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

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Message-ID: <dPp-BlyzEqZTTJfOrFzxSsFZj1eDOH4k4Lx7Qc0DGBdsRXPJHP-K5866dzAoZTvw4-GsBRHc4bfkL2zrajh8-KXWeR2B0pLIy0SIjK0-K5c=@protonmail.ch>
Date:   Tue, 26 Mar 2019 13:54:40 +0000
From:   Jordan Glover <Golden_Miller83@...tonmail.ch>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Matthew Garrett <matthewgarrett@...gle.com>,
        "jmorris@...ei.org" <jmorris@...ei.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Chun-Yi Lee <jlee@...e.com>, Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>
Subject: Re: [PATCH 23/27] bpf: Restrict kernel image access functions when the kernel is locked down

On Tuesday, March 26, 2019 12:00 AM, Daniel Borkmann <daniel@...earbox.net> wrote:

> On 03/26/2019 12:42 AM, Stephen Hemminger wrote:
>
> > On Mon, 25 Mar 2019 15:09:50 -0700
> > Matthew Garrett matthewgarrett@...gle.com wrote:
> >
> > > From: David Howells dhowells@...hat.com
> > > There are some bpf functions can be used to read kernel memory:
> > > bpf_probe_read, bpf_probe_write_user and bpf_trace_printk. These allow
> > > private keys in kernel memory (e.g. the hibernation image signing key) to
> > > be read by an eBPF program and kernel memory to be altered without
> > > restriction.
>
> I'm not sure where 'kernel memory to be altered without restriction' comes
> from, but it's definitely a wrong statement.
>
> > > Completely prohibit the use of BPF when the kernel is locked down.
>
> In which scenarios will the lock-down mode be used? Mostly niche? I'm asking
> as this would otherwise break a lot of existing stuff ... I'd prefer you find
> a better solution to this than this straight -EPERM rejection.


AFAIK this change breaks IPAddressAllow/IPAddressDeny usage in systemd services
which makes them LESS secure.

https://www.freedesktop.org/software/systemd/man/systemd.resource-control.html
https://github.com/systemd/systemd/blob/04d7ca022843913fba5170c40be07acf2ab5902b/README#L96

Jordan