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Date:   Wed, 27 Mar 2019 07:47:55 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Matthew Garrett <mjg59@google.com>
Cc:     James Morris <jmorris@namei.org>,
        LSM List <linux-security-module@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        David Howells <dhowells@redhat.com>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        "Naveen N . Rao" <naveen.n.rao@linux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
        davem@davemloft.net
Subject: Re: [PATCH 22/27] Lock down kprobes
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On Tue, 26 Mar 2019 10:41:23 -0700
Matthew Garrett <mjg59@google.com> wrote:

> On Tue, Mar 26, 2019 at 5:30 AM Masami Hiramatsu <mhiramat@kernel.org> wrote:
> >
> > On Mon, 25 Mar 2019 15:09:49 -0700
> > Matthew Garrett <matthewgarrett@google.com> wrote:
> >
> > > From: David Howells <dhowells@redhat.com>
> > >
> > > Disallow the creation of kprobes when the kernel is locked down by
> > > preventing their registration.  This prevents kprobes from being used to
> > > access kernel memory, either to make modifications or to steal crypto data.
> >
> > Hmm, if you enforce signature check of modules, those modules
> > should be allowed to use kprobes?
> > I think we should introduce some kind of trust inheritance from
> > signed (trusted) modules.
> 
> Is there any way to install a kprobe /without/ it coming from a
> module? The presumption in lockdown mode is that module signing is
> enforced, so I'll admit to not being entirely clear on why this patch
> is needed in that case.

Yes, there are 2 paths, ftrace and perf(bpf). If you want to disable ftrace
path (which start from user's input via tracefs), this should be done in
trace_kprobe_create()@kernel/trace/trace_kprobe.c.
If you want to disable both, __register_trace_kprobe()@kernel/trace/trace_kprobe.c
is the best place.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>