From: Dominique Martinet asmadeus@codewreck.org

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: a3be076dc174 ("net/9p/usbg: Add new usb gadget function transport") Cc: stable@vger.kernel.org Signed-off-by: Dominique Martinet asmadeus@codewreck.org --- Not actually tested, I'll try to find time to figure out how to run with qemu for real this time...

Changes in v2: - run through p9_client_cb() on error - Link to v1: https://lore.kernel.org/r/20250616132539.63434-1-danisjiang@gmail.com --- 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..43078e0d4ca3f4063660f659d28452c81bef10b4 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_sizel; + + 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,