linux-kernel - Re: [PATCH V40 12/29] x86: Lock down IO port access when the kernel is locked down

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Date:   Wed, 5 Jan 2022 18:05:26 +0800
From:   Kai-Heng Feng <kai.heng.feng@...onical.com>
To:     Matthew Garrett <mjg59@...f.ucam.org>
Cc:     jmorris@...ei.org, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        David Howells <dhowells@...hat.com>,
        Kees Cook <keescook@...omium.org>, x86@...nel.org
Subject: Re: [PATCH V40 12/29] x86: Lock down IO port access when the kernel
 is locked down

On Wed, Jan 5, 2022 at 3:20 PM Matthew Garrett <mjg59@...f.ucam.org> wrote:
>
> On Wed, Jan 05, 2022 at 02:57:57PM +0800, Kai-Heng Feng wrote:
>
> > The affected system from the customer has SecureBoot enabled (and
> > hence lockdown), and the kernel upgrade surprisingly broke ioperm()
> > usage.
>
> Which kernel was being used that was signed but didn't implement
> lockdown? That sounds, uh, bad.

It was upgraded from older distro release. Older kernels don't have lockdown.

>
> > The userspace program is proprietary so I can't share it here.
>
> Ok. Are you able to describe anything about what it does so we can
> figure out a better solution?
>
> > Basically this patch makes ioperm() a noop on SecureBoot enabled x86 systems.
> > If reverting is not an option, what else can we do to circumvent the regression?
>
> There's two main choices:
>
> 1) Disable secure boot on the system in question - if there's a need to
> run userland that can do arbitrary port IO then secure boot isn't
> providing any meaningful security benefit in any case.

How so?
Other security features are still incredible valuable, we don't want
to toss them out just because someone has to use ioperm().

>
> 2) Implement a kernel driver that abstracts the hardware access away
> from userland, and ensures that all the accesses are performed in a safe
> way.
>
> Doing port IO from userland is almost always a terrible idea - it
> usually involves indexed accesses (you write an address to one port and
> then write or read data from another), and if two processes are trying
> to do this simultaneously (either because SMP or because one process
> gets preempted after writing the address but before accessing the data
> register), and in that case you can end up with accesses to the wrong
> register as a result. You really want this sort of thing to be mediated
> by the kernel, both from a safety perspective and to ensure appropriate
> synchronisation.

Agree, let me start a discussion with them.

Kai-Heng