[PATCH] lockdown, selinux: fix bogus SELinux lockdown permission checks

[Ondrej Mosnacek]

On Wed, May 12, 2021 at 7:12 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
>
> On 5/12/2021 9:44 AM, Ondrej Mosnacek wrote:
> > On Wed, May 12, 2021 at 6:18 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
> >> On 5/12/2021 6:21 AM, Ondrej Mosnacek wrote:
> >>> On Sat, May 8, 2021 at 12:17 AM Casey Schaufler <casey at schaufler-ca.com> wrote:
> >>>> On 5/7/2021 4:40 AM, Ondrej Mosnacek wrote:
> >>>>> Commit 59438b46471a ("security,lockdown,selinux: implement SELinux
> >>>>> lockdown") added an implementation of the locked_down LSM hook to
> >>>>> SELinux, with the aim to restrict which domains are allowed to perform
> >>>>> operations that would breach lockdown.
> >>>>>
> >>>>> However, in several places the security_locked_down() hook is called in
> >>>>> situations where the current task isn't doing any action that would
> >>>>> directly breach lockdown, leading to SELinux checks that are basically
> >>>>> bogus.
> >>>>>
> >>>>> Since in most of these situations converting the callers such that
> >>>>> security_locked_down() is called in a context where the current task
> >>>>> would be meaningful for SELinux is impossible or very non-trivial (and
> >>>>> could lead to TOCTOU issues for the classic Lockdown LSM
> >>>>> implementation), fix this by adding a separate hook
> >>>>> security_locked_down_globally()
> >>>> This is a poor solution to the stated problem. Rather than adding
> >>>> a new hook you should add the task as a parameter to the existing hook
> >>>> and let the security modules do as they will based on its value.
> >>>> If the caller does not have an appropriate task it should pass NULL.
> >>>> The lockdown LSM can ignore the task value and SELinux can make its
> >>>> own decision based on the task value passed.
> >>> The problem with that approach is that all callers would then need to
> >>> be updated and I intended to keep the patch small as I'd like it to go
> >>> to stable kernels as well.
> >>>
> >>> But it does seem to be a better long-term solution - would it work for
> >>> you (and whichever maintainer would be taking the patch(es)) if I just
> >>> added another patch that refactors it to use the task parameter?
> >> I can't figure out what you're suggesting. Are you saying that you
> >> want to add a new hook *and* add the task parameter?
> > No, just to keep this patch as-is (and let it go to stable in this
> > form) and post another (non-stable) patch on top of it that undoes the
> > new hook and re-implements the fix using your suggestion. (Yeah, it'll
> > look weird, but I'm not sure how better to handle such situation - I'm
> > open to doing it whatever different way the maintainers prefer.)
>
> James gets to make the call on this one. If it was my call I would
> tell you to make the task parameter change and accept the backport
> pain. I think that as a security developer community we spend way too
> much time and effort trying to avoid being noticed in source trees.

Hm... actually, what about this attached patch? It switches to a
single hook with a cred argument (I figured cred makes more sense than
task_struct, since the rest of task_struct should be irrelevant for
the LSM, anyway...) right from the start and keeps the original
security_locked_down() function only as a simple wrapper around the
main hook.

At that point I think converting the other callers to call
security_cred_locked_down() directly isn't really worth it, since the
resulting calls would just be more verbose without much benefit. So
I'm tempted to just leave the security_locked_down() helper as is, so
that the more common pattern can be still achieved with a simpler
call.

What do you think?

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