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Subject: Re: [PATCH 23/27] bpf: Restrict kernel image access functions when
 the kernel is locked down
To:     Stephen Hemminger <stephen@networkplumber.org>,
        Matthew Garrett <matthewgarrett@google.com>
Cc:     jmorris@namei.org, linux-security-module@vger.kernel.org,
        linux-kernel@vger.kernel.org, dhowells@redhat.com,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        netdev@vger.kernel.org, Chun-Yi Lee <jlee@suse.com>,
        Kees Cook <keescook@chromium.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Will Drewry <wad@chromium.org>
References: <20190325220954.29054-1-matthewgarrett@google.com>
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From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <1cfa7345-c807-db76-f50a-ea3ba70f07b2@iogearbox.net>
Date:   Tue, 26 Mar 2019 01:00:29 +0100
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On 03/26/2019 12:42 AM, Stephen Hemminger wrote:
> On Mon, 25 Mar 2019 15:09:50 -0700
> Matthew Garrett <matthewgarrett@google.com> wrote:
> 
>> From: David Howells <dhowells@redhat.com>
>>
>> There are some bpf functions can be used to read kernel memory:
>> bpf_probe_read, bpf_probe_write_user and bpf_trace_printk.  These allow
>> 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
>> restriction.

I'm not sure where 'kernel memory to be altered without restriction' comes
from, but it's definitely a wrong statement.

>> Completely prohibit the use of BPF when the kernel is locked down.

In which scenarios will the lock-down mode be used? Mostly niche? I'm asking
as this would otherwise break a lot of existing stuff ... I'd prefer you find
a better solution to this than this straight -EPERM rejection.

>> Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
>> Signed-off-by: David Howells <dhowells@redhat.com>
>> cc: netdev@vger.kernel.org
>> cc: Chun-Yi Lee <jlee@suse.com>
>> cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
>> Cc: Daniel Borkmann <daniel@iogearbox.net>
>> Signed-off-by: Matthew Garrett <matthewgarrett@google.com>
> 
> Wouldn't this mean that Seccomp won't work in locked down mode?
>