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

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Date:   Tue, 3 Apr 2018 19:34:25 -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 3, 2018 at 9:26 AM, Andy Lutomirski <luto@...nel.org> wrote:
> On Tue, Apr 3, 2018 at 8:41 AM, Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
>> 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.
>
> Fair enough.

Actually looking at the patch again:
https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/commit/?h=efi-lock-down&id=78bb0059c3b8304a8d124b55feebc780fb3e0500

If the only thing that folks are paranoid about is reading
arbitrary kernel memory with bpf_probe_read() helper
then preferred patch would be to disable it during verification
when in lockdown mode.
No run-time overhead and android folks will be happy
that lockdown doesn't break their work.
They converted out-of-tree networking accounting
module and corresponding user daemon to use bpf:
https://www.linuxplumbersconf.org/2017/ocw/system/presentations/4791/original/eBPF%20cgroup%20filters%20for%20data%20usage%20accounting%20on%20Android.pdf