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

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Casey Schaufler <casey@schaufler-ca.com>
Cc: "Mickaël Salaün" <mic@digikod.net>,
	linux-kernel@vger.kernel.org,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Andy Lutomirski" <luto@amacapital.net>,
	"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>,
	"Mickaël Salaün" <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
Date: Tue, 5 Nov 2019 11:31:32 -0800
Message-ID: <20191105193130.qam2eafnmgvrvjwk@ast-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <c5c6b433-7e6a-c8f8-f063-e704c3df4cc6@schaufler-ca.com>

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?

> It's got
> to perform well and integrate with the rest of the system. 

what do you mean by that?

> I see many issues with a BPF <-> vfs interface.

There is no such interface today. What do you have in mind?

> 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.