[PATCH v2] lockdown,selinux: avoid bogus SELinux lockdown permission checks

[Daniel Borkmann]

On 5/28/21 9:09 AM, Daniel Borkmann wrote:
> On 5/28/21 3:37 AM, Paul Moore wrote:
>> On Mon, May 17, 2021 at 5:22 AM Ondrej Mosnacek <omosnace at redhat.com> 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 modifying the hook to accept a struct cred
>>> pointer as argument, where NULL will be interpreted as a request for a
>>> "global", task-independent lockdown decision only. Then modify SELinux
>>> to ignore calls with cred == NULL.
>>
>> I'm not overly excited about skipping the access check when cred is
>> NULL.  Based on the description and the little bit that I've dug into
>> thus far it looks like using SECINITSID_KERNEL as the subject would be
>> much more appropriate.  *Something* (the kernel in most of the
>> relevant cases it looks like) is requesting that a potentially
>> sensitive disclosure be made, and ignoring it seems like the wrong
>> thing to do.  Leaving the access control intact also provides a nice
>> avenue to audit these requests should users want to do that.
> 
> I think the rationale/workaround for ignoring calls with cred == NULL (or the previous
> patch with the unimplemented hook) from Ondrej was two-fold, at least speaking for his
> seen tracing cases:
> 
>    i) The audit events that are triggered due to calls to security_locked_down()
>       can OOM kill a machine, see below details [0].
> 
>   ii) It seems to be causing a deadlock via slow_avc_audit() -> audit_log_end()
>       when presumingly trying to wake up kauditd [1].

Ondrej / Paul / Jiri: at least for the BPF tracing case specifically (I haven't looked
at the rest but it's also kind of independent), the attached fix should address both
reported issues, please take a look & test.

Thanks a lot,
Daniel