On 10-Jan 05:11, James Morris wrote:
> On Wed, 8 Jan 2020, Stephen Smalley wrote:
>
> > The cover letter subject line and the Kconfig help text refer to it as a
> > BPF-based "MAC and Audit policy". It has an enforce config option that
> > enables the bpf programs to deny access, providing access control. IIRC, in
> > the earlier discussion threads, the BPF maintainers suggested that Smack and
> > other LSMs could be entirely re-implemented via it in the future, and that
> > such an implementation would be more optimal.
>
> In this case, the eBPF code is similar to a kernel module, rather than a
> loadable policy file. It's a loadable mechanism, rather than a policy, in
> my view.
>
> This would be similar to the difference between iptables rules and
> loadable eBPF networking code. I'd be interested to know how the
> eBPF networking scenarios are handled wrt kernel ABI.
>
>
> > Again, not arguing for or against, but wondering if people fully understand
> > the implications. If it ends up being useful, people will build access
> > control systems with it, and it directly exposes a lot of kernel internals to
> > userspace. There was a lot of concern originally about the LSM hook interface
> > becoming a stable ABI and/or about it being misused. Exposing that interface
> > along with every kernel data structure exposed through it to userspace seems
> > like a major leap.
>
> Agreed this is a leap, although I'm not sure I'd characterize it as
> exposure to userspace -- it allows dynamic extension of the LSM API from
> userland, but the code is executed in the kernel.
>
> KP: One thing I'd like to understand better is the attack surface
> introduced by this. IIUC, the BTF fields are read only, so the eBPF code
> should not be able to modify any LSM parameters, correct?
>
That's correct, the verifier does not allow writes to BTF types:
from kernel/bpf/verifier.c:
case PTR_TO_BTF_ID:
if (type == BPF_WRITE) {
verbose(env, "Writes through BTF pointers are not allowed\n");
return -EINVAL;
}
We can also add additional checks on top of those added by the
verifier using the verifier_ops that each BPF program type can define.
- KP
>
> > Even if the mainline kernel doesn't worry about any kind
> > of stable interface guarantees for it, the distros might be forced to provide
> > some kABI guarantees for it to appease ISVs and users...
>
> How is this handled currently for other eBPF use-cases?
>
> --
> James Morris
> <jmorris@xxxxxxxxx>
>
