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 <[email protected]>
Closes: https://lkml.kernel.org/r/[email protected]
Fixes: a3be076 ("net/9p/usbg: Add new usb gadget function transport")
Cc: [email protected]
Message-ID: <[email protected]>
Signed-off-by: Dominique Martinet <[email protected]>

Author: martinetd

net/9p/trans_usbg.c

@@ -231,6 +231,8 @@ static void usb9pfs_rx_complete(struct usb_ep *ep, struct usb_request *req)
 	struct f_usb9pfs *usb9pfs = ep->driver_data;
 	struct usb_composite_dev *cdev = usb9pfs->function.config->cdev;
 	struct p9_req_t *p9_rx_req;
+	unsigned int req_size = req->actual;
+	int status = REQ_STATUS_RCVD;
 
 	if (req->status) {
 		dev_err(&cdev->gadget->dev, "%s usb9pfs complete --> %d, %d/%d\n",
@@ -242,11 +244,19 @@ static void usb9pfs_rx_complete(struct usb_ep *ep, struct usb_request *req)
 	if (!p9_rx_req)
 		return;
 
-	memcpy(p9_rx_req->rc.sdata, req->buf, req->actual);
+	if (req_size > p9_rx_req->rc.capacity) {
+		dev_err(&cdev->gadget->dev,
+			"%s received data size %u exceeds buffer capacity %zu\n",
+			ep->name, req_size, p9_rx_req->rc.capacity);
+		req_size = 0;
+		status = REQ_STATUS_ERROR;
+	}
+
+	memcpy(p9_rx_req->rc.sdata, req->buf, req_size);
 
-	p9_rx_req->rc.size = req->actual;
+	p9_rx_req->rc.size = req_size;
 
-	p9_client_cb(usb9pfs->client, p9_rx_req, REQ_STATUS_RCVD);
+	p9_client_cb(usb9pfs->client, p9_rx_req, status);
 	p9_req_put(usb9pfs->client, p9_rx_req);
 
 	complete(&usb9pfs->received);