linux-kernel - Re: [GIT PULL] Kernel lockdown for secure boot

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Message-ID: <CALCETrX3D3jrr=Vq70H+ttdCKSevU-cC-cvZrfaoahEQoO7wzA@mail.gmail.com>
Date:   Tue, 3 Apr 2018 09:26:26 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        James Morris <jmorris@...ei.org>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Garrett <mjg59@...gle.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Justin Forbes <jforbes@...hat.com>,
        linux-man <linux-man@...r.kernel.org>, joeyli <jlee@...e.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        linux-efi <linux-efi@...r.kernel.org>
Subject: Re: [GIT PULL] Kernel lockdown for secure boot

On Tue, Apr 3, 2018 at 8:41 AM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> On Tue, Apr 03, 2018 at 08:11:07AM -0700, Andy Lutomirski wrote:
>> >
>> >> "bpf: Restrict kernel image access functions when the kernel is locked down":
>> >> This patch just sucks in general.
>> >
>> > Yes - but that's what Alexei Starovoitov specified.  bpf kind of sucks since
>> > it gives you unrestricted access to the kernel.
>>
>> bpf, in certain contexts, gives you unrestricted access to *reading*
>> kernel memory.  bpf should, under no circumstances, let you write to
>> the kernel unless you're using fault injection or similar.
>>
>> I'm surprised that Alexei acked this patch.  If something like XDP or
>> bpfilter starts becoming widely used, this patch will require a lot of
>> reworking to avoid breaking standard distros.
>
> my understanding was that this lockdown set attemps to disallow _reads_
> of kernel memory from anything, so first version of patch was adding
> run-time checks for bpf_probe_read() which is no-go
> and without this helper the bpf for tracing is losing a lot of its power,
> so the easiest is to disable it all.

Fair enough.

> I think lockdown suppose to disable xdp, bpfilter, nflog, raw sockets + pcap too
> otherwise even cap_net_admin can see traffic coming into host.
> Similarly kprobe, perf_event, ftrace should be off as well?
>

I'm reasonably sure that lockdown is not intended to be this far
reaching.  cap_net_admin can see traffic coming into the host, and I
don't think lockdown is intended to change that.

David, I think this is exactly why you need to define what "lockdown"
means.  As it stands, the best argument I've seen involves
"blacklisting", but that's a political thing and almost no one
involved has any ability to evaluate it.  Right now there's a series
of patches that check for "lockdown" and seem to disable things that
make someone uncomfortable.  That's not a good way to design a
security feature.