Re: [PATCH] cgroup: Add new capability to allow a process to migrate other tasks between cgroups

[John Stultz]

On Mon, Oct 17, 2016 at 3:40 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Mon, Oct 17, 2016 at 3:35 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
>> This patch adds CAP_GROUP_MIGRATE 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.
>>
>> The approach taken there overloads CAP_SYS_NICE a bit much, and
>> is maybe more complicated then needed.
>>
>> So this patch, as suggested by Tejun,  simply adds a new process
>> capability flag (CAP_CGROUP_MIGRATE), 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 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!
>>
>> Cc: Tejun Heo <tj@xxxxxxxxxx>
>> Cc: Li Zefan <lizefan@xxxxxxxxxx>
>> Cc: Jonathan Corbet <corbet@xxxxxxx>
>> Cc: cgroups@xxxxxxxxxxxxxxx
>> Cc: Android Kernel Team <kernel-team@xxxxxxxxxxx>
>> Cc: Rom Lemarchand <romlem@xxxxxxxxxxx>
>> Cc: Colin Cross <ccross@xxxxxxxxxxx>
>> Cc: Dmitry Shmidt <dimitrysh@xxxxxxxxxx>
>> Cc: Ricky Zhou <rickyz@xxxxxxxxxxxx>
>> Cc: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
>> Cc: Todd Kjos <tkjos@xxxxxxxxxx>
>> Cc: Christian Poetzsch <christian.potzsch@xxxxxxxxxx>
>> Cc: Amit Pundir <amit.pundir@xxxxxxxxxx>
>> Cc: Serge E. Hallyn <serge@xxxxxxxxxx>
>> Cc: linux-api@xxxxxxxxxxxxxxx
>> Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
>> ---
>> v2: Renamed to just CAP_CGROUP_MIGRATE as reccomended by Tejun
>> ---
>>  include/uapi/linux/capability.h | 5 ++++-
>>  kernel/cgroup.c                 | 3 ++-
>>  2 files changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
>> index 49bc062..44d7ff4 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 migrating tasks between cgroups */
>>
>> -#define CAP_LAST_CAP         CAP_AUDIT_READ
>> +#define CAP_CGROUP_MIGRATE     38
>> +
>> +#define CAP_LAST_CAP         CAP_CGROUP_MIGRATE
>>
>>  #define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP)
>>
>> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
>> index 85bc9be..09f84d2 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_MIGRATE))
>>                 ret = -EACCES;
>
> This logic seems rather confused to me.  Without this patch, a user
> can write to procs if it's root *or* it matches the target uid *or* it
> matches the target suid.  How does this make sense?  How about
> ptrace_may_access(...) || ns_capable(tcred->user_ns,
> CAP_CGROUP_MIGRATE)?

Though ptrace_may_access would open it also to apps with
CAP_SYS_PTRACE as well, no?

Would pulling out from __ptrace_may_access the:
 if (uid_eq(caller_uid, tcred->euid) &&
            uid_eq(caller_uid, tcred->suid) &&
            uid_eq(caller_uid, tcred->uid)  &&
            gid_eq(caller_gid, tcred->egid) &&
            gid_eq(caller_gid, tcred->sgid) &&
            gid_eq(caller_gid, tcred->gid))
                goto ok;

check and creating a new helper that could be shared between them be
the right approach?

thanks
-john