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

From: Casey Schaufler <casey@schaufler-ca.com>
To: Ondrej Mosnacek <omosnace@redhat.com>
Cc: Linux Security Module list 
	<linux-security-module@vger.kernel.org>,
	James Morris <jmorris@namei.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@redhat.com>,
	Stephen Smalley <stephen.smalley.work@gmail.com>,
	SElinux list <selinux@vger.kernel.org>,
	linuxppc-dev@lists.ozlabs.org,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	bpf <bpf@vger.kernel.org>, network dev <netdev@vger.kernel.org>,
	Linux kernel mailing list <linux-kernel@vger.kernel.org>,
	Casey Schaufler <casey@schaufler-ca.com>
Subject: Re: [PATCH] lockdown,selinux: fix bogus SELinux lockdown permission checks
Date: Wed, 12 May 2021 09:17:50 -0700	[thread overview]
Message-ID: <24a61ff1-e415-adf8-17e8-d212364d4b97@schaufler-ca.com> (raw)
In-Reply-To: <CAFqZXNtr1YjzRg7fTm+j=0oZF+7C5xEu5J0mCZynP-dgEzvyUg@mail.gmail.com>

On 5/12/2021 6:21 AM, Ondrej Mosnacek wrote:
> On Sat, May 8, 2021 at 12:17 AM Casey Schaufler <casey@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?

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