[PATCH] USB: usbmon: fix null-ptr-deref in mon_bus_remove() when mon_bus_init() fails

[Jiangong . Han]

When mon_bus_init() fails to allocate the mon_bus struct, it returns
without setting ubus->mon_bus, so mbus->mon_bus remains NULL.
mon_bus_add() intentionally ignores this failure because the USB core
does not require monitoring to be established for normal operation.
However, when the HCD is later removed, usb_remove_hcd() fires the
USB_BUS_REMOVE notifier which calls mon_bus_remove(), it dereferences
ubus->mon_bus without checking for NULL, causing a null-ptr deference.

Fix this by adding a NULL check for mbus at the top of mon_bus_remove().
If mon_bus_init() failed, ubus->mon_bus is NULL, meaning the bus was
never tracked by the monitor, so there is nothing to clean up and
returning early is correct.

Reported-by: syzbot+5f4b29d9fe710a964482@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=5f4b29d9fe710a964482
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Jiangong.Han <jiangong.han@xxxxxxxxxxxxx>
---
 drivers/usb/mon/mon_main.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/usb/mon/mon_main.c b/drivers/usb/mon/mon_main.c
index e4fff194fea0..600c9de84c7e 100644
--- a/drivers/usb/mon/mon_main.c
+++ b/drivers/usb/mon/mon_main.c
@@ -199,6 +199,9 @@ static void mon_bus_remove(struct usb_bus *ubus)
 {
 	struct mon_bus *mbus = ubus->mon_bus;
 
+	if (mbus == NULL)
+		return;
+
 	mutex_lock(&mon_lock);
 	list_del(&mbus->bus_link);
 	if (mbus->text_inited)
-- 
2.37.3