linux-kernel - Re: [PATCH] Allow userspace control of runtime disabling/enabling of driver probing

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Message-ID: <CAGXu5jKuTf-d1sES3vdB7i1dqKWxc1=ukULPr11v6sgtjDYQuQ@mail.gmail.com>
Date:   Tue, 3 Jan 2017 15:38:24 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Matthew Garrett <mjg59@...eos.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Lukas Wunner <lukas@...ner.de>,
        Madalin Bucur <madalin.bucur@....com>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <rmk+kernel@....linux.org.uk>,
        Petr Tesarik <ptesarik@...e.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH] Allow userspace control of runtime disabling/enabling of
 driver probing

On Tue, Jan 3, 2017 at 3:34 PM, Rafael J. Wysocki <rafael@...nel.org> wrote:
> On Tue, Jan 3, 2017 at 11:58 PM, Kees Cook <keescook@...omium.org> wrote:
>> From: Matthew Garrett <mjg59@...eos.com>
>>
>> Various attacks are made possible due to the large attack surface of
>> kernel drivers and the easy availability of hotpluggable hardware that can
>> be programmed to mimic arbitrary devices. This allows attackers to find a
>> single vulnerable driver and then produce a device that can exploit it by
>> plugging into a hotpluggable bus (such as PCI or USB). This violates user
>> assumptions about unattended systems being secure as long as the screen
>> is locked.
>>
>> The kernel already has support for deferring driver binding in order
>> to avoid problems over suspend/resume. By exposing this to userspace we
>> can disable probing when the screen is locked and simply reenable it on
>> unlock.
>>
>> This is not a complete solution - since this still permits device
>> creation and simply blocks driver binding, it won't stop userspace
>> drivers from attaching to devices and it won't protect against any kernel
>> vulnerabilities in the core bus code. However, it should be sufficient to
>> block attacks like Poisontap (https://samy.pl/poisontap/).
>
> It also looks like this may be worked around by tricking the user to
> unlock the screen while the malicious device is still attached to the
> system.

It certainly changes the temporal aspect of the attack (i.e. there is
a delay and must be "silent" in that the local user cannot notice it).

> If that really is the case, I wonder if it's worth the extra complexity.

I think so, since it's not that much more complexity (it uses the
existing deferral mechanism).

-Kees

-- 
Kees Cook
Nexus Security