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Subject: Re: [PATCH v5 01/10] capabilities: introduce CAP_PERFMON to kernel
 and user space
To:     Stephen Smalley <sds@tycho.nsa.gov>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>
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From:   Alexey Budankov <alexey.budankov@linux.intel.com>
Organization: Intel Corp.
Message-ID: <ac0dbab7-de47-ee34-bb88-4c43d3538b7d@linux.intel.com>
Date:   Wed, 22 Jan 2020 17:25:09 +0300
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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:
>>>>>>>
>>>>>>> Introduce CAP_PERFMON capability designed to secure system performance
>>>>>>> monitoring and observability operations so that CAP_PERFMON would assist
>>>>>>> CAP_SYS_ADMIN capability in its governing role for perf_events, i915_perf
>>>>>>> and other performance monitoring and observability subsystems.
>>>>>>>
>>>>>>> CAP_PERFMON intends to harden system security and integrity during system
>>>>>>> performance monitoring and observability operations by decreasing attack
>>>>>>> surface that is available to a CAP_SYS_ADMIN privileged process [1].
>>>>>>> Providing access to system performance monitoring and observability
>>>>>>> operations under CAP_PERFMON capability singly, without the rest of
>>>>>>> CAP_SYS_ADMIN credentials, excludes chances to misuse the credentials and
>>>>>>> makes operation more secure.
>>>>>>>
>>>>>>> CAP_PERFMON intends to take over CAP_SYS_ADMIN credentials related to
>>>>>>> system performance monitoring and observability operations and balance
>>>>>>> amount of CAP_SYS_ADMIN credentials following the recommendations in the
>>>>>>> capabilities man page [1] for CAP_SYS_ADMIN: "Note: this capability is
>>>>>>> overloaded; see Notes to kernel developers, below."
>>>>>>>
>>>>>>> Although the software running under CAP_PERFMON can not ensure avoidance
>>>>>>> of related hardware issues, the software can still mitigate these issues
>>>>>>> following the official embargoed hardware issues mitigation procedure [2].
>>>>>>> The bugs in the software itself could be fixed following the standard
>>>>>>> kernel development process [3] to maintain and harden security of system
>>>>>>> performance monitoring and observability operations.
>>>>>>>
>>>>>>> [1] http://man7.org/linux/man-pages/man7/capabilities.7.html
>>>>>>> [2] https://www.kernel.org/doc/html/latest/process/embargoed-hardware-issues.html
>>>>>>> [3] https://www.kernel.org/doc/html/latest/admin-guide/security-bugs.html
>>>>>>>
>>>>>>> Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
>>>>>>> ---
>>>>>>>    include/linux/capability.h          | 12 ++++++++++++
>>>>>>>    include/uapi/linux/capability.h     |  8 +++++++-
>>>>>>>    security/selinux/include/classmap.h |  4 ++--
>>>>>>>    3 files changed, 21 insertions(+), 3 deletions(-)
>>>>>>>
>>>>>>> diff --git a/include/linux/capability.h b/include/linux/capability.h
>>>>>>> index ecce0f43c73a..8784969d91e1 100644
>>>>>>> --- a/include/linux/capability.h
>>>>>>> +++ b/include/linux/capability.h
>>>>>>> @@ -251,6 +251,18 @@ extern bool privileged_wrt_inode_uidgid(struct user_namespace *ns, const struct
>>>>>>>    extern bool capable_wrt_inode_uidgid(const struct inode *inode, int cap);
>>>>>>>    extern bool file_ns_capable(const struct file *file, struct user_namespace *ns, int cap);
>>>>>>>    extern bool ptracer_capable(struct task_struct *tsk, struct user_namespace *ns);
>>>>>>> +static inline bool perfmon_capable(void)
>>>>>>> +{
>>>>>>> +    struct user_namespace *ns = &init_user_ns;
>>>>>>> +
>>>>>>> +    if (ns_capable_noaudit(ns, CAP_PERFMON))
>>>>>>> +        return ns_capable(ns, CAP_PERFMON);
>>>>>>> +
>>>>>>> +    if (ns_capable_noaudit(ns, CAP_SYS_ADMIN))
>>>>>>> +        return ns_capable(ns, CAP_SYS_ADMIN);
>>>>>>> +
>>>>>>> +    return false;
>>>>>>> +}
>>>>>>
>>>>>> 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.

perf security [1] document can be updated, at least, to align and document 
this audit logging specifics.

~Alexey

[1] https://www.kernel.org/doc/html/latest/admin-guide/perf-security.html