Login broken with old userspace (was Re: [PATCH v2] selinux: introduce an initial SID for early boot processes)

Ondrej Mosnacek omosnace at redhat.com
Fri Jul 28 21:02:58 AEST 2023

On Fri, Jul 28, 2023 at 4:12 AM Michael Ellerman <mpe at ellerman.id.au> wrote:
> Ondrej Mosnacek <omosnace at redhat.com> writes:
> > Currently, SELinux doesn't allow distinguishing between kernel threads
> > and userspace processes that are started before the policy is first
> > loaded - both get the label corresponding to the kernel SID. The only
> > way a process that persists from early boot can get a meaningful label
> > is by doing a voluntary dyntransition or re-executing itself.
> Hi,
> This commit breaks login for me when booting linux-next kernels with old
> userspace, specifically Ubuntu 16.04 on ppc64le. 18.04 is OK.
> The symptom is that login never accepts the root password, it just
> always says "Login incorrect".
> Bisect points to this commit.
> Reverting this commit on top of next-20230726, fixes the problem
> (ie. login works again).
> Booting with selinux=0 also fixes the problem.
> Is this expected? The change log below suggests backward compatibility
> was considered, is 16.04 just too old?

Hi Michael,

I can reproduce it on Fedora 38 when I boot with SELINUX=disabled in
/etc/selinux/config (+ a kernel including that commit), so it likely
isn't caused by the userspace being old. Can you check what you have
in /etc/selinux/config (or if it exists at all)?

We have deprecated and removed the "runtime disable" functionality in
SELinux recently [1], which was used to implement "disabling" SELinux
via the /etc/selinux/config file, so now the situation (selinux=0 +
SELINUX=disabled in /etc/selinux/config) leads to a state where
SELinux is enabled, but no policy is loaded (and no enforcement is
done). Such a state mostly behaves as if SElinux was truly disabled
(via kernel command line), but there are some subtle differences and I
believe we don't officially support it (Paul might clarify). With
latest kernels it is recommended to either disable SELinux via the
kernel command line (or Kconfig[2]) or to boot it in Enforcing or
Permissive mode with a valid/usable policy installed.

So I wonder if Ubuntu ships by default with the bad configuration or
if it's just a result of using the custom-built linux-next kernel (or
some changes on your part). If Ubuntu's stock kernel is configured to
boot with SELinux enabled by default, they should also by default ship
a usable policy and SELINUX=permissive/enforcing in
/etc/selinux/config (or configure the kernel[2] or bootloader to boot
with SELinux disabled by default). (Although if they ship a pre-[1]
kernel, they may continue to rely on the runtime disable
functionality, but it means people will have to be careful when
booting newer or custom kernels.)

That said, I'd like to get to the bottom of why the commit causes the
login to fail and fix it somehow. I presume something in PAM chokes on
the fact that userspace tasks now have "init" instead of "kernel" as
the pre-policy-load security context, but so far I haven't been able
to pinpoint the problem. I'll keep digging...

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f22f9aaf6c3d92ebd5ad9e67acc03afebaaeb289

Ondrej Mosnacek
Senior Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.

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