linux-kernel - Re: [PATCH 16/18] LSM: Allow arbitrary LSM ordering

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Date:   Mon, 17 Sep 2018 17:00:45 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Mickaël Salaün <mic@...ikod.net>,
        Kees Cook <keescook@...omium.org>
Cc:     John Johansen <john.johansen@...onical.com>,
        James Morris <jmorris@...ei.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        "Schaufler, Casey" <casey.schaufler@...el.com>,
        LSM <linux-security-module@...r.kernel.org>,
        LKLM <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 16/18] LSM: Allow arbitrary LSM ordering

On 9/17/2018 4:47 PM, Mickaël Salaün wrote:
> On 9/18/18 01:30, Casey Schaufler wrote:
>> On 9/17/2018 4:20 PM, Kees Cook wrote:
>>> On Mon, Sep 17, 2018 at 4:10 PM, Mickaël Salaün <mic@...ikod.net> wrote:
>>>> Landlock, because it target unprivileged users, should only be called
>>>> after all other major (access-control) LSMs. The admin or distro must
>>>> not be able to change that order in any way. This constraint doesn't
>>>> apply to current LSMs, though.
>> What harm would it cause for Landlock to get called before SELinux?
>> I certainly see why it seems like it ought to get called after, but
>> would it really make a difference?
> If an unprivileged process is able to infer some properties of a file
> being requested (thanks to one of its eBPF program doing checks on this
> process accesses), whereas this file access would be denied by a
> privileged LSM, then there is a side channel attack allowing this
> process to indirectly get information otherwise inaccessible.
>
> In other words, an unprivileged process should not be allowed to sneak
> itself (via an eBPF program) before SELinux for instance. SELinux should
> be able to block such information gathering the same way it can block a
> fstat(2) requested by a process.

The argument would feel a bit stronger if LSM checks happened before
the DAC checks. The opportunity to sneak a check in already exists, but
not with the tools you get with eBPF. For now at least I'll grant that
there's good reason for Landlock to go last.