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

[Günther Noack]

Hello!

I think this is a good idea.
Some minor implementation remarks below.

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 configurations with outdated "lsm" kernel's command-line parameter.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;

Optional stylistic remark; I try to avoid predicate functions which
have a "negated" meaning, because double negations are slightly more
error prone.  (We return false here, so Landlock is not not
initialized.)

> +
> +	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");
> +	return true;
> +}
> +
>  /**
>   * copy_min_struct_from_user - Safe future-proof argument copying
>   *
> @@ -173,7 +185,7 @@ SYSCALL_DEFINE3(landlock_create_ruleset,
>  	/* Build-time checks. */
>  	build_check_abi();
>  
> -	if (!landlock_initialized)
> +	if (is_not_initialized())
>  		return -EOPNOTSUPP;

Technically, any Landlock user needs to go through the
landlock_create_ruleset() system call anyway; it might be enough to
just add it in that place and leave the other system calls as they
were.  Then you could also omit the special function.

Reviewed-by: Günther Noack <gnoack3000 at gmail.com>

–Günther