Re: [GIT PULL] Kernel lockdown for secure boot

[joeyli]

On Tue, Apr 03, 2018 at 07:34:25PM -0700, Alexei Starovoitov wrote:
> On Tue, Apr 3, 2018 at 9:26 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> > On Tue, Apr 3, 2018 at 8:41 AM, Alexei Starovoitov
> > <alexei.starovoitov@xxxxxxxxx> 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
> 

The bpf is totally disabled in this patch.

> 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

Sorry for I didn't fully understand your idea...
Do you mean that using bpf verifier to filter out bpf program that
uses bpf_probe_read()?
.
> 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

Thanks
Joey Lee