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

[Stephen Smalley]

On 2/12/20 11:56 AM, Alexey Budankov wrote:
> 
> 
> On 12.02.2020 18:45, Stephen Smalley wrote:
>> 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 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.
>>
>> 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).
> 
> It is user_t for local terminal session:
> ps -Z
> LABEL                             PID TTY          TIME CMD
> user_u:user_r:user_t            11317 pts/9    00:00:00 bash
> user_u:user_r:user_t            11796 pts/9    00:00:00 ps
> 
> For local terminal root session:
> ps -Z
> LABEL                             PID TTY          TIME CMD
> user_u:user_r:user_su_t          2926 pts/3    00:00:00 bash
> user_u:user_r:user_su_t         10995 pts/3    00:00:00 ps
> 
> For remote ssh session:
> ps -Z
> LABEL                             PID TTY          TIME CMD
> user_u:user_r:user_t             7540 pts/8    00:00:00 ps
> user_u:user_r:user_systemd_t     8875 pts/8    00:00:00 bash

That's a bug in either your policy or your userspace/distro integration. 
  In any event, unless user_systemd_t is allowed all capability2 
permissions by your policy, you should see the denials if CAP_PERFMON is 
set in the effective capability set of the process.