffer overflow in USB transport layer

A buffer overflow vulnerability exists in the USB 9pfs transport layer
where inconsistent size validation between packet header parsing and
actual data copying allows a malicious USB host to overflow heap buffers.

The issue occurs because:
- usb9pfs_rx_header() validates only the declared size in packet header
- usb9pfs_rx_complete() uses req->actual (actual received bytes) for
memcpy

This allows an attacker to craft packets with small declared size
(bypassing validation) but large actual payload (triggering overflow
in memcpy).

Add validation in usb9pfs_rx_complete() to ensure req->actual does not
exceed the buffer capacity before copying data.

Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Closes: https://lkml.kernel.org/r/20250616132539.63434-1-danisjiang@gmail.com


Fixes: a3be076d ("net/9p/usbg: Add new usb gadget function transport")
Cc: stable@vger.kernel.org
Message-ID: <20250622-9p-usb_overflow-v3-1-ab172691b946@codewreck.org>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>