linux-kernel - Re: [kernel-hardening] Re: [PATCH 2/2] security, perf: Add config option to restrict perf_event

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Date:	Tue, 2 Aug 2016 08:19:38 -0700
From:	Kees Cook <keescook@...omium.org>
To:	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>
Cc:	Jeff Vander Stoep <jeffv@...gle.com>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [kernel-hardening] Re: [PATCH 2/2] security, perf: Add config
 option to restrict perf_event_open

On Tue, Aug 2, 2016 at 2:55 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, Jul 27, 2016 at 07:46:23AM -0700, Jeff Vander Stoep wrote:
>> +++ b/kernel/events/core.c
>> @@ -355,7 +355,11 @@ static struct srcu_struct pmus_srcu;
>>   *   2 - disallow kernel profiling for unpriv
>>   *   3 - disallow all unpriv perf event use
>>   */
>> +#ifdef CONFIG_SECURITY_PERF_EVENTS_RESTRICT
>> +int sysctl_perf_event_paranoid __read_mostly = 3;
>> +#else
>>  int sysctl_perf_event_paranoid __read_mostly = 2;
>> +#endif
>>
>>  /* Minimum for 512 kiB + 1 user control page */
>>  int sysctl_perf_event_mlock __read_mostly = 512 + (PAGE_SIZE / 1024); /* 'free' kiB per user */
>> diff --git a/security/Kconfig b/security/Kconfig
>> index df28f2b..2a93551 100644
>> --- a/security/Kconfig
>> +++ b/security/Kconfig
>> @@ -18,6 +18,15 @@ config SECURITY_DMESG_RESTRICT
>>
>>         If you are unsure how to answer this question, answer N.
>>
>> +config SECURITY_PERF_EVENTS_RESTRICT
>> +     bool "Restrict unprivileged use of performance events"
>> +     depends on PERF_EVENTS
>> +     help
>> +       If you say Y here, the kernel.perf_event_paranoid sysctl
>> +       will be set to 3 by default, and no unprivileged use of the
>> +       perf_event_open syscall will be permitted unless it is
>> +       changed.
>
> NAK.
>
> Apart from the fact that I hate the 3 thing this is not how you do
> default CONFIG knobs for !bool state variables.
>
> Use an "int" config not a "bool" config and allow all options to be
> default.

How about leaving off 2/2 and just keeping 1/2 of this series?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security