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

Casey Schaufler
2019-11-05T19:55:43Z

On 11/5/2019 11:31 AM, Alexei Starovoitov wrote:
> On Tue, Nov 05, 2019 at 09:55:42AM -0800, Casey Schaufler wrote:
>> On 11/5/2019 9:18 AM, 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.
>>>
>>> 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. BPF is capable
>>> to be a superset of all existing LSMs
>> I can't agree with this. Nope. There are many security models
>> for which BPF introduces excessive complexity. You don't need
>> or want the generality of a general purpose programming language
>> to implement Smack or TOMOYO. Or a simple Bell & LaPadula for
>> that matter. SELinux? I can't imagine anyone trying to do that
>> in eBPF, although I'm willing to be surprised. Being able to
>> enforce a policy isn't the only criteria for an LSM.
> what are the other criteria?

They include, but are not limited to, performance impact
and the ability to be analyzed. The interactions with other
subsystems meeting the requirements thereof is always a concern.

>
>> It's got
>> to perform well and integrate with the rest of the system.
> what do you mean by that?

It has to be fast, or the networking people are
going to have fits. You can't require the addition
of a pointer into the skb because it'll get rejected
out of hand. You can't completely refactor the vfs locking
to accommodate you needs.

>
>> I see many issues with a BPF <-> vfs interface.
> There is no such interface today. What do you have in mind?

You can't implement SELinux or Smack using BPF without a way
to manipulate inode data.

>
>> the mechanisms needed for the concerns of the day. Ideally,
>> we should be able to drop mechanisms when we decide that they
>> no longer add value.
> Exactly. bpf-based lsm must not add to kernel abi.

Huh? I have no idea where that came from.