[PATCH v3] net/9p: Fix buffer overflow in USB transport layer

[Dominique Martinet via B4 Relay]

From: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>

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@xxxxxxxxx>
Closes: https://lkml.kernel.org/r/20250616132539.63434-1-danisjiang@xxxxxxxxx
Fixes: a3be076dc174 ("net/9p/usbg: Add new usb gadget function transport")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
---
(still not actually tested, can't get dummy_hcd/gt to create a device
listed by p9_fwd.py/useable in qemu, I give up..)

Changes in v3:
- fix typo s/req_sizel/req_size/ -- sorry for that, module wasn't
  built...
- Link to v2: https://lore.kernel.org/r/20250620-9p-usb_overflow-v2-1-026c6109c7a1@xxxxxxxxxxxxx

Changes in v2:
- run through p9_client_cb() on error
- Link to v1: https://lore.kernel.org/r/20250616132539.63434-1-danisjiang@xxxxxxxxx
---
 net/9p/trans_usbg.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/net/9p/trans_usbg.c b/net/9p/trans_usbg.c
index 6b694f117aef296a66419fed5252305e7a1d0936..468f7e8f0277b9ae5f1bb3c94c649fca97d28857 100644
--- a/net/9p/trans_usbg.c
+++ b/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;
+	}
 
-	p9_rx_req->rc.size = req->actual;
+	memcpy(p9_rx_req->rc.sdata, req->buf, req_size);
 
-	p9_client_cb(usb9pfs->client, p9_rx_req, REQ_STATUS_RCVD);
+	p9_rx_req->rc.size = req_size;
+
+	p9_client_cb(usb9pfs->client, p9_rx_req, status);
 	p9_req_put(usb9pfs->client, p9_rx_req);
 
 	complete(&usb9pfs->received);

---
base-commit: 74b4cc9b8780bfe8a3992c9ac0033bf22ac01f19
change-id: 20250620-9p-usb_overflow-25bfc5e9bef3

Best regards,
-- 
Dominique Martinet <asmadeus@xxxxxxxxxxxxx>