Re: [kernel-hardening] [PATCH 1/2] security, perf: allow further restriction of perf_event_open

From: Kees Cook
Date: Tue Oct 18 2016 - 16:49:05 EST

On Mon, Oct 17, 2016 at 6:44 AM, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> Hi,
> Attempt to revive discussions below...
> On Wed, Jul 27, 2016 at 07:45:46AM -0700, Jeff Vander Stoep wrote:
>> When kernel.perf_event_paranoid is set to 3 (or greater), disallow
>> all access to performance events by users without CAP_SYS_ADMIN.
>> This new level of restriction is intended to reduce the attack
>> surface of the kernel. Perf is a valuable tool for developers but
>> is generally unnecessary and unused on production systems. Perf may
>> open up an attack vector to vulnerable device-specific drivers as
>> recently demonstrated in CVE-2016-0805, CVE-2016-0819,
>> CVE-2016-0843, CVE-2016-3768, and CVE-2016-3843. This new level of
>> restriction allows for a safe default to be set on production systems
>> while leaving a simple means for developers to grant access [1].
>> This feature is derived from CONFIG_GRKERNSEC_PERF_HARDEN by Brad
>> Spengler. It is based on a patch by Ben Hutchings [2]. Ben's patches
>> have been modified and split up to address on-list feedback.
>> kernel.perf_event_paranoid=3 is the default on both Debian [2] and
>> Android [3].
> While people weren't particularly happy with this global toggle
> approach, my understanding from face-to-face discussions at LSS2016 was
> that people were happy with a more scoped restriction (e.g. using
> capabilities or some other access control mechanism), but no-one had the
> time to work on that.
> Does that match everyone's understanding, or am I mistaken?

That's correct: some kind of finer-grain control would be preferred to
the maintainer, but no one has had time to work on it. (The =3 sysctl
setting present in Android, Debian, and Ubuntu satisfies most people.)


Kees Cook
Nexus Security