linux-kernel - Re: [PATCH 23/27] bpf: Restrict kernel image access functions when the kernel is locked down

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Message-ID: <CACdnJusN0XwKAFhCk-z2+u8Wn1OOzbSqQYYACFikcAtFqOA+WA@mail.gmail.com>
Date:   Thu, 28 Mar 2019 13:08:39 -0700
From:   Matthew Garrett <mjg59@...gle.com>
To:     James Morris <jmorris@...ei.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Linux API <linux-api@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        LKML <linux-kernel@...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>,
        Kees Cook <keescook@...omium.org>,
        Will Drewry <wad@...omium.org>
Subject: Re: [PATCH 23/27] bpf: Restrict kernel image access functions when
 the kernel is locked down

On Thu, Mar 28, 2019 at 12:23 PM James Morris <jmorris@...ei.org> wrote:
>
> On Thu, 28 Mar 2019, Matthew Garrett wrote:
>
> > On Wed, Mar 27, 2019 at 8:15 PM James Morris <jmorris@...ei.org> wrote:
> > > OTOH, this seems like a combination of mechanism and policy. The 3 modes
> > > are a help here, but I wonder if they may be too coarse grained still,
> > > e.g. if someone wants to allow a specific mechanism according to their own
> > > threat model and mitigations.
> >
> > In general the interfaces blocked by these patches could also be
> > blocked with an LSM, and I'd guess that people with more fine-grained
> > requirements would probably take that approach.
>
> So... I have to ask, why not use LSM for this in the first place?
>
> Either with an existing module or perhaps a lockdown LSM?

Some of it isn't really achievable that way - for instance, enforcing
module or kexec signatures. We have other mechanisms that can be used
to enable that which could be done at the more fine-grained level, but
a design goal was to make it possible to automatically enable a full
set of integrity protections under specified circumstances.