[PATCH] landlock: Warn once if a Landlock action is requested while disabled

[Mickaël Salaün]

On Mon, Feb 19, 2024 at 01:07:48PM -0800, Kees Cook wrote:
> On Mon, Feb 19, 2024 at 08:18:04PM +0100, Mickaël Salaün wrote:
> > Because sandboxing can be used as an opportunistic security measure,
> > user space may not log unsupported features.  Let the system
> > administrator know if an application tries to use Landlock but failed
> > because it isn't enabled at boot time.  This may be caused by bootloader
> > configurations with outdated "lsm" kernel's command-line parameter.
> > 
> > Cc: Günther Noack <gnoack at google.com>
> > Cc: stable at vger.kernel.org
> > Fixes: 265885daf3e5 ("landlock: Add syscall implementations")
> > Signed-off-by: Mickaël Salaün <mic at digikod.net>
> > ---
> >  security/landlock/syscalls.c | 18 +++++++++++++++---
> >  1 file changed, 15 insertions(+), 3 deletions(-)
> > 
> > diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
> > index f0bc50003b46..b5b424819dee 100644
> > --- a/security/landlock/syscalls.c
> > +++ b/security/landlock/syscalls.c
> > @@ -33,6 +33,18 @@
> >  #include "ruleset.h"
> >  #include "setup.h"
> >  
> > +static bool is_not_initialized(void)
> > +{
> > +	if (likely(landlock_initialized))
> > +		return false;
> > +
> > +	pr_warn_once(
> > +		"Disabled but requested by user space. "
> > +		"You should enable Landlock at boot time: "
> > +		"https://docs.kernel.org/userspace-api/landlock.html#kernel-support\n");
> 
> Perhaps update this docs to be really explicit with a example, maybe...
> 
> If `landlock` is not present in `CONFIG_LSM`, you can add it. For
> example, if this was the current config::
> 
>   $ zgrep -h ^CONFIG_LSM= /boot/config-$(uname -r) /proc/config.gz 2>/dev/null
>   CONFIG_LSM="lockdown,yama,integrity,apparmor"
> 
> You can boot with::
> 
>   lsm=landlock,lockdown,yama,integrity,apparmor
> 

Indeed, I'll send a dedicated patch and update the link accordingly.

> 
> I *still* wish we had the "+" operator for lsm=. It would be SO much
> easier to say "boot with lsm=+landlock". *shrug*

I guess it's still possible and it would need to be backported to be
more useful.

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