[PATCH] serial: core: fix suspicious security_locked_down() call

[Kees Cook]

On Fri, May 07, 2021 at 01:57:19PM +0200, Ondrej Mosnacek wrote:
> The commit that added this check did so in a very strange way - first
> security_locked_down() is called, its value stored into retval, and if
> it's nonzero, then an additional check is made for (change_irq ||
> change_port), and if this is true, the function returns. However, if
> the goto exit branch is not taken, the code keeps the retval value and
> continues executing the function. Then, depending on whether
> uport->ops->verify_port is set, the retval value may or may not be reset
> to zero and eventually the error value from security_locked_down() may
> abort the function a few lines below.
> 
> I will go out on a limb and assume that this isn't the intended behavior
> and that an error value from security_locked_down() was supposed to
> abort the function only in case (change_irq || change_port) is true.
> 
> Note that security_locked_down() should be called last in any series of
> checks, since the SELinux implementation of this hook will do a check
> against the policy and generate an audit record in case of denial. If
> the operation was to carry on after calling security_locked_down(), then
> the SELinux denial record would be bogus.
> 
> See commit 59438b46471a ("security,lockdown,selinux: implement SELinux
> lockdown") for how SELinux implements this hook.
> 
> Fixes: 794edf30ee6c ("lockdown: Lock down TIOCSSERIAL")
> Signed-off-by: Ondrej Mosnacek <omosnace at redhat.com>
> ---
>  drivers/tty/serial/serial_core.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index ba31e97d3d96..d7d8e7dbda60 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -865,9 +865,11 @@ static int uart_set_info(struct tty_struct *tty, struct tty_port *port,
>  		goto check_and_exit;
>  	}
>  
> -	retval = security_locked_down(LOCKDOWN_TIOCSSERIAL);
> -	if (retval && (change_irq || change_port))
> -		goto exit;
> +	if (change_irq || change_port) {
> +		retval = security_locked_down(LOCKDOWN_TIOCSSERIAL);
> +		if (retval)
> +			goto exit;
> +	}
>  
>  	/*
>  	 * Ask the low level driver to verify the settings.

Oops. Yeah, good catch -- I missed the kind of weird handling of retval
in this function when I originally reviewed it.

I think the goals of just covering IRQ/IO port changes originate from here:
https://lore.kernel.org/lkml/26173.1479769852@warthog.procyon.org.uk/

And I think the "Reported-by: Greg KH" originates from here:
https://lore.kernel.org/lkml/20161206071104.GA10292@kroah.com/

So, yes, I think your fix is correct.

Acked-by: Kees Cook <keescook at chromium.org>

-- 
Kees Cook