Re: [PATCH] usb: gadget: mv_u3d: Fix a NULL pointer dereference in mv_u3d_req_to_trb()

[Johan Hovold <johan@kernel.org>]

On Wed, Dec 01, 2021 at 01:29:17AM +0800, Zhou Qingyang wrote:
> In mv_u3d_req_to_trb(), mv_u3d_build_trb_one() is assigned to trb and
> there is a dereference of it in mv_u3d_req_to_trb(), which could lead
> to a NULL pointer dereference on failure of mv_u3d_build_trb_one().
> 
> Fix this bug by adding a check of trb.
> 
> This bug was found by a static analyzer. The analysis employs
> differential checking to identify inconsistent security operations
> (e.g., checks or kfrees) between two code paths and confirms that the
> inconsistent operations are not recovered in the current function or
> the callers, so they constitute bugs.
> 
> Note that, as a bug found by static analysis, it can be a false
> positive or hard to trigger. Multiple researchers have cross-reviewed
> the bug.
> 
> Builds with CONFIG_USB_MV_U3D=m show no new warnings,
> and our static analyzer no longer warns about this code.
> 
> Fixes: 3d4eb9dfa3e8 ("usb: gadget: mv: Add USB 3.0 device driver for Marvell PXA2128 chip.")
> Signed-off-by: Zhou Qingyang <zhou1615@umn.edu>
> ---
>  drivers/usb/gadget/udc/mv_u3d_core.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/usb/gadget/udc/mv_u3d_core.c b/drivers/usb/gadget/udc/mv_u3d_core.c
> index a1057ddfbda3..e90ef4046a9d 100644
> --- a/drivers/usb/gadget/udc/mv_u3d_core.c
> +++ b/drivers/usb/gadget/udc/mv_u3d_core.c
> @@ -417,6 +417,12 @@ static int mv_u3d_req_to_trb(struct mv_u3d_req *req)
>  	 */
>  	if (length <= (unsigned)MV_U3D_EP_MAX_LENGTH_TRANSFER) {
>  		trb = mv_u3d_build_trb_one(req, &count, &dma);
> +		if (!trb) {
> +			dev_err(u3d->dev, "%s, mv_u3d_build_trb_one fail\n",
> +				__func__);

This looks correct, but the error message isn't needed as failures would
already have been logged by the helper (allocator).

> +			return -ENOMEM;
> +		}
> +
>  		list_add_tail(&trb->trb_list, &req->trb_list);
>  		req->trb_head = trb;
>  		req->trb_count = 1;

Johan