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

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Message-ID: <CALCETrWmZX3R1L88Gz9vLY68gcK8zSXL4cA4GqAzQoyqSR7rRQ@mail.gmail.com>
Date:   Mon, 24 Jun 2019 13:08:44 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Matthew Garrett <mjg59@...gle.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        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 12:54 PM Matthew Garrett <mjg59@...gle.com> wrote:
>
> 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.

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