linux-kernel - Re: [PATCH v2 2/3] clone3: allow spawning processes into cgroups

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Message-ID: <20200116122944.nj3e66eusxu6sb44@wittgenstein>
Date:   Thu, 16 Jan 2020 13:29:44 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Li Zefan <lizefan@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>, cgroups@...r.kernel.org
Subject: Re: [PATCH v2 2/3] clone3: allow spawning processes into cgroups

On Wed, Jan 08, 2020 at 07:09:07PM +0100, Christian Brauner wrote:
> On Tue, Jan 07, 2020 at 08:32:04AM -0800, Tejun Heo wrote:
> > On Mon, Dec 23, 2019 at 07:15:03AM +0100, Christian Brauner wrote:
> > > +static struct cgroup *cgroup_get_from_file(struct file *f)
> > > +{
> > > +	struct cgroup_subsys_state *css;
> > > +	struct cgroup *cgrp;
> > > +
> > > +	css = css_tryget_online_from_dir(f->f_path.dentry, NULL);
> > > +	if (IS_ERR(css))
> > > +		return ERR_CAST(css);
> > > +
> > > +	cgrp = css->cgroup;
> > > +	if (!cgroup_on_dfl(cgrp)) {
> > > +		cgroup_put(cgrp);
> > > +		return ERR_PTR(-EBADF);
> > > +	}
> > > +
> > > +	return cgrp;
> > > +}
> > 
> > It's minor but can you put this refactoring into a separate patch?
> 
> Yep, will do.
> 
> > 
> > ...
> > > +static int cgroup_css_set_fork(struct task_struct *parent,
> > > +			       struct kernel_clone_args *kargs)
> > > +	__acquires(&cgroup_mutex) __acquires(&cgroup_threadgroup_rwsem)
> > > +{
> > > +	int ret;
> > > +	struct cgroup *dst_cgrp = NULL, *src_cgrp;
> > > +	struct css_set *cset;
> > > +	struct super_block *sb;
> > > +	struct file *f;
> > > +
> > > +	if (kargs->flags & CLONE_INTO_CGROUP) {
> > > +		ret = mutex_lock_killable(&cgroup_mutex);
> > > +		if (ret)
> > > +			return ret;
> > > +	}
> > 
> > I don't think this is necessary.  cgroup_mutex should always only be
> > held for a finite enough time; otherwise, processes would get stuck on
> > random cgroupfs accesses or even /proc/self/cgroup.
> 
> Ok, so a simple mutex_lock() should suffice then.
> 
> > 
> > ...
> > > +	spin_lock_irq(&css_set_lock);
> > > +	src_cgrp = task_cgroup_from_root(parent, &cgrp_dfl_root);
> > > +	spin_unlock_irq(&css_set_lock);
> > 
> > You can simply do cset->dfl_root here, which is consistent with other
> > code paths which know that they want the dfl cgroup.
> 
> Ah, great!
> 
> > 
> > > +	ret = cgroup_attach_permissions(src_cgrp, dst_cgrp, sb,
> > > +					!!(kargs->flags & CLONE_THREAD));
> > > +	if (ret)
> > > +		goto err;
> > 
> > So, the existing perm check depends on the fact that for the write
> > operation to have started, it already should have passed write perm
> > check on the destination cgroup.procs file.  We're missing that here,
> > so we prolly need to check that explicitly.
> 
> I need to look into this before I can say yay or nay. :)

Could it be that you misread cgroup_attach_permissions()? Because it
does check for write permissions on the destination cgroup.procs file.
That's why I've added the cgroup_get_from_file() helper. :) See:

static int cgroup_attach_permissions(struct cgroup *src_cgrp,
				     struct cgroup *dst_cgrp,
				     struct super_block *sb, bool thread)
{
	int ret = 0;

	ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp, sb);
	if (ret)
		return ret;

	ret = cgroup_migrate_vet_dst(dst_cgrp);
	if (ret)
		return ret;

	if (thread &&
	    !cgroup_same_domain(src_cgrp->dom_cgrp, dst_cgrp->dom_cgrp))
		ret = -EOPNOTSUPP;

	return ret;
}

Maybe I'm misunderstanding though. :)

Thanks!
Christian