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Subject: Re: [PATCH 10/10] LSM: Blob sharing support for S.A.R.A and LandLock
To:     Kees Cook <keescook@chromium.org>,
        John Johansen <john.johansen@canonical.com>
Cc:     Paul Moore <paul@paul-moore.com>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        James Morris <jmorris@namei.org>,
        LKML <linux-kernel@vger.kernel.org>,
        SE Linux <selinux@tycho.nsa.gov>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Stephen Smalley <sds@tycho.nsa.gov>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        "Schaufler, Casey" <casey.schaufler@intel.com>
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From:   Casey Schaufler <casey@schaufler-ca.com>
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Date:   Thu, 13 Sep 2018 17:03:50 -0700
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On 9/13/2018 4:51 PM, Kees Cook wrote:
> 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?

Do we have a case where it matters? I know that I could write a
module that would have issues if one hook got called and another
didn't because because a precursor module hook failed. I don't
think that any of the existing modules have this problem.