Re: [PATCH 22/27] Lock down kprobes

[Masami Hiramatsu]

On Tue, 26 Mar 2019 10:41:23 -0700
Matthew Garrett <mjg59@xxxxxxxxxx> wrote:

> On Tue, Mar 26, 2019 at 5:30 AM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> >
> > On Mon, 25 Mar 2019 15:09:49 -0700
> > Matthew Garrett <matthewgarrett@xxxxxxxxxx> wrote:
> >
> > > From: David Howells <dhowells@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx>