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

[Alexey Budankov <alexey.budankov@linux.intel.com>]

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.
>>
>> 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.
>