linux-kernel - Re: [PATCH] lockdown,selinux: fix bogus SELinux lockdown permission checks

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Date:   Wed, 12 May 2021 15:21:37 +0200
From:   Ondrej Mosnacek <omosnace@...hat.com>
To:     Casey Schaufler <casey@...aufler-ca.com>
Cc:     Linux Security Module list 
        <linux-security-module@...r.kernel.org>,
        James Morris <jmorris@...ei.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        SElinux list <selinux@...r.kernel.org>,
        linuxppc-dev@...ts.ozlabs.org,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, network dev <netdev@...r.kernel.org>,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lockdown,selinux: fix bogus SELinux lockdown permission checks

On Sat, May 8, 2021 at 12:17 AM Casey Schaufler <casey@...aufler-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?

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