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

[Paul Moore]

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.

-- 
paul-moore.com