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

Casey Schaufler casey at schaufler-ca.com
Thu May 13 03:12:10 AEST 2021

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.

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