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Date: Tue, 3 Jan 2017 15:38:24 -0800
From: Kees Cook <>
To: "Rafael J. Wysocki" <>
Cc: Greg Kroah-Hartman <>, 
	Linux Kernel Mailing List <>, "Rafael J. Wysocki" <>, 
	Len Brown <>, Pavel Machek <>, Matthew Garrett <>, 
	Ulf Hansson <>, Mauro Carvalho Chehab <>, 
	Tomeu Vizoso <>, Lukas Wunner <>, 
	Madalin Bucur <>, Sudip Mukherjee <>, 
	Rasmus Villemoes <>, Arnd Bergmann <>, 
	Andrew Morton <>, Russell King <>, 
	Petr Tesarik <>, Linux PM <>, 
	Kernel Hardening <>
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 <> wrote:
> On Tue, Jan 3, 2017 at 11:58 PM, Kees Cook <> wrote:
>> From: Matthew Garrett <>
>> 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 (
> 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 Cook
Nexus Security

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