Re: [RFC PATCH v1 06/11] landlock: Enforce capability restrictions

["Günther Noack" <gnoack3000@gmail.com>]

Hello!

On Thu, Mar 12, 2026 at 11:04:39AM +0100, Mickaël Salaün wrote:
> diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
> index 152d952e98f6..38a4bf92781a 100644
> --- a/security/landlock/syscalls.c
> +++ b/security/landlock/syscalls.c
> [...]

> +	/*
> +	 * Stores only the capabilities this kernel knows about.
> +	 * Unknown bits are silently accepted for forward compatibility:
> +	 * user space compiled against newer headers can pass new
> +	 * CAP_* bits without getting EINVAL on older kernels.
> +	 * Unknown bits have no effect because no hook checks them.
> +	 */
> +	mutex_lock(&ruleset->lock);
> +	ruleset->layers[0].allowed.caps |=
> +		landlock_caps_to_bits(cap_attr.capabilities & CAP_VALID_MASK);
> +	mutex_unlock(&ruleset->lock);

See the example in the documentation patch set [1]; I think it can be
an incompatibility if we ignore the unknown bits here (and I don't
know of a scenario where it would be a problem to reject them).

[1] https://lore.kernel.org/all/20260422.5a7059c06fb0@gnoack.org/

–Günther