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

Kees Cook
2021-05-10T19:37:17Z

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@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@chromium.org>

--
Kees Cook