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

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Date:   Mon, 24 Jun 2019 12:54:00 -0700
From:   Matthew Garrett <mjg59@...gle.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     James Morris <jmorris@...ei.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        Chun-Yi Lee <jlee@...e.com>, Jann Horn <jannh@...gle.com>,
        bpf@...r.kernel.org
Subject: Re: [PATCH V34 23/29] bpf: Restrict bpf when kernel lockdown is in
 confidentiality mode

On Mon, Jun 24, 2019 at 8:37 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 06/22/2019 02:03 AM, Matthew Garrett wrote:
> > From: David Howells <dhowells@...hat.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.