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

Kees Cook
2017-01-03T23:38:24Z

On Tue, Jan 3, 2017 at 3:34 PM, Rafael J. Wysocki <rafael@kernel.org> wrote:
> On Tue, Jan 3, 2017 at 11:58 PM, Kees Cook <keescook@chromium.org> wrote:
>> From: Matthew Garrett <mjg59@coreos.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