Re: [PATCH] serial: Lock console when calling into driver before registration

[John Ogness]

On 2024-02-22, Peter Collingbourne <pcc@xxxxxxxxxx> wrote:
> During the handoff from earlycon to the real console driver, we have
> two separate drivers operating on the same device concurrently. In the
> case of the 8250 driver these concurrent accesses cause problems due
> to the driver's use of banked registers, controlled by LCR.DLAB. It is
> possible for the setup(), config_port(), pm() and set_mctrl() callbacks
> to set DLAB, which can cause the earlycon code that intends to access
> TX to instead access DLL, leading to missed output and corruption on
> the serial line due to unintended modifications to the baud rate.
>
> In particular, for setup() we have:
>
> univ8250_console_setup()
> -> serial8250_console_setup()
> -> uart_set_options()
> -> serial8250_set_termios()
> -> serial8250_do_set_termios()
> -> serial8250_do_set_divisor()
>
> For config_port() we have:
>
> serial8250_config_port()
> -> autoconfig()
>
> For pm() we have:
>
> serial8250_pm()
> -> serial8250_do_pm()
> -> serial8250_set_sleep()
>
> For set_mctrl() we have (for some devices):
>
> serial8250_set_mctrl()
> -> omap8250_set_mctrl()
> -> __omap8250_set_mctrl()
>
> To avoid such problems, let's make it so that the console is locked
> during pre-registration calls to these callbacks, which will prevent
> the earlycon driver from running concurrently.

Even after the current atomic/threaded print rework is completed, the
console lock will be used to synchronize boot consoles with non-boot
consoles. I am fine with this solution.

Comments below...

> Remove the partial solution to this problem in the 8250 driver
> that locked the console only during autoconfig_irq(), as this would
> result in a deadlock with the new approach. The console continues
> to be locked during autoconfig_irq() because it can only be called
> through uart_configure_port().
>
> Although this patch introduces more locking than strictly necessary
> (and in particular it also locks during the call to rs485_config()
> which is not affected by this issue as far as I can tell), it follows
> the principle that it is the responsibility of the generic console
> code to manage the earlycon handoff by ensuring that earlycon and real
> console driver code cannot run concurrently, and not the individual
> drivers.
>
> Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx>
> Link: https://linux-review.googlesource.com/id/I7cf8124dcebf8618e6b2ee543fa5b25532de55d8
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  drivers/tty/serial/8250/8250_port.c |  6 ------
>  drivers/tty/serial/serial_core.c    | 10 ++++++++++
>  kernel/printk/printk.c              | 20 +++++++++++++++++---
>  3 files changed, 27 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 8ca061d3bbb9..1d65055dde27 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -1329,9 +1329,6 @@ static void autoconfig_irq(struct uart_8250_port *up)
>  		inb_p(ICP);
>  	}
>  
> -	if (uart_console(port))
> -		console_lock();
> -
>  	/* forget possible initially masked and pending IRQ */
>  	probe_irq_off(probe_irq_on());
>  	save_mcr = serial8250_in_MCR(up);
> @@ -1371,9 +1368,6 @@ static void autoconfig_irq(struct uart_8250_port *up)
>  	if (port->flags & UPF_FOURPORT)
>  		outb_p(save_ICP, ICP);
>  
> -	if (uart_console(port))
> -		console_unlock();
> -
>  	port->irq = (irq > 0) ? irq : 0;
>  }
>  
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index d6a58a9e072a..128aa0e0ae24 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -2608,7 +2608,11 @@ uart_configure_port(struct uart_driver *drv, struct uart_state *state,
>  			port->type = PORT_UNKNOWN;
>  			flags |= UART_CONFIG_TYPE;
>  		}

It would be nice to add a comment here mentioning why the console_lock
is taken. Even if it is something brief like:

     /* Sychronize with possible boot console. */

> +		if (uart_console(port))
> +			console_lock();
>  		port->ops->config_port(port, flags);
> +		if (uart_console(port))
> +			console_unlock();
>  	}
>  
>  	if (port->type != PORT_UNKNOWN) {
> @@ -2616,6 +2620,9 @@ uart_configure_port(struct uart_driver *drv, struct uart_state *state,
>  
>  		uart_report_port(drv, port);
>  

Also, here a brief comment.

> +		if (uart_console(port))
> +			console_lock();
> +
>  		/* Power up port for set_mctrl() */
>  		uart_change_pm(state, UART_PM_STATE_ON);
>  
> @@ -2632,6 +2639,9 @@ uart_configure_port(struct uart_driver *drv, struct uart_state *state,
>  
>  		uart_rs485_config(port);
>  
> +		if (uart_console(port))
> +			console_unlock();
> +
>  		/*
>  		 * If this driver supports console, and it hasn't been
>  		 * successfully registered yet, try to re-register it.
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index f2444b581e16..db69545e6250 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3263,6 +3263,20 @@ static int __init keep_bootcon_setup(char *str)
>  
>  early_param("keep_bootcon", keep_bootcon_setup);
>  

And here. Maybe slightly more verbose since there is an entire wrapper
function created for the purpose.

> +static int console_call_setup(struct console *newcon, char *options)
> +{
> +	int err;
> +
> +	if (!newcon->setup)
> +		return 0;
> +
> +	console_lock();
> +	err = newcon->setup(newcon, options);
> +	console_unlock();
> +
> +	return err;
> +}
> +
>  /*
>   * This is called by register_console() to try to match
>   * the newly registered console with any of the ones selected
> @@ -3298,8 +3312,8 @@ static int try_enable_preferred_console(struct console *newcon,
>  			if (_braille_register_console(newcon, c))
>  				return 0;
>  
> -			if (newcon->setup &&
> -			    (err = newcon->setup(newcon, c->options)) != 0)
> +			err = console_call_setup(newcon, c->options);
> +			if (err != 0)
>  				return err;
>  		}
>  		newcon->flags |= CON_ENABLED;
> @@ -3325,7 +3339,7 @@ static void try_enable_default_console(struct console *newcon)
>  	if (newcon->index < 0)
>  		newcon->index = 0;
>  
> -	if (newcon->setup && newcon->setup(newcon, NULL) != 0)
> +	if (console_call_setup(newcon, NULL) != 0)
>  		return;
>  
>  	newcon->flags |= CON_ENABLED;
> -- 
> 2.44.0.rc1.240.g4c46232300-goog

With comments added:

Reviewed-by: John Ogness <john.ogness@xxxxxxxxxxxxx>