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

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Message-ID: <20180403154156.f37wrzjhuvijzi2i@ast-mbp.dhcp.thefacebook.com>
Date:   Tue, 3 Apr 2018 08:41:58 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     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 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.
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?