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

[Stephen Smalley <sds@tycho.nsa.gov>]

On 2/12/20 10:21 AM, Stephen Smalley wrote:
> 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@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.

Also, the fact that your denials are showing up in user_systemd_t 
suggests that something is off in your policy or userspace/distro; I 
assume that is a domain type for the systemd --user instance, but your 
shell and commands shouldn't be running in that domain (user_t would be 
more appropriate for that).