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

[Alexey Budankov]

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@xxxxxxxxxxxxxxx> 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)
>>>> {
>>>> ÂÂÂÂÂreturn 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

> 
>