Re: [PATCH V34 23/29] bpf: Restrict bpf when kernel lockdown is in confidentiality mode

Andy Lutomirski
2019-06-24T20:09:01Z

On Mon, Jun 24, 2019 at 12:54 PM Matthew Garrett <mjg59@google.com> wrote:
>
> On Mon, Jun 24, 2019 at 8:37 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
> >
> > On 06/22/2019 02:03 AM, Matthew Garrett wrote:
> > > From: David Howells <dhowells@redhat.com>
> > >
> > > There are some bpf functions can be used to read kernel memory:
> >
> > Nit: that
>
> Fixed.
>
> > > bpf_probe_read, bpf_probe_write_user and bpf_trace_printk. These allow
> >
> > Please explain how bpf_probe_write_user reads kernel memory ... ?!
>
> Ha.
>
> > > private keys in kernel memory (e.g. the hibernation image signing key) to
> > > be read by an eBPF program and kernel memory to be altered without
> >
> > ... and while we're at it, also how they allow "kernel memory to be
> > altered without restriction". I've been pointing this false statement
> > out long ago.
>
> Yup. How's the following description:
>
> bpf: Restrict bpf when kernel lockdown is in confidentiality mode
>
> There are some bpf functions that can be used to read kernel memory and
> exfiltrate it to userland: bpf_probe_read, bpf_probe_write_user and
> bpf_trace_printk. These could be abused to (eg) allow private
> keys in kernel
> memory to be leaked. Disable them if the kernel has been locked
> down in confidentiality
> mode.

I'm confused. I understand why we're restricting bpf_probe_read().
Why are we restricting bpf_probe_write_user() and bpf_trace_printk(),
though?

--Andy