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

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Date:   Mon, 24 Jun 2019 14:30:46 -0700
From:   Matthew Garrett <mjg59@...gle.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Andy Lutomirski <luto@...nel.org>,
        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 2:22 PM Daniel Borkmann <daniel@...earbox.net> wrote:
> Agree, for example, bpf_probe_write_user() can never write into
> kernel memory (only user one). Just thinking out loud, wouldn't it
> be cleaner and more generic to perform this check at the actual function
> which performs the kernel memory without faulting? All three of these
> are in mm/maccess.c, and the very few occasions that override the
> probe_kernel_read symbol are calling eventually into __probe_kernel_read(),
> so this would catch all of them wrt lockdown restrictions. Otherwise
> you'd need to keep tracking every bit of new code being merged that
> calls into one of these, no? That way you only need to do it once like
> below and are guaranteed that the check catches these in future as well.

Not all paths into probe_kernel_read/write are from entry points that
need to be locked down (eg, as far as I can tell ftrace can't leak
anything interesting here).