[PATCH 16/18] LSM: Allow arbitrary LSM ordering

[Casey Schaufler]

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 at digikod.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.