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

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Message-ID: <dd6a1382-7b2f-a6e6-a1ac-009566d7f556@tycho.nsa.gov>
Date:   Wed, 12 Feb 2020 08:32:07 -0500
From:   Stephen Smalley <sds@...ho.nsa.gov>
To:     Alexey Budankov <alexey.budankov@...ux.intel.com>
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>,
        "joonas.lahtinen@...ux.intel.com" <joonas.lahtinen@...ux.intel.com>,
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        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        "james.bottomley@...senpartnership.com" 
        <james.bottomley@...senpartnership.com>,
        Serge Hallyn <serge@...lyn.com>,
        James Morris <jmorris@...ei.org>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Robert Richter <rric@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
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        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
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        linux-kernel <linux-kernel@...r.kernel.org>,
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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 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.