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

Casey Schaufler casey at schaufler-ca.com
Thu May 13 02:17:50 AEST 2021

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?

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

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