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

Ondrej Mosnacek omosnace at redhat.com
Mon May 17 18:34:12 AEST 2021

On Sat, May 15, 2021 at 2:57 AM Casey Schaufler <casey at schaufler-ca.com> wrote:
> On 5/14/2021 8:12 AM, Ondrej Mosnacek wrote:
> > 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?
> It's still a bit kludgy, but a big improvement over the previous version.
> I wouldn't object to this approach.

Ok, thanks. I'll post it as a v2 then.

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

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