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

Matthew Garrett
2019-06-24T19:54:15Z

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.

> This whole thing is still buggy as has been pointed out before by
> Jann. For helpers like above and few others below, error conditions
> must clear the buffer ...

Sorry, yes. My fault.