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Subject: Re: [PATCH v5 01/10] capabilities: introduce CAP_PERFMON to kernel
 and user space
From:   Alexey Budankov <alexey.budankov@linux.intel.com>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Stephen Smalley <sds@tycho.nsa.gov>
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Date:   Wed, 22 Jan 2020 13:45:45 +0300
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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.

~Alexey

>>>
>>> Some of ideas from v4 review.
>>
>> well, in the requested changes form v4 I wrote:
>> return capable(CAP_PERFMON);
>> instead of
>> return false;
> 
> Aww, indeed. I was concerning exactly about it when updating the patch
> and simply put false, missing the fact that capable() also logs.
> 
> I suppose the idea is originally from here [1].
> BTW, Has it already seen any _more optimal_ implementation?
> Anyway, original or optimized version could be reused for CAP_PERFMON.
> 
> ~Alexey
> 
> [1] https://patchwork.ozlabs.org/patch/1159243/
> 
>>
>> That's what Andy suggested earlier for CAP_BPF.
>> I think that should resolve Stephen's concern.
>>