linux-kernel - Re: [PATCH V33 24/30] bpf: Restrict bpf when kernel lockdown is in confidentiality mode

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Message-ID: <CACdnJuv2sePuGBtTM6UL4S2k1UATcAk517o6vPx2EWF0Uxt8iw@mail.gmail.com>
Date:   Fri, 21 Jun 2019 13:05:44 -0700
From:   Matthew Garrett <mjg59@...gle.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     James Morris <jmorris@...ei.org>, linux-security@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        Chun-Yi Lee <jlee@...e.com>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH V33 24/30] bpf: Restrict bpf when kernel lockdown is in
 confidentiality mode

On Thu, Jun 20, 2019 at 10:22 PM Andy Lutomirski <luto@...nel.org> wrote:
> On Thu, Jun 20, 2019 at 6:21 PM Matthew Garrett
> <matthewgarrett@...gle.com> wrote:
> > --- a/security/lockdown/lockdown.c
> > +++ b/security/lockdown/lockdown.c
> > @@ -33,6 +33,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
> >         [LOCKDOWN_INTEGRITY_MAX] = "integrity",
> >         [LOCKDOWN_KCORE] = "/proc/kcore access",
> >         [LOCKDOWN_KPROBES] = "use of kprobes",
> > +       [LOCKDOWN_BPF] = "use of bpf",
> >         [LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
>
> The text here says "use of bpf", but what this patch is *really* doing
> is locking down use of BPF to read kernel memory.  If the details
> change, then every LSM needs to get updated, and we risk breaking user
> policies that are based on LSMs that offer excessively fine
> granularity.

The text is descriptive rather than normative, and no changes should
be made that alter the semantics of a reason - it makes more sense to
just add another reason.

> I'd be more comfortable if the LSM only got to see "confidentiality"
> or "integrity".

If LSM authors can be trusted to do the right thing here, then I see
no problem in providing additional data. I'm happy to defer to James
on that.