linux-kernel - Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task

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Date:   Wed, 1 Sep 2021 06:45:06 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Zhang Qiao <zhangqiao22@...wei.com>
Cc:     peterz@...radead.org, juri.lelli@...hat.com,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        vincent.guittot@...aro.org
Subject: Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid
 sched_task_group

Hello,

On Wed, Sep 01, 2021 at 03:43:00PM +0800, Zhang Qiao wrote:
> 在 2021/9/1 6:59, Tejun Heo 写道:
> > On Tue, Aug 31, 2021 at 03:58:42PM +0800, Zhang Qiao wrote:
> >>> I think this would allow cgroup migrations to take place before
> >>> sched_post_fork() is run, which likely will break stuff. The right
> 
> cgroup migrations? Do you mean child process set its cgroups at
> cgroup_subsys->fork()?

As soon as cgroup_post_fork() is complete, userspace can try moving the
process to a different cgroup which can get confusing for sched_post_fork.

> > Between cgroup_can_fork() and cgroup_post_fork(), the cgroup membership and
> > thus sched_task_group can't change, so if the child sets it to the parent's
> > inbetween, the sched_task_group can't go away.
> 
> so the child just need to update its sched_task_group after cgroup_can_fork(),
> then call sched_fork(), is it right?

Yeah, the bug here is that a field which is dependent on cgroup membership
is being read before the cgroup membership is fixated.

Thanks.

-- 
tejun