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

[Yuhao Jiang <danisjiang@gmail.com>]

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>
Fixes: a3be076dc174 ("net/9p/usbg: Add new usb gadget function transport")
Cc: stable@vger.kernel.org
Signed-off-by: Yuhao Jiang <danisjiang@gmail.com>
---
 net/9p/trans_usbg.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/9p/trans_usbg.c b/net/9p/trans_usbg.c
index 6b694f117aef..047a2862fc84 100644
--- a/net/9p/trans_usbg.c
+++ b/net/9p/trans_usbg.c
@@ -242,6 +242,15 @@ static void usb9pfs_rx_complete(struct usb_ep *ep, struct usb_request *req)
 	if (!p9_rx_req)
 		return;
 
+	/* Validate actual received size against buffer capacity */
+	if (req->actual > p9_rx_req->rc.capacity) {
+		dev_err(&cdev->gadget->dev,
+			"received data size %u exceeds buffer capacity %zu\n",
+			req->actual, p9_rx_req->rc.capacity);
+		p9_req_put(usb9pfs->client, p9_rx_req);
+		return;
+	}
+
 	memcpy(p9_rx_req->rc.sdata, req->buf, req->actual);
 
 	p9_rx_req->rc.size = req->actual;
-- 
2.43.0