Date:   Sun, 15 Dec 2019 14:53:35 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Stephen Smalley <sds@...ho.nsa.gov>,
        Andi Kleen <ak@...ux.intel.com>,
        Casey Schaufler <casey@...aufler-ca.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
        elena.reshetova@...el.com,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Stephane Eranian <eranian@...gle.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        James Morris <jmorris@...ei.org>
Subject: Re: [PATCH v1 0/3] Introduce CAP_SYS_PERFMON capability for secure
 Perf users groups


On 12.12.2019 17:24, Stephen Smalley wrote:
> On 12/11/19 3:36 PM, Andi Kleen wrote:
>>>> In this circumstances CAP_SYS_PERFMON looks like smart balanced advancement that
>>>> trade-offs between perf_events subsystem extensions, required level of control
>>>> and configurability of perf_events, existing users adoption effort, and it brings
>>>> security hardening benefits of decreasing attack surface for the existing users
>>>> and use cases.
>>>
>>> I'm not 100% opposed to CAP_SYS_PERFMON. I am 100% opposed to new capabilities
>>> that have a single use. Surely there are other CAP_SYS_ADMIN users that [cs]ould
>>> be converted to CAP_SYS_PERFMON as well. If there is a class of system performance
>>> privileged operations, say a dozen or so, you may have a viable argument.
>>
>> perf events is not a single use. It has a bazillion of sub functionalities,
>> including hardware tracing, software tracing, pmu counters, software counters,
>> uncore counters, break points and various other stuff in its PMU drivers.
>>
>> See it more as a whole quite heterogenous driver subsystem.
>>
>> I guess CAP_SYS_PERFMON is not a good name because perf is much more
>> than just Perfmon. Perhaps call it CAP_SYS_PERF_EVENTS
> 
> That seems misleading since it isn't being checked for all perf_events operations IIUC (CAP_SYS_ADMIN is still required for some?) and it is even more specialized than CAP_SYS_PERFMON, making it less likely that we could ever use this capability as a check for other kernel performance monitoring facilities beyond perf_events.
> 
> I'm not as opposed to fine-grained capabilities as Casey is but I do recognize that there are a limited number of available bits (although we do have a fair number of unused ones currently given the extension to 64-bits) and that it would be easy to consume them all if we allocated one for every kernel feature.  That said, this might be a sufficiently important use case to justify it.
> 
> Obviously I'd encourage you to consider leveraging SELinux as well but I understand that you are looking for a solution that doesn't depend on a distro using a particular LSM or a particular policy.  I will note that SELinux doesn't suffer from the limited bits problem because one can always define a new SELinux security class with its own access vector permissions bitmap, as has been done for the recently added LSM/SELinux perf_event hooks.
> 
> I don't know who actually gets to decide when/if a new capability is allocated.  Maybe Serge and/or James as capabilities and LSM maintainers.
> 
> I have no objections to these patches from a SELinux POV.

Stephen, thanks for meaningful input!

~Alexey

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