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

Ondrej Mosnacek omosnace at redhat.com
Thu May 13 02:44:21 AEST 2021

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.)

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

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