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

[Stephen Smalley]

On 2/12/20 8:53 AM, Alexey Budankov wrote:
> 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 at 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.
>>
>> 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.

Some denials may be silenced by dontaudit rules; semodule -DB will strip 
those and semodule -B will restore them.  Other possibility is that the 
process doesn't have CAP_PERFMON in its effective set and therefore 
never reaches SELinux at all; denied first by the capability module.