[RFC PATCH 29/29] lsm: add support for counting lsm_prop support among LSMs

[Paul Moore]

On Thu, May 15, 2025 at 10:12 AM Casey Schaufler <casey at schaufler-ca.com> wrote:
>
> On 5/14/2025 3:11 PM, Paul Moore wrote:
> > On Wed, May 14, 2025 at 5:16 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
> >> On 5/14/2025 1:57 PM, Paul Moore wrote:
> >>> On Wed, May 14, 2025 at 3:30 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
> >>>> On 5/13/2025 1:23 PM, Paul Moore wrote:
> >>>>> On Tue, May 13, 2025 at 12:39 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
> >>>>>> On 4/9/2025 11:50 AM, Paul Moore wrote:
> > ..
> >
> >>>> In my coming audit patch I changed where the counts of properties are
> >>>> maintained from the LSM infrastructure to the audit subsystem, where they are
> >>>> actually used. Instead of the LSM init code counting the property users, the
> >>>> individual LSM init functions call an audit function that keeps track. BPF
> >>>> could call that audit function if it loads a program that uses contexts. That
> >>>> could happen after init, and the audit system would handle it properly.
> >>>> Unloading the bpf program would be problematic. I honestly don't know whether
> >>>> that's permitted.
> >>> BPF programs can definitely go away, so that is something that would
> >>> need to be accounted for in any solution.  My understanding is that
> >>> once all references to a BPF program are gone, the BPF program is
> >>> unloaded from the kernel.
> >>>
> >>> Perhaps the answer is that whenever the BPF LSM is enabled at boot,
> >>> the audit subsystem always queries for subj/obj labels from the BPF
> >>> LSM and instead of using the normal audit placeholder for missing
> >>> values, "?", we simply don't log the BPF subj/obj fields.  I dislike
> >>> the special case nature of the solution, but the reality is that the
> >>> BPF is a bit "special" and we are going to need to have some special
> >>> code to deal with it.
> >> If BPF never calls audit_lsm_secctx() everything is fine, and the BPF
> >> context(s) never result in an aux record. If BPF does call audit_lsm_secctx()
> >> and there is another LSM that uses contexts you get the aux record, even
> >> if the BPF program goes away. You will get an aux record with only one context.
> >> This is not ideal, but provides the correct information. This all assumes that
> >> BPF programs can call into the audit system, and that they deal with multiple
> >> contexts within BPF. There could be a flag to audit_lsm_secctx() to delete the
> >> entry, but that seems potentially dangerous.
> > I think the answer to "can BPF programs call into the audit subsystem"
> > is dependent on if they have the proper BPF kfuncs for the audit API.
> > I don't recall seeing them post anything to the audit list about that,
> > but it's also possible they did it without telling anyone (ala move
> > fast, break things).  I don't think we would want to prevent BPF
> > programs from calling into the normal audit API that other subsystems
> > use, but we would need to look at that as it comes up.
>
> I suggest that until the "BPF auditing doesn't work!!!" crisis hits
> there's not a lot of point in going to heroic efforts to ensure all
> the bases are covered. I'll move forward assuming that an LSM could
> dynamically decide to call audit_lsm_secctx(), and that once it does
> it will always show up in the aux record, even if that means subj_bpf=?
> shows up every time.

My only concern is that I suspect most/all of the major distro enable
the BPF LSM by default which means that suddenly a lot of users/admins
are going to start seeing the multi-subj/obj labeling scheme only to
have an empty field logged.

-- 
paul-moore.com