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Date:   Tue, 5 Nov 2019 11:34:47 -0800
From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
To:     =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= <mic@digikod.net>
Cc:     linux-kernel@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Casey Schaufler <casey@schaufler-ca.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        David Drysdale <drysdale@google.com>,
        Florent Revest <revest@chromium.org>,
        James Morris <jmorris@namei.org>, Jann Horn <jann@thejh.net>,
        John Johansen <john.johansen@canonical.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        KP Singh <kpsingh@chromium.org>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= <mickael.salaun@ssi.gouv.fr>,
        Paul Moore <paul@paul-moore.com>,
        Sargun Dhillon <sargun@sargun.me>,
        "Serge E . Hallyn" <serge@hallyn.com>,
        Shuah Khan <shuah@kernel.org>,
        Stephen Smalley <sds@tycho.nsa.gov>, Tejun Heo <tj@kernel.org>,
        Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        Tycho Andersen <tycho@tycho.ws>,
        Will Drewry <wad@chromium.org>, bpf@vger.kernel.org,
        kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
        linux-security-module@vger.kernel.org, netdev@vger.kernel.org
Subject: Re: [PATCH bpf-next v13 4/7] landlock: Add ptrace LSM hooks
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On Tue, Nov 05, 2019 at 07:01:41PM +0100, Micka?l Sala?n wrote:
> 
> On 05/11/2019 18:18, Alexei Starovoitov wrote:
> > On Mon, Nov 04, 2019 at 06:21:43PM +0100, Micka?l Sala?n 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: Micka?l Sala?n <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?

It's a pointer leak.

> 
> > 
> > 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.

I would like KRSI folks to speak up. So far I don't see any sharing happening
between landlock and KRSI. You're claiming this set is a first step. They're
claiming the same about their patches. I'd like to set a patchset that was
jointly developed.