linux-kernel - Re: [PATCH v5 01/10] capabilities: introduce CAP

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Message-ID: <8141da2e-49cf-c02d-69e9-8a7cbdc91431@linux.intel.com>
Date:   Wed, 12 Feb 2020 16:53:48 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Stephen Smalley <sds@...ho.nsa.gov>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "jani.nikula@...ux.intel.com" <jani.nikula@...ux.intel.com>,
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        Paul Mackerras <paulus@...ba.org>,
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        <james.bottomley@...senpartnership.com>,
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        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
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        oprofile-list@...ts.sf.net, Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v5 01/10] capabilities: introduce CAP_PERFMON to kernel
 and user space

On 12.02.2020 16:32, Stephen Smalley wrote:
> On 2/12/20 3:53 AM, Alexey Budankov wrote:
>> Hi Stephen,
>>
>> On 22.01.2020 17:07, Stephen Smalley wrote:
>>> On 1/22/20 5:45 AM, Alexey Budankov wrote:
>>>>
>>>> On 21.01.2020 21:27, Alexey Budankov wrote:
>>>>>
>>>>> On 21.01.2020 20:55, Alexei Starovoitov wrote:
>>>>>> On Tue, Jan 21, 2020 at 9:31 AM Alexey Budankov
>>>>>> <alexey.budankov@...ux.intel.com> wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 21.01.2020 17:43, Stephen Smalley wrote:
>>>>>>>> On 1/20/20 6:23 AM, Alexey Budankov wrote:
>>>>>>>>>
>> <SNIP>
>>>>>>>>> Introduce CAP_PERFMON capability designed to secure system performance
>>>>>>>>
>>>>>>>> Why _noaudit()?  Normally only used when a permission failure is non-fatal to the operation.  Otherwise, we want the audit message.
>>>>
>>>> So far so good, I suggest using the simplest version for v6:
>>>>
>>>> static inline bool perfmon_capable(void)
>>>> {
>>>>      return capable(CAP_PERFMON) || capable(CAP_SYS_ADMIN);
>>>> }
>>>>
>>>> It keeps the implementation simple and readable. The implementation is more
>>>> performant in the sense of calling the API - one capable() call for CAP_PERFMON
>>>> privileged process.
>>>>
>>>> Yes, it bloats audit log for CAP_SYS_ADMIN privileged and unprivileged processes,
>>>> but this bloating also advertises and leverages using more secure CAP_PERFMON
>>>> based approach to use perf_event_open system call.
>>>
>>> I can live with that.  We just need to document that when you see both a CAP_PERFMON and a CAP_SYS_ADMIN audit message for a process, try only allowing CAP_PERFMON first and see if that resolves the issue.  We have a similar issue with CAP_DAC_READ_SEARCH versus CAP_DAC_OVERRIDE.
>>
>> I am trying to reproduce this double logging with CAP_PERFMON.
>> I am using the refpolicy version with enabled perf_event tclass [1], in permissive mode.
>> When running perf stat -a I am observing this AVC audit messages:
>>
>> type=AVC msg=audit(1581496695.666:8691): avc:  denied  { open } for  pid=2779 comm="perf" scontext=user_u:user_r:user_systemd_t tcontext=user_u:user_r:user_systemd_t tclass=perf_event permissive=1
>> type=AVC msg=audit(1581496695.666:8691): avc:  denied  { kernel } for  pid=2779 comm="perf" scontext=user_u:user_r:user_systemd_t tcontext=user_u:user_r:user_systemd_t tclass=perf_event permissive=1
>> type=AVC msg=audit(1581496695.666:8691): avc:  denied  { cpu } for  pid=2779 comm="perf" scontext=user_u:user_r:user_systemd_t tcontext=user_u:user_r:user_systemd_t tclass=perf_event permissive=1
>> type=AVC msg=audit(1581496695.666:8692): avc:  denied  { write } for  pid=2779 comm="perf" scontext=user_u:user_r:user_systemd_t tcontext=user_u:user_r:user_systemd_t tclass=perf_event permissive=1
>>
>> However there is no capability related messages around. I suppose my refpolicy should
>> be modified somehow to observe capability related AVCs.
>>
>> Could you please comment or clarify on how to enable caps related AVCs in order
>> to test the concerned logging.
> 
> The new perfmon permission has to be defined in your policy; you'll have a message in dmesg about "Permission perfmon in class capability2 not defined in policy.".  You can either add it to the common cap2 definition in refpolicy/policy/flask/access_vectors and rebuild your policy or extract your base module as CIL, add it there, and insert the updated module.

Yes, I already have it like this:
common cap2
{
<------>mac_override<--># unused by SELinux
<------>mac_admin
<------>syslog
<------>wake_alarm
<------>block_suspend
<------>audit_read
<------>perfmon
}

dmesg stopped reporting perfmon as not defined but audit.log still doesn't report CAP_PERFMON denials.
BTW, audit even doesn't report CAP_SYS_ADMIN denials, however perfmon_capable() does check for it.

~Alexey

> 
>