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

Casey Schaufler casey at schaufler-ca.com
Sat May 15 10:57:08 AEST 2021

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.

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

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