Re: [PATCH v3] scripts/selinux: add basic mls support to mdp

From: Dominick Grift <dac.override@gmail.com>
To: Stephen Smalley <stephen.smalley@gmail.com>
Cc: Paul Moore <paul@paul-moore.com>,
	"Stephen D. Smalley" <sds@tycho.nsa.gov>,
	selinux@vger.kernel.org, Petr Lautrbach <plautrba@redhat.com>
Subject: Re: [PATCH v3] scripts/selinux: add basic mls support to mdp
Date: Tue, 19 Feb 2019 09:15:09 +0100	[thread overview]
Message-ID: <871s448a9e.fsf@gmail.com> (raw)
In-Reply-To: <CAB9W1A3f1jxJQPrU-o=gEKzgjRGmbThoqPvzbK7QNqprdE-LAw@mail.gmail.com> (Stephen Smalley's message of "Mon, 18 Feb 2019 19:07:33 -0500")

Stephen Smalley <stephen.smalley@gmail.com> writes:

> On Mon, Feb 18, 2019, 2:09 AM Dominick Grift <dac.override@gmail.com wrote:
>
>  Paul Moore <paul@paul-moore.com> writes:
>
>  > On Sat, Feb 16, 2019 at 7:12 AM Dominick Grift <dac.override@gmail.com> wrote:
>  >>
>  >> On Sat, Feb 16, 2019 at 01:04:12PM +0100, Dominick Grift wrote:
>  >> > On Fri, Feb 15, 2019 at 02:48:45PM -0500, Stephen Smalley wrote:
>  >> > <snip>
>  >> >
>  >> > >
>  >> > > Oh, I see: scripts/selinux/install_policy.sh just invokes checkpolicy
>  >> > > without specifying -U / --handle-unknown, so the policy defaults to deny,
>  >> > > and that would indeed render dbus-daemon and systemd broken with that
>  >> > > policy.  Might be as simple to fix as passing -U allow.
>  >> >
>  >> > I have looked a litte into this and here are some observations:
>  >> >
>  >> > 1. You can boot mdp as-is in permissive mode if you use `checkpolicy` with `-U allow`
>  >> >
>  >> > 2. You need *at least* an `/etc/selinux/dummy/seusers` with
>  >> > `__default__:user_u` and an accompanying
>  >> > `/etc/selinux/dummy/contexts/failsafe_context` with
>  >> > `base_r:base_t` to boot mdp in enforcing
>  >
>  > Wow.  I didn't expect we would get to this point so quickly.
>  >
>  > Originally my plan had been to just merge the mdp changes that Stephen
>  > submitted, and leave the rest for some other time.  Although based on
>  > everything in this thread, it looks like we are really close to having
>  > something that you can build and boot without too many hacks.
>  >
>  >> > 3. There is an issue with checkpolicy and object_r:
>  >> >
>  >> > PAM libselinux clients such as `login` try to associate `object_r` with the tty and fail.
>  >> >
>  >> > if you try to append: `role object_r; role object_r types base_t;`
>  >> > to policy.conf and compile that with `checkpolicy` then the
>  >> > `roletype-rule` does *not* end up in the compiled policy for some
>  >> > reason.
>  >
>  > This sounds like a bug in checkpolicy ... ?
>
>  Yes, looks like it
>
> I don't think so. object_r has always been handled specially. The kernel ignores the role-type definition for it and always exempts contexts
> that contain it from user-role, role-type, and user-range restrictions. We didn't use to include it in the policy at all; I think CIL does but
> we only generate a stub in the kernel policy with the role name and value but no types and the kernel ignores it. What exactly breaks with
> pam_selinux? 

The login program (pam_selinux) is not able to relabel the login user tty
(/dev/ttys0: user_u:base_r:base_t -> user_u:object_r:base_t) and so the user cannot log into the system in enforcing mode.

Maybe a missing contexts config file? I suppose I should look at it
again since you sound confident that this is not a bug. I also suppose
Android uses checkpolicy so they would have noticed?

>
>  >
>  >> > thus, you cannot log in because object_r:base_t is not valid.
>  >> >
>  >> > To hack around this add `default_role * source` rules to policy.conf and recompile.
>  >> >
>  >> > This will allow you to log into the system locally in enforcing mode.
>  >> >
>  >> > 4. I also noticed that fedoras' ssh seems to hardcode `sshd_net_t`
>  >> > for its "privsep" functionality so, while untested, you probably
>  >> > need an `openssh_contexts` with `privsep_preauth=base_t`
>  >
>  > Petr, what's the deal with ssh on Fedora?
>
>  I wonder whether it would be possible (and feasible) to not transition on
>  privsep_preauth at all *unless* a privsep preauth type is specified in
>  openssh_context.
>
>  Currently it falls back to a hardcoded type to transition to if
>  openssh_contexts does not exist.
>
>  Then again, i would not want to risk breaking or regressing some of the nice
>  functionality openssh in fedora has for selinux. It's state is currently
>  very good even compared to RHEL.
>
>  >
>  >> The `install_policy.sh` script should probably also do a bash file test for `checkpolicy` and fail gracefully if its not found
>
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>  Dominick Grift
>

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Dominick Grift