[PATCH 1/3] xhci: dbgtty: Fix unregister on tty_register_driver() failure

[Lucas De Marchi]

If tty_register_driver() fails, it drops the reference, but fails to set
the global dbc_tty_driver to NULL, causing the unregister to be called
again when module exits.

On module unload dbc_tty_exit() only gates its cleanup on the driver
pointer being non-NULL, so it operates on the already-freed driver:

    module_init(xhci_hcd_init)
      xhci_hcd_init()
        xhci_dbc_init()                       [return value ignored]
          dbc_tty_init()
            tty_register_driver() fails
              tty_driver_kref_put()           -> driver freed
              (dbc_tty_driver left dangling)
    ...
    module_exit(xhci_hcd_fini)
      xhci_hcd_fini()
        xhci_dbc_exit()
          dbc_tty_exit()
            if (dbc_tty_driver)               -> true (dangling)
              tty_unregister_driver()         -> use-after-free

Fixes: 4521f1613940 ("xhci: dbctty: split dbc tty driver registration and unregistration functions.")
Cc: stable@xxxxxxxxxxxxxxx # v5.10
Cc: Mathias Nyman <mathias.nyman@xxxxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Lucas De Marchi <ldemarchi@xxxxxxxxxx>
---
 drivers/usb/host/xhci-dbgtty.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/usb/host/xhci-dbgtty.c b/drivers/usb/host/xhci-dbgtty.c
index 2e7384c6b6ec..01decd46f313 100644
--- a/drivers/usb/host/xhci-dbgtty.c
+++ b/drivers/usb/host/xhci-dbgtty.c
@@ -651,6 +651,7 @@ int dbc_tty_init(void)
 		pr_err("Can't register dbc tty driver\n");
 		tty_driver_kref_put(dbc_tty_driver);
 		idr_destroy(&dbc_tty_minors);
+		dbc_tty_driver = NULL;
 	}
 
 	return ret;
-- 
2.54.0