This patch adds CAP_GROUP and logic to allows a process to migrate other tasks between cgroups.
In Android (where this feature originated), the ActivityManager tracks various application states (TOP_APP, FOREGROUND, BACKGROUND, SYSTEM, etc), and then as applications change states, the SchedPolicy logic will migrate the application tasks between different cgroups used to control the different application states (for example, there is a background cpuset cgroup which can limit background tasks to stay on one low-power cpu, and the bg_non_interactive cpuctrl cgroup can then further limit those background tasks to a small percentage of that one cpu's cpu time). However, for security reasons, Android doesn't want to make the system_server (the process that runs the ActivityManager and SchedPolicy logic), run as root. So in the Android common.git kernel, they have some logic to allow cgroups to loosen their permissions so CAP_SYS_NICE tasks can migrate other tasks between cgroups. I feel the approach taken there overloads CAP_SYS_NICE a bit much for non-android environments. Efforts to re-use CAP_SYS_RESOURCE for this purpose (which Android has since adopted) was also stymied by concerns about risks from future cgroups that could be considered "dangerous" by how they might change system semantics. So to avoid overlapping usage, this patch adds a brand new process capability flag (CAP_CGROUP), and uses it when checking if a task can migrate other tasks between cgroups. I've tested this with AOSP master (though its a bit hacked in as I still need to properly get the selinux userspace bits aware of the new capability bit) with selinux set to permissive and it seems to be working well. Thoughts and feedback would be appreciated! (Note, I'm going on holiday break after today, so I may not respond to feedback immediately, but I figured it would be better to give folks the chance to review this rather then sit it for two weeks. I'll resend after the new-year, addressing any feedback I do get.) Cc: Tejun Heo <[email protected]> Cc: Li Zefan <[email protected]> Cc: Jonathan Corbet <[email protected]> Cc: [email protected] Cc: Android Kernel Team <[email protected]> Cc: Rom Lemarchand <[email protected]> Cc: Colin Cross <[email protected]> Cc: Dmitry Shmidt <[email protected]> Cc: Todd Kjos <[email protected]> Cc: Christian Poetzsch <[email protected]> Cc: Amit Pundir <[email protected]> Cc: Dmitry Torokhov <[email protected]> Cc: Kees Cook <[email protected]> Cc: Serge E. Hallyn <[email protected]> Cc: Andy Lutomirski <[email protected]> Cc: [email protected] Cc: Paul Moore <[email protected]> Cc: Stephen Smalley <[email protected]> Cc: Eric Paris <[email protected]> Cc: James Morris <[email protected]> Cc: Jeff Vander Stoep <[email protected]> Cc: [email protected] Cc: [email protected] Signed-off-by: John Stultz <[email protected]> --- v2: Renamed to just CAP_CGROUP_MIGRATE as reccomended by Tejun v3: Switched to just using CAP_SYS_RESOURCE as suggested by Michael v4: Send out properly folded down version of the patch. :P v5: Switch back to CAP_CGROUP_MIGRATE due to concerns from Andy v6: Rename to CAP_CGROUP, as it might be used for other purposes in the future. Also added selinux mappings for the new cap. --- include/uapi/linux/capability.h | 5 ++++- kernel/cgroup.c | 3 ++- security/selinux/include/classmap.h | 4 ++-- 3 files changed, 8 insertions(+), 4 deletions(-) diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h index 49bc062..726f767 100644 --- a/include/uapi/linux/capability.h +++ b/include/uapi/linux/capability.h @@ -349,8 +349,11 @@ struct vfs_cap_data { #define CAP_AUDIT_READ 37 +/* Allow migration of other tasks between cgroups */ -#define CAP_LAST_CAP CAP_AUDIT_READ +#define CAP_CGROUP 38 + +#define CAP_LAST_CAP CAP_CGROUP #define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP) diff --git a/kernel/cgroup.c b/kernel/cgroup.c index 2ee9ec3..8b42ae3 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -2856,7 +2856,8 @@ static int cgroup_procs_write_permission(struct task_struct *task, */ if (!uid_eq(cred->euid, GLOBAL_ROOT_UID) && !uid_eq(cred->euid, tcred->uid) && - !uid_eq(cred->euid, tcred->suid)) + !uid_eq(cred->euid, tcred->suid) && + !ns_capable(tcred->user_ns, CAP_CGROUP)) ret = -EACCES; if (!ret && cgroup_on_dfl(dst_cgrp)) { diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h index e2d4ad3a..ee8c1ed 100644 --- a/security/selinux/include/classmap.h +++ b/security/selinux/include/classmap.h @@ -22,9 +22,9 @@ "audit_control", "setfcap" #define COMMON_CAP2_PERMS "mac_override", "mac_admin", "syslog", \ - "wake_alarm", "block_suspend", "audit_read" + "wake_alarm", "block_suspend", "audit_read", "cgroup" -#if CAP_LAST_CAP > CAP_AUDIT_READ +#if CAP_LAST_CAP > CAP_CGROUP #error New capability defined, please update COMMON_CAP2_PERMS. #endif -- 2.7.4
