Re: [PATCH bpf-next v13 4/7] landlock: Add ptrace LSM hooks

KP Singh
2019-11-06T10:35:08Z

On 05-Nov 19:01, Mickaël Salaün wrote:
>
> On 05/11/2019 18:18, Alexei Starovoitov wrote:
> > On Mon, Nov 04, 2019 at 06:21:43PM +0100, Mickaël Salaün wrote:
> >> Add a first Landlock hook that can be used to enforce a security policy
> >> or to audit some process activities. For a sandboxing use-case, it is
> >> needed to inform the kernel if a task can legitimately debug another.
> >> ptrace(2) can also be used by an attacker to impersonate another task
> >> and remain undetected while performing malicious activities.
> >>
> >> Using ptrace(2) and related features on a target process can lead to a
> >> privilege escalation. A sandboxed task must then be able to tell the
> >> kernel if another task is more privileged, via ptrace_may_access().
> >>
> >> Signed-off-by: Mickaël Salaün <mic@digikod.net>
> > ...
> >> +static int check_ptrace(struct landlock_domain *domain,
> >> + struct task_struct *tracer, struct task_struct *tracee)
> >> +{
> >> + struct landlock_hook_ctx_ptrace ctx_ptrace = {
> >> + .prog_ctx = {
> >> + .tracer = (uintptr_t)tracer,
> >> + .tracee = (uintptr_t)tracee,
> >> + },
> >> + };
> >
> > So you're passing two kernel pointers obfuscated as u64 into bpf program
> > yet claiming that the end goal is to make landlock unprivileged?!
> > The most basic security hole in the tool that is aiming to provide security.
>
> How could you used these pointers without dedicated BPF helpers? This
> context items are typed as PTR_TO_TASK and can't be used without a
> dedicated helper able to deal with ARG_PTR_TO_TASK. Moreover, pointer
> arithmetic is explicitly forbidden (and I added tests for that). Did I
> miss something?
>
> >
> > I think the only way bpf-based LSM can land is both landlock and KRSI
> > developers work together on a design that solves all use cases.
>
> As I said in a previous cover letter [1], that would be great. I think
> that the current Landlock bases (almost everything from this series
> except the seccomp interface) should meet both needs, but I would like
> to have the point of view of the KRSI developers.

As I mentioned we are willing to collaborate but the current landlock
patches does not meet the needs for KRSI:

* One program type per use-case (eg. LANDLOCK_PROG_PTRACE) as opposed to
a single program type. This is something that KRSI proposed in it's
initial design [1] and the new common "eBPF + LSM" based approach
[2] would maintain as well.

* Landlock chooses to have multiple LSM hooks per landlock hook which is
more restrictive. It's not easy to write precise MAC and Audit
policies for a privileged LSM based on this and this ends up bloating
the context that needs to be maintained and requires avoidable
boilerplate work in the kernel.

[1] https://lore.kernel.org/patchwork/project/lkml/list/?series=410101
[2] https://lore.kernel.org/bpf/20191106100655.GA18815@chromium.org/T/#u

- KP Singh

>
> [1] https://lore.kernel.org/lkml/20191029171505.6650-1-mic@digikod.net/
>
> > BPF is capable
> > to be a superset of all existing LSMs whereas landlock and KRSI propsals today
> > are custom solutions to specific security concerns. BPF subsystem was extended
> > with custom things in the past. In networking we have lwt, skb, tc, xdp, sk
> > program types with a lot of overlapping functionality. We couldn't figure out
> > how to generalize them into single 'networking' program. Now we can and we
> > should. Accepting two partially overlapping bpf-based LSMs would be repeating
> > the same mistake again.
>
> I'll let the LSM maintainers comment on whether BPF could be a superset
> of all LSM, but given the complexity of an access-control system, I have
> some doubts though. Anyway, we need to start somewhere and then iterate.
> This patch series is a first step.