Hi John, On 10/18/2016 06:54 PM, John Stultz wrote: > On Tue, Oct 18, 2016 at 1:17 AM, Michael Kerrisk (man-pages) > <mtk.manpa...@gmail.com> wrote: >> Hi John, >> >> On 18 October 2016 at 01:35, John Stultz <john.stu...@linaro.org> wrote: >>> On Mon, Oct 17, 2016 at 3:40 PM, Andy Lutomirski <l...@amacapital.net> >>> wrote: >>>> On Mon, Oct 17, 2016 at 3:35 PM, John Stultz <john.stu...@linaro.org> >>>> 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 <t...@kernel.org> >>>>> Cc: Li Zefan <lize...@huawei.com> >>>>> Cc: Jonathan Corbet <cor...@lwn.net> >>>>> Cc: cgro...@vger.kernel.org >>>>> Cc: Android Kernel Team <kernel-t...@android.com> >>>>> Cc: Rom Lemarchand <rom...@android.com> >>>>> Cc: Colin Cross <ccr...@android.com> >>>>> Cc: Dmitry Shmidt <dimitr...@google.com> >>>>> Cc: Ricky Zhou <ric...@chromium.org> >>>>> Cc: Dmitry Torokhov <dmitry.torok...@gmail.com> >>>>> Cc: Todd Kjos <tk...@google.com> >>>>> Cc: Christian Poetzsch <christian.potz...@imgtec.com> >>>>> Cc: Amit Pundir <amit.pun...@linaro.org> >>>>> Cc: Serge E. Hallyn <se...@hallyn.com> >>>>> Cc: linux-...@vger.kernel.org >>>>> Signed-off-by: John Stultz <john.stu...@linaro.org> >>>>> --- >>>>> 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? >> >> So, is creating a new capability here necessarily the right approach? >> Is this operation so unique, or is there an existing silo (not >> CAP_SYS_ADMIN) that we can re-use? I ask, because we currently use 38 >> silos out of a possible 64 capabilities, and when everyone chooses >> single-use capabilities, we will quickly exhaust the silos. > > Agreed this is a concern, and CGROUP_MIGRATE is maybe too narrow of a > specification for something so limited. > >> I'm not saying that creating a new capability here is wrong, but it is >> worth further considering the existing silos to see if there is one >> that is a suitable match. >> >> Looking at http://man7.org/linux/man-pages/man7/capabilities.7.html >> throws up the following possibilities: >> >> CAP_SYS_NICE > > Again, for Android uses, CAP_SYS_NICE would be fine (ideal really), > but I worry that it might be too commonly given in other systems to > allow a task to migrate potential cgroup restrictions in container > focused environments. > >> CAP_SYS_PTRACE > > For Android, PTRACE requires too much privilege given to the > controlling task, as that would allow the system_server to also be > able to inspect memory of all other tasks, which raises security > concerns. (We already went through this with the > proc/<tid>/timerslack_ns interface, and had to move back to > CAP_SYS_NICE there). > > >> CAP_SYS_RESOURCE >> >> I'm aware that you said above that use CAP_SYS_NICE overloads that >> capability a bit too much. Maybe it's true, but on the other hand, by >> my count from dome rough grepping of the kernel source, there are a >> total of 14 capable() checks for CAP_SYS_NICE, out of a total of >> around 1256 capable() checks altogether. So, I think this does need to >> be balanced against the limited number of silos. >> >> Also, CAP_SYS_RESOURCE deserves consideration (34 uses in capable() >> checks). I'd say, since cgroups are about resources, so there's >> something of a match there., so it's also worth considering. > > I'll try to look into CAP_SYS_RESOURCE. > > Colin/Todd: Any objection from the Android side on CAP_SYS_RESOURCE?
Just to reiterate my perspective: I'm suggesting that one of the existing silos be considered only. It may be that because of the smearing issues you allude to (where the fact that a process may have the capability for another purpose that inadvertently allows it also to cgroup migration), that a new capability is in order. I just want to make sure that the issue is considered (and--importantly--that the rationale for the eventual decision is documented in the commit message!). Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/