Re: [PATCH] tty: serial: kgdboc: fix mutex locking order for configure_kgdboc()

[Petr Mladek]

On Wed 2023-01-11 15:57:10, John Ogness wrote:
> Several mutexes are taken while setting up console serial ports. In
> particular, the tty_port->mutex and @console_mutex are taken:
> 
>   serial_pnp_probe
>     serial8250_register_8250_port
>       uart_add_one_port (locks tty_port->mutex)
>         uart_configure_port
>           register_console (locks @console_mutex)
> 
> In order to synchronize kgdb's tty_find_polling_driver() with
> register_console(), commit 6193bc90849a ("tty: serial: kgdboc:
> synchronize tty_find_polling_driver() and register_console()") takes
> the @console_mutex. However, this leads to the following call chain
> (with locking):
> 
>   platform_probe
>     kgdboc_probe
>       configure_kgdboc (locks @console_mutex)
>         tty_find_polling_driver
>           uart_poll_init (locks tty_port->mutex)
>             uart_set_options
> 
> This is clearly deadlock potential due to the reverse lock ordering.

Great catch!

> Since uart_set_options() requires holding @console_mutex in order to
> serialize early initialization of the serial-console lock, take the
> @console_mutex in uart_poll_init() instead of configure_kgdboc().
> 
> Since configure_kgdboc() was using @console_mutex for safe traversal
> of the console list, change it to use the SRCU iterator instead.
> 
> Add comments to uart_set_options() kerneldoc mentioning that it
> requires holding @console_mutex (aka the console_list_lock).
> 
> Fixes: 6193bc90849a ("tty: serial: kgdboc: synchronize tty_find_polling_driver() and register_console()")
> Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>

> --- a/drivers/tty/serial/kgdboc.c
> +++ b/drivers/tty/serial/kgdboc.c
> @@ -189,15 +190,6 @@ static int configure_kgdboc(void)
>  	if (kgdboc_register_kbd(&cptr))
>  		goto do_register;
>  
> -	/*
> -	 * tty_find_polling_driver() can call uart_set_options()
> -	 * (via poll_init) to configure the uart. Take the console_list_lock
> -	 * in order to synchronize against register_console(), which can also
> -	 * configure the uart via uart_set_options(). This also allows safe
> -	 * traversal of the console list.
> -	 */
> -	console_list_lock();
> -
>  	p = tty_find_polling_driver(cptr, &tty_line);
>  	if (!p) {
>  		console_list_unlock();

This console_list_unlock() should be removed here as well.

Best Regards,
Petr

PS: I still have to double check all the dependencies. I think that I need
to do it with a fresh head tomorrow.