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From:   Kees Cook <keescook@chromium.org>
Date:   Thu, 13 Sep 2018 16:51:43 -0700
Message-ID: <CAGXu5jKZ7rquTux5fS+5tQ==0w_9rDTRaJWS_mS7C+Fy2dXRRQ@mail.gmail.com>
Subject: Re: [PATCH 10/10] LSM: Blob sharing support for S.A.R.A and LandLock
To:     John Johansen <john.johansen@canonical.com>
Cc:     Paul Moore <paul@paul-moore.com>,
        Casey Schaufler <casey@schaufler-ca.com>,
        linux-security-module <linux-security-module@vger.kernel.org>,
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        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
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On Thu, Sep 13, 2018 at 4:32 PM, John Johansen
<john.johansen@canonical.com> wrote:
> On 09/13/2018 04:06 PM, Kees Cook wrote:
>> - what order should any stacking happen? Makefile? security=?
>>
> Preferably not. For the single LSM we have the ability to choose the default LSM, ideally we let the distro decide in the Kconfig and the user with security=...

I can't find a non-crazy way to do this in Kconfig. Right now, if I
threw out all the _DEFAULT stuff, I could do:

config SECURITY_SELINUX_ENABLED
        bool "SELinux LSM enabled at boot time"
        depends on SECURITY_SELINUX
        depends on !SECURITY_APPARMOR_ENABLED && !SECURITY_SMACK_ENABLED
        default SECURITY_SELINUX

config SECURITY_SMACK_ENABLED
        bool "SMACK LSM enabled at boot time"
        depends on SECURITY_SMACK
        depends on !SECURITY_APPARMOR_ENABLED && !SECURITY_SELINUX_ENABLED
        default SECURITY_SMACK

config SECURITY_APPARMOR_ENABLED
        bool "AppArmor LSM enabled at boot time"
        depends on SECURITY_APPARMOR
        depends on !SECURITY_SMACK_ENABLED && !SECURITY_SELINUX_ENABLED
        default SECURITY_APPARMOR

config SECURITY_TOMOYO_ENABLED
        bool "TOMOYO LSM enabled at boot time"
        depends on SECURITY_TOMOYO
        default y if !SECURITY_SELINUX_ENABLED &&
!SECURITY_SMACK_ENABLED && !SECURITY_APPARMOR_ENABLED

config DEFAULT_SECURITY
        string
        default "selinux" if SECURITY_SELINUX_ENABLED
        default "smack" if SECURITY_SMACK_ENABLED
        default "apparmor" if SECURITY_APPARMOR_ENABLED
        default "tomoyo" if SECURITY_TOMOYO_ENABLED

(As before CONFIG_DEFAULT_SECURITY basically means the effective
"security=" contents. Reminder than Kconfig default are "first match",
so tomoyo would only happen if all others are not enabled by default.)

But this doesn't provide a way for Kconfig to declare the ordering of
TOMOYO followed by SELinux. If we just declare ordering is a function
of the Makefile, then the above would work as expected. The
"conflicting major LSM" could be specified on "security=" and stacked
could be enabled with $lsm.enable=1 (or disabled).

So, before we can really make a decision, I think we have to decide:
should ordering be arbitrary for even this level of stacking?

-Kees

-- 
Kees Cook
Pixel Security