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

[Stephen Smalley]

On Fri, Jul 28, 2023 at 7:36 AM Ondrej Mosnacek <omosnace at redhat.com> wrote:
>
> 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
> [2] via CONFIG_LSM (or CONFIG_SECURITY_SELINUX_BOOTPARAM_VALUE on older kernels)

Prior to selinux userspace commit
685f4aeeadc0b60f3770404d4f149610d656e3c8 ("libselinux:
is_selinux_enabled(): drop no-policy-loaded test.") libselinux was
checking the result of reading /proc/self/attr/current to see if it
returned the "kernel" string as a means of detecting a system with
SELinux enabled but no policy loaded, and treated that as if SELinux
were disabled. Hence, this does break old userspace. Not sure though
why you'd see the same behavior with modern libselinux.