Re: [PATCH v6 01/10] capabilities: introduce CAP_PERFMON to kernel and user space

[Stephen Smalley <sds@tycho.nsa.gov>]

On 2/5/20 12:30 PM, 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 performance monitoring
> and observability subsystems.
> 
> CAP_PERFMON hardens system security and integrity during performance
> monitoring and observability operations by decreasing attack surface that
> is available to a CAP_SYS_ADMIN privileged process [2]. Providing the 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 the operation more secure.
> Thus, CAP_PERFMON implements the principal of least privilege for performance
> monitoring and observability operations (POSIX IEEE 1003.1e: 2.2.2.39 principle
> of least privilege: A security design principle that states that a process
> or program be granted only those privileges (e.g., capabilities) necessary
> to accomplish its legitimate function, and only for the time that such
> privileges are actually required)
> 
> CAP_PERFMON meets the demand to secure system performance monitoring and
> observability operations for adoption in security sensitive, restricted,
> multiuser production environments (e.g. HPC clusters, cloud and virtual compute
> environments), where root or CAP_SYS_ADMIN credentials are not available to
> mass users of a system, and securely unblocks accessibility of system performance monitoring and observability operations beyond root and CAP_SYS_ADMIN use cases.
> 
> CAP_PERFMON takes over CAP_SYS_ADMIN credentials related to system performance
> monitoring and observability operations and balances 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." For backward compatibility reasons access to system
> performance monitoring and observability subsystems of the kernel remains
> open for CAP_SYS_ADMIN privileged processes but CAP_SYS_ADMIN capability
> usage for secure system performance monitoring and observability operations
> is discouraged with respect to the designed CAP_PERFMON capability.
> 
> 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 hardware issues mitigation procedure [2]. The bugs
> in the software itself can 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>

This will require a small update to the selinux-testsuite to correctly 
reflect the new capability requirements, but that's easy enough.

Acked-by: Stephen Smalley <sds@tycho.nsa.gov>

> ---
>   include/linux/capability.h          | 4 ++++
>   include/uapi/linux/capability.h     | 8 +++++++-
>   security/selinux/include/classmap.h | 4 ++--
>   3 files changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index ecce0f43c73a..027d7e4a853b 100644
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -251,6 +251,10 @@ 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)
> +{
> +	return capable(CAP_PERFMON) || capable(CAP_SYS_ADMIN);
> +}
>   
>   /* audit system wants to get cap info from files as well */
>   extern int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data *cpu_caps);
> diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
> index 240fdb9a60f6..8b416e5f3afa 100644
> --- a/include/uapi/linux/capability.h
> +++ b/include/uapi/linux/capability.h
> @@ -366,8 +366,14 @@ struct vfs_ns_cap_data {
>   
>   #define CAP_AUDIT_READ		37
>   
> +/*
> + * Allow system performance and observability privileged operations
> + * using perf_events, i915_perf and other kernel subsystems
> + */
> +
> +#define CAP_PERFMON		38
>   
> -#define CAP_LAST_CAP         CAP_AUDIT_READ
> +#define CAP_LAST_CAP         CAP_PERFMON
>   
>   #define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP)
>   
> diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
> index 7db24855e12d..c599b0c2b0e7 100644
> --- a/security/selinux/include/classmap.h
> +++ b/security/selinux/include/classmap.h
> @@ -27,9 +27,9 @@
>   	    "audit_control", "setfcap"
>   
>   #define COMMON_CAP2_PERMS  "mac_override", "mac_admin", "syslog", \
> -		"wake_alarm", "block_suspend", "audit_read"
> +		"wake_alarm", "block_suspend", "audit_read", "perfmon"
>   
> -#if CAP_LAST_CAP > CAP_AUDIT_READ
> +#if CAP_LAST_CAP > CAP_PERFMON
>   #error New capability defined, please update COMMON_CAP2_PERMS.
>   #endif
>   
>