Re: [PATCH v2 3/3] security: Add LSMs dependencies to CONFIG_LSM

[Masahiro Yamada <masahiroy@kernel.org>]

On Sun, Feb 21, 2021 at 8:11 PM Mickaël Salaün <mic@digikod.net> wrote:
>
>
> On 21/02/2021 09:50, Masahiro Yamada wrote:
> > On Tue, Feb 16, 2021 at 4:03 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> >>
> >> On Mon, Feb 15, 2021 at 7:17 PM Mickaël Salaün <mic@digikod.net> wrote:
> >>> From: Mickaël Salaün <mic@linux.microsoft.com>
> >>>
> >>> Thanks to the previous commit, this gives the opportunity to users, when
> >>> running make oldconfig, to update the list of enabled LSMs at boot time
> >>> if an LSM has just been enabled or disabled in the build.  Moreover,
> >>> this list only makes sense if at least one LSM is enabled.
> >>>
> >>> Cc: Casey Schaufler <casey@schaufler-ca.com>
> >>> Cc: James Morris <jmorris@namei.org>
> >>> Cc: Masahiro Yamada <masahiroy@kernel.org>
> >>> Cc: Serge E. Hallyn <serge@hallyn.com>
> >>> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
> >>> Link: https://lore.kernel.org/r/20210215181511.2840674-4-mic@digikod.net
> >>> ---
> >>>
> >>> Changes since v1:
> >>> * Add CONFIG_SECURITY as a dependency of CONFIG_LSM.  This prevent an
> >>>   error when building without any LSMs.
> >>> ---
> >>>  security/Kconfig | 4 ++++
> >>>  1 file changed, 4 insertions(+)
> >>>
> >>> diff --git a/security/Kconfig b/security/Kconfig
> >>> index 7561f6f99f1d..addcc1c04701 100644
> >>> --- a/security/Kconfig
> >>> +++ b/security/Kconfig
> >>> @@ -277,6 +277,10 @@ endchoice
> >>>
> >>>  config LSM
> >>>         string "Ordered list of enabled LSMs"
> >>> +       depends on SECURITY || SECURITY_LOCKDOWN_LSM || SECURITY_YAMA || \
> >>> +               SECURITY_LOADPIN || SECURITY_SAFESETID || INTEGRITY || \
> >>> +               SECURITY_SELINUX || SECURITY_SMACK || SECURITY_TOMOYO || \
> >>> +               SECURITY_APPARMOR || BPF_LSM
> >>
> >> This looks really awkward, since all of these already depend on
> >> SECURITY (if not, it's a bug)... I guarantee you that after some time
> >> someone will come, see that the weird boolean expression is equivalent
> >> to just SECURITY, and simplify it.
> >
> >
> > Currently, LSM does not depend on SECURITY.
> > So you can always define LSM irrespective of SECURITY,
> > which seems a bug.
> >
> > So, I agree with adding 'depends on SECURITY'.
> >
> > What he is trying to achieve in this series
> > seems wrong, of course.
>
> This may be wrong in the general case, but not for CONFIG_LSM.
>
> >
> >
> >> I assume the new mechanism wouldn't work as intended if there is just
> >> SECURITY? If not, then maybe you should rather specify this value
> >> dependency via some new  field rather than abusing "depends on" (say,
> >> "value depends on"?). The fact that a seemingly innocent change to the
> >> config definition breaks your mechanism suggests that the design is
> >> flawed.
>
> Masahiro, what do you think about this suggested "value depends on"?


Of course, no.


See the help text in init/Kconfig:

          This choice is there only for converting CONFIG_DEFAULT_SECURITY
          in old kernel configs to CONFIG_LSM in new kernel configs. Don't
          change this choice unless you are creating a fresh kernel config,
          for this choice will be ignored after CONFIG_LSM has been set.


When CONFIG_LSM is already set in the .config,
this choice is just ignored.
So, oldconfig is working as the help message says.

If you think 2623c4fbe2ad1341ff2d1e12410d0afdae2490ca
is a pointless commit, you should ask Kees about it.






> >>
> >> I do think this would be a useful feature, but IMHO shouldn't be
> >> implemented like this.
> >>
> >>>         default "lockdown,yama,loadpin,safesetid,integrity,smack,selinux,tomoyo,apparmor,bpf" if DEFAULT_SECURITY_SMACK
> >>>         default "lockdown,yama,loadpin,safesetid,integrity,apparmor,selinux,smack,tomoyo,bpf" if DEFAULT_SECURITY_APPARMOR
> >>>         default "lockdown,yama,loadpin,safesetid,integrity,tomoyo,bpf" if DEFAULT_SECURITY_TOMOYO
> >>> --
> >>> 2.30.0
> >>>
> >>
> >> --
> >> Ondrej Mosnacek
> >> Software Engineer, Linux Security - SELinux kernel
> >> Red Hat, Inc.
> >>
> >
> >
--
Best Regards
Masahiro Yamada