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

[Stephen Smalley]

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@xxxxxxxxxxxxxxx>
> ---
>   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.