[PATCH v2] usb: gadget: core: Check for unset descriptor

[crwulff@gmail.com]

From: Chris Wulff <crwulff@gmail.com>

Make sure the descriptor has been set before looking at maxpacket.
This fixes a null pointer panic in this case.

This may happen if the gadget doesn't properly set up the endpoint
for the current speed, or the gadget descriptors are malformed and
the descriptor for the speed/endpoint are not found.

No current gadget driver is known to have this problem, but this
may cause a hard-to-find bug during development of new gadgets.

Fixes: 54f83b8c8ea9 ("USB: gadget: Reject endpoints with 0 maxpacket value")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Wulff <crwulff@gmail.com>
---
v2: Added WARN_ONCE message & clarification on causes
v1: https://lore.kernel.org/all/20240721192048.3530097-2-crwulff@gmail.com/
---
 drivers/usb/gadget/udc/core.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/drivers/usb/gadget/udc/core.c b/drivers/usb/gadget/udc/core.c
index 2dfae7a17b3f..81f9140f3681 100644
--- a/drivers/usb/gadget/udc/core.c
+++ b/drivers/usb/gadget/udc/core.c
@@ -118,12 +118,10 @@ int usb_ep_enable(struct usb_ep *ep)
 		goto out;
 
 	/* UDC drivers can't handle endpoints with maxpacket size 0 */
-	if (usb_endpoint_maxp(ep->desc) == 0) {
-		/*
-		 * We should log an error message here, but we can't call
-		 * dev_err() because there's no way to find the gadget
-		 * given only ep.
-		 */
+	if (!ep->desc || usb_endpoint_maxp(ep->desc) == 0) {
+		WARN_ONCE(1, "%s: ep%d (%s) has %s\n", __func__, ep->address, ep->name,
+			  (!ep->desc) ? "NULL descriptor" : "maxpacket 0");
+
 		ret = -EINVAL;
 		goto out;
 	}
-- 
2.43.0