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

On Tue, Jun 17, 2025 at 7:00 AM asmadeus@codewreck.org wrote:

Yuhao Jiang wrote on Mon, Jun 16, 2025 at 09:25:39PM +0800:

...

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.

Thanks for this check!

Did you reproduce this or was this static analysis found? (to knowi if you tested wrt question below)

I found this by static analysis.

...

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);
  

I still haven't gotten around to setting up something to test this, and even less the error case, but I'm not sure a single put is enough -- p9_client_cb does another put. Conceptually I think it's better to mark the error and move on e.g. (not even compile tested)

        int status = REQ_STATUS_RCVD;

        [...]

        if (req->actual > p9_rx_req->rc.capacity) {
                dev_err(...)
                req->actual = 0;
                status = REQ_STATUS_ERROR;
        }

        memcpy(..)

        p9_rx_req->rc.size = req->actual;

        p9_client_cb(usb9pfs->client, p9_rx_req, status);
        p9_req_put(usb9pfs->client, p9_rx_req);

        complete(&usb9pfs->received);

(I'm not sure overriding req->actual is allowed, might be safer to use an intermediate variable like status instead)

What do you think?

Thanks,

Dominique Martinet | Asmadeus

Yes, I think your patch is better, my initial patch forgot p9_client_cb.

Thanks, Yuhao Jiang