[PATCH v4 2/5] security: Count the LSMs enabled at compile time

[KP Singh]

On Fri, Sep 22, 2023 at 5:50 PM Kees Cook <keescook at chromium.org> wrote:
>
> On Fri, Sep 22, 2023 at 04:55:02PM +0200, KP Singh wrote:
> > These macros are a clever trick to determine a count of the number of
> > LSMs that are enabled in the config to ascertain the maximum number of
> > static calls that need to be configured per LSM hook.
> >
> > Without this one would need to generate static calls for the total
> > number of LSMs in the kernel (even if they are not compiled) times the
> > number of LSM hooks which ends up being quite wasteful.
> >
> > Suggested-by: Kui-Feng Lee <sinquersw at gmail.com>
> > Suggested-by: Andrii Nakryiko <andrii at kernel.org>
> > Acked-by: Song Liu <song at kernel.org>
> > Signed-off-by: KP Singh <kpsingh at kernel.org>
>
> Thought below, but regardless of result:
>
> Reviewed-by: Kees Cook <keescook at chromium.org>
>
>
> > ---
> >  include/linux/lsm_count.h | 107 ++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 107 insertions(+)
> >  create mode 100644 include/linux/lsm_count.h
> >
> > diff --git a/include/linux/lsm_count.h b/include/linux/lsm_count.h
> > new file mode 100644
> > index 000000000000..4d6dac6efb75
> > --- /dev/null
> > +++ b/include/linux/lsm_count.h
> > @@ -0,0 +1,107 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +
> > +/*
> > + * Copyright (C) 2023 Google LLC.
> > + */
> > +
> > +#ifndef __LINUX_LSM_COUNT_H
> > +#define __LINUX_LSM_COUNT_H
> > +
> > +#include <linux/args.h>
> > +
> > +#ifdef CONFIG_SECURITY
> > +
> > +/*
> > + * Macros to count the number of LSMs enabled in the kernel at compile time.
> > + */
> > +
> > +/*
> > + * Capabilities is enabled when CONFIG_SECURITY is enabled.
> > + */
> > +#if IS_ENABLED(CONFIG_SECURITY)
> > +#define CAPABILITIES_ENABLED 1,
> > +#else
> > +#define CAPABILITIES_ENABLED
> > +#endif
>
> We're in an #ifdef CONFIG_SECURITY, so CAPABILITIES_ENABLED will always
> be set. As such, we could leave off the trailing comma and list it
> _last_ in the macro, and then ...
>
> > +/*
> > + *  There is a trailing comma that we need to be accounted for. This is done by
> > + *  using a skipped argument in __COUNT_LSMS
> > + */
> > +#define __COUNT_LSMS(skipped_arg, args...) COUNT_ARGS(args)
> > +#define COUNT_LSMS(args...) __COUNT_LSMS(args)
>
> This wouldn't be needed...

Slight preference for explicitly having the capabilities listed than
implicitly over counting. But no strong opinion, fine with either
approches.

>
> > +
> > +#define MAX_LSM_COUNT                        \
> > +     COUNT_LSMS(                     \
> > +             CAPABILITIES_ENABLED    \
> > +             SELINUX_ENABLED         \
> > +             SMACK_ENABLED           \
> > +             APPARMOR_ENABLED        \
> > +             TOMOYO_ENABLED          \
> > +             YAMA_ENABLED            \
> > +             LOADPIN_ENABLED         \
> > +             LOCKDOWN_ENABLED        \
> > +             BPF_LSM_ENABLED         \
> > +             LANDLOCK_ENABLED)
>
>
>         COUNT_ARGS(                     \
>                 SELINUX_ENABLED         \
>                 SMACK_ENABLED           \
>                 ...
>                 CAPABILITIES_ENABLED)
>
> -Kees
>
> --
> Kees Cook