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

Mickaël Salaün
2019-11-05T18:02:39Z

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.

[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.