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

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Date:   Thu, 27 Jun 2019 06:22:47 +1000 (AEST)
From:   James Morris <jmorris@...ei.org>
To:     Andy Lutomirski <luto@...nel.org>
cc:     Matthew Garrett <matthewgarrett@...gle.com>,
        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>,
        Matthew Garrett <mjg59@...gle.com>,
        Network Development <netdev@...r.kernel.org>,
        Chun-Yi Lee <jlee@...e.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH V33 24/30] bpf: Restrict bpf when kernel lockdown is in
 confidentiality mode

[Adding the LSM mailing list: missed this patchset initially]

On Thu, 20 Jun 2019, Andy Lutomirski wrote:

> This patch exemplifies why I don't like this approach:
> 
> > @@ -97,6 +97,7 @@ enum lockdown_reason {
> >         LOCKDOWN_INTEGRITY_MAX,
> >         LOCKDOWN_KCORE,
> >         LOCKDOWN_KPROBES,
> > +       LOCKDOWN_BPF,
> >         LOCKDOWN_CONFIDENTIALITY_MAX,
> 
> > --- 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.

Can you give an example of how the details might change?

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

These are not sufficient for creating a useful policy for the SELinux 
case.

-- 
James Morris
<jmorris@...ei.org>