linux-kernel - Re: [PATCH v5 01/10] capabilities: introduce CAP

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Message-ID: <220e87bf-226e-ca35-acc0-89dc82dc7811@linux.intel.com>
Date:   Wed, 12 Feb 2020 19:16:31 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Stephen Smalley <sds@...ho.nsa.gov>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "jani.nikula@...ux.intel.com" <jani.nikula@...ux.intel.com>,
        "joonas.lahtinen@...ux.intel.com" <joonas.lahtinen@...ux.intel.com>,
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        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
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        <james.bottomley@...senpartnership.com>,
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        James Morris <jmorris@...ei.org>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Robert Richter <rric@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
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        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
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        <linux-security-module@...r.kernel.org>,
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        oprofile-list@...ts.sf.net, Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v5 01/10] capabilities: introduce CAP_PERFMON to kernel
 and user space

On 12.02.2020 18:21, 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@...ux.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.

Yes, that all makes sense.
selinux_capable() calls avc_audit() logging but cap_capable() doesn't, so proper order matters.
I am doing debug tracing of the kernel code to reveal the exact reasons.

~Alexey