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

[Alexey Budankov]

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@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)
>>>>>>> {
>>>>>>> ÂÂÂÂÂÂreturn 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

~Alexey