linux-kernel - Re: [PATCH v1 3/3] perf-security: document perf

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Date:   Thu, 7 Feb 2019 16:14:16 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Jonathan Corbet <corbet@....net>
Cc:     Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Jann Horn <jannh@...gle.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>,
        Andi Kleen <ak@...ux.intel.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 3/3] perf-security: document perf_events/Perf resource
 control


On 07.02.2019 3:01, Jonathan Corbet wrote:
> On Fri, 1 Feb 2019 10:30:58 +0300
> Alexey Budankov <alexey.budankov@...ux.intel.com> wrote:
> 
>> Elaborate on possible perf_event/Perf privileged users groups
>> and document steps about creating such groups.
>>
>> Signed-off-by: Alexey Budankov <alexey.budankov@...ux.intel.com>
>> ---
>>  Documentation/admin-guide/perf-security.rst | 43 +++++++++++++++++++++
>>  1 file changed, 43 insertions(+)
>>
>> diff --git a/Documentation/admin-guide/perf-security.rst b/Documentation/admin-guide/perf-security.rst
>> index 7da7fa459718..fe90f8952be9 100644
>> --- a/Documentation/admin-guide/perf-security.rst
>> +++ b/Documentation/admin-guide/perf-security.rst
>> @@ -73,6 +73,48 @@ enable capturing of additional data required for later performance analysis of
>>  monitored processes or a system. For example, CAP_SYSLOG capability permits
>>  reading kernel space memory addresses from /proc/kallsyms file.
>>  
>> +perf_events/Perf privileged users
>> +---------------------------------
>> +
>> +Mechanisms of capabilities, privileged capability-dumb files [6]_ and file system
>> +ACLs [10]_ can be used to create a dedicated group of perf_events/Perf privileged
>> +users who are permitted to execute performance monitoring without *scope* limits.
>> +The following steps can be taken to create such a group of privileged Perf user
>> +
>> +1. Create perf_users group of privileged Perf users, assign perf_users group to
>> +   Perf tool executable and limit *access* to the executable for other users in
>> +   the system:
>> +
>> +::
>> +
>> +   # groupadd perf_users
>> +   # ls -alhF
>> +   -rwxr-xr-x  2 root root  11M Oct 19 15:12 perf
>> +   # chgrp perf_users perf
>> +   # ls -alhF
>> +   -rwxr-xr-x  2 root perf_users  11M Oct 19 15:12 perf
>> +   # chmod o-rwx perf
>> +   # ls -alhF
>> +   -rwxr-x---  2 root perf_users  11M Oct 19 15:12 perf
> 
> Since we're giving basic sysadmin info here, we should probably say
> explicitly that this will block access to the perf binary to anybody who
> is not in the perf_users group.

We already say that above:
	"limit *access* to the executable for other users in the system".

Let's have it this way then:
	"limit access to the executable for other users in the system 
         who are not in the perf_users group".

> 
>> +2. Assign required capabilities to the Perf tool executable file and enable
> 
> Assign *the* required

Accepted.

> 
>> +   members of perf_users group with performance monitoring privileges [6]_ :
>> +
>> +::
>> +
>> +   # setcap "cap_sys_admin,cap_sys_ptrace,cap_syslog=ep" perf
>> +   # setcap -v "cap_sys_admin,cap_sys_ptrace,cap_syslog=ep" perf
>> +   perf: OK
>> +   # getcap perf
>> +   perf = cap_sys_ptrace,cap_sys_admin,cap_syslog+ep
>> +
>> +As a result, members of perf_users group are capable of conducting performance
>> +monitoring by using functionality of the configured Perf tool executable that,
>> +when executes, passes perf_events subsystem *scope* checks.
>> +
>> +This specific *access* control management is only available to superuser or root
> 
> Why the *emphasis* here?  We prefer to minimize this kind of markup
> whenever possible.

Avoided emphasis here and in the other places of this paragraph.

> 
> Thanks,
> 
> jon
> 

Thanks,
Alexey