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SubjectRe: [PATCH] sched, cgroup: Use exit hook to avoid use-after-free crash
* Peter Zijlstra <peterz@infradead.org> [2010-12-24 16:59:13]:

> On Fri, 2010-12-24 at 13:16 +0100, Mike Galbraith wrote:
> > On Fri, 2010-12-24 at 11:54 +0100, Peter Zijlstra wrote:
>
> > > Right, so the cgroup core is supposed to already emit -EBUSY when there
> > > are associated tasks with the cgroup, that _should_ be sufficient, the
> > > pre_destroy() method is to frob some extra constraints or somesuch.
> > >
> > > Our problem looks to be that a task (afaict usually current) changes
> > > cgroups without us getting notified of it. On destruction the task is
> > > still enqueued in the cfs_rq being destroyed but is not actually part of
> > > that cgroup according to the task->css bits.
> >
> > Could it be an exiting task? We're still preemptible, and iirc, you run
> > a CONFIG_PREEMPT kernel. (grasp at all straws;)
> >
> > cgroup_exit:
> > /* Reassign the task to the init_css_set. */
> > task_lock(tsk);
> > cg = tsk->cgroups;
> > tsk->cgroups = &init_css_set;
> > task_unlock(tsk);
> > if (cg)
> > put_css_set_taskexit(cg);
> >
>
> This straw appears true:
>
> $ grep -e cpu_cgroup\\\|f491447c log9
>
> ...
>
> kworker/-1196 0d..2. 1601180us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /system/systemd-modules-load.service
> kworker/-1196 0d..2. 1601186us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /system/systemd-modules-load.service
> kworker/-1196 0d..2. 1601188us : __dequeue_entity: f491447c from f492a480, 1 left
> kworker/-1196 0d..2. 1601188us : pick_next_task_fair: picked: f491447c, modprobe/1210
> kworker/-1196 0d..2. 1601192us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /system/systemd-modules-load.service
> modprobe-1210 0d..5. 1601802us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /
> modprobe-1210 0d..5. 1601807us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /
> modprobe-1210 0d..2. 1601817us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /
> modprobe-1210 0d..2. 1601819us : __enqueue_entity: f491447c to f492a480, 1 tasks
> modprobe-1210 0d..2. 1601826us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /
> modprobe-1210 0d..2. 1601832us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /
> modprobe-1210 0d..2. 1601839us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /
> kworker/-1196 0d..2. 1601848us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /
> kworker/-1196 0d..2. 1601854us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /
> kworker/-1196 0d..2. 1601860us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /
> kworker/-1196 0d..2. 1601865us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /
> kworker/-1196 0d..2. 1601871us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /
> kworker/-1196 0d..2. 1601872us : __dequeue_entity: f491447c from f492a480, 1 left
> kworker/-1196 0d..2. 1601873us : pick_next_task_fair: picked: f491447c, modprobe/1210
> kworker/-1196 0d..2. 1601876us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 0, load: 1024, cgroup: /
> modprobe-1210 0d..7. 1601895us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: /
> modprobe-1210 0d..7. 1601900us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: /
> modprobe-1210 0d..2. 1601909us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: /
> modprobe-1210 0d..2. 1601911us : __enqueue_entity: f491447c to f492a480, 1 tasks
> modprobe-1210 0d..2. 1601918us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: /
> modprobe-1210 0d..2. 1601924us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: /
> modprobe-1210 0d..2. 1601931us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: /
> kworker/-1196 0d..2. 1602071us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: /
> kworker/-1196 0d..2. 1602080us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: /
> kworker/-1196 0d..2. 1602089us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: /
> kworker/-1196 0d..2. 1602097us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: /
> kworker/-1196 0d..2. 1602105us : __print_runqueue: se: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: /
> kworker/-1196 0d..2. 1602107us : __dequeue_entity: f491447c from f492a480, 1 left
> kworker/-1196 0d..2. 1602108us : pick_next_task_fair: picked: f491447c, modprobe/1210
> kworker/-1196 0d..2. 1602114us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 16, load: 1024, cgroup: /
> modprobe-1210 0d..3. 1602128us : __print_runqueue: curr: f491447c, comm: modprobe/1210, state: 80, load: 1024, cgroup: /
>
>
> So cgroup moves a task without calling cgroup_subsys::attach() which is
> odd, but it does have an ::exit method, sadly it calls that _before_
> re-assigning the task, which means we have to jump through some hoops.
>
> The below seems to fix the problem for me..
>
> ---
> Subject: sched, cgroup: Use exit hook to avoid use-after-free crash
>
> By not notifying the controller of the on-exit move back to
> init_css_set, we fail to move the task out of the previous cgroup's
> cfs_rq. This leads to an opportunity for a cgroup-destroy to come in and
> free the cgroup (there are no active tasks left in it after all) to
> which the not-quite dead task is still enqueued.
>
> Cc: stable@kernel.org
> Reported-by: Miklos Vajna <vmiklos@frugalware.org>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
> kernel/sched.c | 10 ++++++++++
> 1 files changed, 10 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 7e401f8..572625c 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -611,6 +611,9 @@ static inline struct task_group *task_group(struct task_struct *p)
> struct task_group *tg;
> struct cgroup_subsys_state *css;
>
> + if (p->flags & PF_EXITING)
> + return &root_task_group;
> +
> css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
> lockdep_is_held(&task_rq(p)->lock));
> tg = container_of(css, struct task_group, css);
> @@ -8887,6 +8890,12 @@ cpu_cgroup_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
> }
> }
>
> +static void
> +cpu_cgroup_exit(struct cgroup_subsys *ss, struct task_struct *task)
> +{
> + sched_move_task(task);
> +}
> +
> #ifdef CONFIG_FAIR_GROUP_SCHED
> static int cpu_shares_write_u64(struct cgroup *cgrp, struct cftype *cftype,
> u64 shareval)
> @@ -8959,6 +8968,7 @@ struct cgroup_subsys cpu_cgroup_subsys = {
> .destroy = cpu_cgroup_destroy,
> .can_attach = cpu_cgroup_can_attach,
> .attach = cpu_cgroup_attach,
> + .exit = cpu_cgroup_exit,
> .populate = cpu_cgroup_populate,
> .subsys_id = cpu_cgroup_subsys_id,
> .early_init = 1,
>
>

Very good catch!

Looks very reasonable and correct to me

Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>



--
Three Cheers,
Balbir


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