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

[Zhou Qingyang]

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@xxxxxxx>
---
 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__);
+			return -ENOMEM;
+		}
+
 		list_add_tail(&trb->trb_list, &req->trb_list);
 		req->trb_head = trb;
 		req->trb_count = 1;
-- 
2.25.1