Re: [PATCH RFC bpf-next 0/4] audit: Expose audit subsystem to BPF LSM programs via BPF kfuncs

Frederick Lawler
2026-03-18T17:34:22Z

Hi Kumar,

On Tue, Mar 17, 2026 at 03:43:36AM +0100, Kumar Kartikeya Dwivedi wrote:
> On Wed, 11 Mar 2026 at 22:31, Frederick Lawler <fred@cloudflare.com> wrote:
> > 3. All bpf_audit_log_*() functions are destructive
> >
> > The audit subsystem allows for AUDIT_FAIL_PANIC to be set when the
> > subsystem can detect that missing events. Further, some call paths may
> > invoke a BUG_ON(). Therefore all the functions are marked destructive.
>
> I think the first part makes sense (i.e., the policy simply configured
> the system to panic on failure).
> However, in the second case, if the program is somehow able to trigger
> BUG_ON() relied upon for internal invariants, it would be considered
> broken.
> I tried grepping through and didn't find anything that would cause
> this, hence the whole thing about BUG_ON() in the cover letter only
> adds to confusion.
> Please drop it or describe cases which you were concerned about.
>

bpf_audit_log_cause()
-> audit_log_untrustedstring()
-> audit_log_n_untrustedstring()
-> audit_log_n_hex()

Was the primary call chain I was referring to, and yes this relies on
internal invariant of the SKB existing. I can remove from cover.

> >
> > 4. Functions are callable once per bpf_audit_context
> >
> > The rationale for this was to prevent abuse. Logs with repeated fields
> > are not helpful, and may not be handled by user space audit coherently.
> >
>
> This rationale feels weak. What abuse are we talking about?
> The LSM program is already written by a trusted entity.
>

I learned through off-list discussions that there's "undocumented" or
"unspoken" rules of log formatting and how auditd expects logs. Without
knowing exactly what these rules are, I can't provide any stronger
arguments other than to reduce duplication of fields in messages, or
exclude wrappers that supply unverified data.

WRT to unverified data, the socket wrappers accepting a netinf int.
In that case, the variable is at least verified via lookup to
report the interface in which it belongs. I left out wrapping
LSM_AUDIT_DATA_ANONINODE due to the passing of arbitrary string,
LSM_AUDIT_DATA_IPC arbitrary int. Others like for infiniband,
lockdown, nlmsgtype, were left out due to specificity, and not general
purpose usage.

Some of the rationale behind this is _not_ trusting BPF LSM authors as
much due to BPF LSM flexibility and accommodating user space audit.

Best,
Fred