Re: [PATCH v5 01/10] capabilities: introduce CAP_PERFMON to kernel and user space

From: Alexey Budankov <alexey.budankov@linux.intel.com>
To: Stephen Smalley <sds@tycho.nsa.gov>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
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Subject: Re: [PATCH v5 01/10] capabilities: introduce CAP_PERFMON to kernel and user space
Date: Wed, 12 Feb 2020 11:53:27 +0300	[thread overview]
Message-ID: <2e38c33d-f085-1320-8cc2-45f74b6ad86d@linux.intel.com> (raw)
In-Reply-To: <d7213569-9578-7201-6106-f5ebc95bd6be@tycho.nsa.gov>

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@linux.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.

Thanks,
Alexey

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[1] https://github.com/SELinuxProject/refpolicy.git