[PATCH v2] usb: gadget: mv_u3d: Fix a NULL pointer dereference in mv_u3d_req_to_trb()

[Zhou Qingyang <zhou1615@umn.edu>]

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.

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>
---
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.

Changes in v2:
  -  Remove unnecessary error message

 drivers/usb/gadget/udc/mv_u3d_core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/usb/gadget/udc/mv_u3d_core.c b/drivers/usb/gadget/udc/mv_u3d_core.c
index a1057ddfbda3..4573233f2835 100644
--- a/drivers/usb/gadget/udc/mv_u3d_core.c
+++ b/drivers/usb/gadget/udc/mv_u3d_core.c
@@ -417,6 +417,9 @@ 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)
+			return -ENOMEM;
+
 		list_add_tail(&trb->trb_list, &req->trb_list);
 		req->trb_head = trb;
 		req->trb_count = 1;
-- 
2.25.1