Re: [PATCH] serial: kgdboc: Fix NMI-safety problems from keyboard reset code

[Doug Anderson]

Hi,

On Fri, Apr 19, 2024 at 3:30 AM Daniel Thompson
<daniel.thompson@xxxxxxxxxx> wrote:
>
> Currently, when kdb is compiled with keyboard support, then we will use
> schedule_work() to provoke reset of the keyboard status.  Unfortunately
> schedule_work() gets called from the kgdboc post-debug-exception
> handler.  That risks deadlock since schedule_work() is not NMI-safe and,
> even on platforms where the NMI is not directly used for debugging, the
> debug trap can have NMI-like behaviour depending on where breakpoints
> are placed.
>
> Fix this by using the irq work system, which is NMI-safe, to defer the
> call to schedule_work() to a point when it is safe to call.
>
> Reported-by: Liuye <liu.yeC@xxxxxxx>
> Closes: https://lore.kernel.org/all/20240228025602.3087748-1-liu.yeC@xxxxxxx/
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
> ---
>  drivers/tty/serial/kgdboc.c | 30 +++++++++++++++++++++++++++++-
>  1 file changed, 29 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
> index 7ce7bb1640054..adcea70fd7507 100644
> --- a/drivers/tty/serial/kgdboc.c
> +++ b/drivers/tty/serial/kgdboc.c
> @@ -19,6 +19,7 @@
>  #include <linux/console.h>
>  #include <linux/vt_kern.h>
>  #include <linux/input.h>
> +#include <linux/irq_work.h>
>  #include <linux/module.h>
>  #include <linux/platform_device.h>
>  #include <linux/serial_core.h>
> @@ -48,6 +49,25 @@ static struct kgdb_io                kgdboc_earlycon_io_ops;
>  static int                      (*earlycon_orig_exit)(struct console *con);
>  #endif /* IS_BUILTIN(CONFIG_KGDB_SERIAL_CONSOLE) */
>
> +/*
> + * When we leave the debug trap handler we need to reset the keyboard status
> + * (since the original keyboard state gets partially clobbered by kdb use of
> + * the keyboard).
> + *
> + * The path to deliver the reset is somewhat circuitous.
> + *
> + * To deliver the reset we register an input handler, reset the keyboard and
> + * then deregister the input handler. However, to get this done right, we do
> + * have to carefully manage the calling context because we can only register
> + * input handlers from task context.
> + *
> + * In particular we need to trigger the action from the debug trap handler with
> + * all its NMI and/or NMI-like oddities. To solve this the kgdboc trap exit code
> + * (the "post_exception" callback) uses irq_work_queue(), which is NMI-safe, to
> + * schedule a callback from a hardirq context. From there we have to defer the
> + * work again, this time using schedule_Work(), to get a callback using the

nit: schedule_work() (no capital "W").

> + * system workqueue, which runs in task context.

Thank you for the comment. It makes the double-jump through IRQ work
and then normal work clearer.


Other than the nit in the comment, this looks good to me.

Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx>