[PATCH] usb: gadget: udc: Fix NULL pointer dereference in gadget_match_driver

[Jimmy Hu <hhhuuu@google.com>]

A NULL pointer dereference occurs in gadget_match_driver() because a
race condition exists between the DRD mode-switch work and the
configfs UDC write path:

1. The DRD mode-switch work invokes __dwc3_set_mode(), which calls
   dwc3_gadget_exit() and subsequently frees the UDC device name via
   device_unregister(&udc->dev).
2. The configfs UDC write path invokes gadget_dev_desc_UDC_store(),
   which calls usb_gadget_register_driver() and subsequently
   compares the UDC device name via gadget_match_driver().

If gadget_match_driver() runs concurrently during UDC unregistration, it
may access the freed UDC device name. Once the freed memory is zeroed,
dev_name(&udc->dev) returns NULL, causing a panic in strcmp().

[39430.908615][ T1171] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
[39430.911397][ T1171] pc : __pi_strcmp+0x20/0x140
[39430.911441][ T1171] lr : gadget_match_driver+0x34/0x60
...
[39430.911890][ T1171]  usb_gadget_register_driver_owner+0x50/0xf8
[39430.911910][ T1171]  gadget_dev_desc_UDC_store+0xf4/0x140
[39430.931308][ T1171]  configfs_write_iter+0xec/0x134
...
[39430.957058][ T1171] Workqueue: events_freezable __dwc3_set_mode
[39430.957287][ T1171]  dwc3_gadget_exit+0x34/0x8c
[39430.957304][ T1171]  __dwc3_set_mode+0xc0/0x664
[39430.957341][ T1171]  worker_thread+0x244/0x334

Fix this by checking dev_name(&udc->dev) before calling strcmp().

Fixes: fc274c1e9973 ("USB: gadget: Add a new bus for gadgets")
Cc: stable@vger.kernel.org
Signed-off-by: Jimmy Hu <hhhuuu@google.com>
---
 drivers/usb/gadget/udc/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/gadget/udc/core.c b/drivers/usb/gadget/udc/core.c
index e8861eaad907..79baed640428 100644
--- a/drivers/usb/gadget/udc/core.c
+++ b/drivers/usb/gadget/udc/core.c
@@ -1594,7 +1594,7 @@ static int gadget_match_driver(struct device *dev, const struct device_driver *d
 			struct usb_gadget_driver, driver);
 
 	/* If the driver specifies a udc_name, it must match the UDC's name */
-	if (driver->udc_name &&
+	if (driver->udc_name && dev_name(&udc->dev) &&
 			strcmp(driver->udc_name, dev_name(&udc->dev)) != 0)
 		return 0;
 

base-commit: 5d6919055dec134de3c40167a490f33c74c12581
-- 
2.54.0.746.g67dd491aae-goog