Re: [PATCH v5 5/6] clone3: allow spawning processes into cgroups

[Christian Brauner <christian.brauner@ubuntu.com>]

On Wed, Jan 29, 2020 at 02:27:19PM +0100, Michal Koutný wrote:
> Hello.
> 
> On Tue, Jan 21, 2020 at 04:48:43PM +0100, Christian Brauner <christian.brauner@ubuntu.com> wrote:
> > +static int cgroup_css_set_fork(struct kernel_clone_args *kargs)
> > +	__acquires(&cgroup_mutex) __acquires(&cgroup_threadgroup_rwsem)
> > +{
> > +	int ret;
> > +	struct cgroup *dst_cgrp = NULL;
> > +	struct css_set *cset;
> > +	struct super_block *sb;
> > +	struct file *f;
> > +
> > +	if (kargs->flags & CLONE_INTO_CGROUP)
> > +		mutex_lock(&cgroup_mutex);
> > +
> > +	cgroup_threadgroup_change_begin(current);
> > +
> > +	spin_lock_irq(&css_set_lock);
> > +	cset = task_css_set(current);
> > +	get_css_set(cset);
> > +	spin_unlock_irq(&css_set_lock);
> > +
> > +	if (!(kargs->flags & CLONE_INTO_CGROUP)) {
> > +		kargs->cset = cset;
> Where is this css_set put when CLONE_INTO_CGROUP isn't used?
> (Aha, it's passed to child's tsk->cgroups but see my other note below.)
> 
> > +	dst_cgrp = cgroup_get_from_file(f);
> > +	if (IS_ERR(dst_cgrp)) {
> > +		ret = PTR_ERR(dst_cgrp);
> > +		dst_cgrp = NULL;
> > +		goto err;
> > +	}
> > +
> > +	/*
> > +	 * Verify that we the target cgroup is writable for us. This is
> > +	 * usually done by the vfs layer but since we're not going through
> > +	 * the vfs layer here we need to do it "manually".
> > +	 */
> > +	ret = cgroup_may_write(dst_cgrp, sb);
> > +	if (ret)
> > +		goto err;
> > +
> > +	ret = cgroup_attach_permissions(cset->dfl_cgrp, dst_cgrp, sb,
> > +					!!(kargs->flags & CLONE_THREAD));
> > +	if (ret)
> > +		goto err;
> > +
> > +	kargs->cset = find_css_set(cset, dst_cgrp);
> > +	if (!kargs->cset) {
> > +		ret = -ENOMEM;
> > +		goto err;
> > +	}
> > +
> > +	if (cgroup_is_dead(dst_cgrp)) {
> > +		ret = -ENODEV;
> > +		goto err;
> > +	}
> I'd move this check right after cgroup_get_from_file. The fork-migration
> path is synchrinized via cgroup_mutex with cgroup_destroy_locked and
> there's no need checking permissions on cgroup that's going away anyway.
> 
> 
> > +static void cgroup_css_set_put_fork(struct kernel_clone_args *kargs)
> > +	__releases(&cgroup_threadgroup_rwsem) __releases(&cgroup_mutex)
> > +{
> > +	cgroup_threadgroup_change_end(current);
> > +
> > +	if (kargs->flags & CLONE_INTO_CGROUP) {
> > +		struct cgroup *cgrp = kargs->cgrp;
> > +		struct css_set *cset = kargs->cset;
> > +
> > +		mutex_unlock(&cgroup_mutex);
> > +
> > +		if (cset) {
> > +			put_css_set(cset);
> > +			kargs->cset = NULL;
> > +		}
> > +
> > +		if (cgrp) {
> > +			cgroup_put(cgrp);
> > +			kargs->cgrp = NULL;
> > +		}
> > +	}
> I don't see any function problem with this ordering, however, I'd
> prefer symmetry with the "allocation" path (in cgroup_css_set_fork),
> i.e. cgroup_put, put_css_set and lastly mutex_unlock.

I prefer to yield the mutex as early as possible.

> 
> > +void cgroup_post_fork(struct task_struct *child,
> > +		      struct kernel_clone_args *kargs)
> > +	__releases(&cgroup_threadgroup_rwsem) __releases(&cgroup_mutex)
> >  {
> >  	struct cgroup_subsys *ss;
> > -	struct css_set *cset;
> > +	struct css_set *cset = kargs->cset;
> >  	int i;
> >  
> >  	spin_lock_irq(&css_set_lock);
> >  
> >  	WARN_ON_ONCE(!list_empty(&child->cg_list));
> > -	cset = task_css_set(current); /* current is @child's parent */
> > -	get_css_set(cset);
> >  	cset->nr_tasks++;
> >  	css_set_move_task(child, NULL, cset, false);
> So, the reference is passed over from kargs->cset to task->cgroups. I
> think it's necessary to zero kargs->cset in order to prevent droping the 
> reference in cgroup_css_set_put_fork.

cgroup_post_fork() is called past the point of no return for fork and
cgroup_css_set_put_fork() is explicitly documented as only being
callable before forks point of no return:

 * Drop references to the prepared css_set and target cgroup if
 * CLONE_INTO_CGROUP was requested. This function can only be
 * called before fork()'s point of no return.

> Perhaps, a general comment about css_set whereabouts during fork and
> kargs passing would be useful.
> 
> > @@ -6016,6 +6146,17 @@ void cgroup_post_fork(struct task_struct *child)
> >  	} while_each_subsys_mask();
> >  
> >  	cgroup_threadgroup_change_end(current);
> > +
> > +	if (kargs->flags & CLONE_INTO_CGROUP) {
> > +		mutex_unlock(&cgroup_mutex);
> > +
> > +		cgroup_put(kargs->cgrp);
> > +		kargs->cgrp = NULL;
> > +	}
> > +
> > +	/* Make the new cset the root_cset of the new cgroup namespace. */
> > +	if (kargs->flags & CLONE_NEWCGROUP)
> > +		child->nsproxy->cgroup_ns->root_cset = cset;
> root_cset reference (from copy_cgroup_ns) seems leaked here and where is
> the additional reference to new cset obtained?

This should be:

if (kargs->flags & CLONE_NEWCGROUP) {
	struct css_set *rcset = child->nsproxy->cgroup_ns->root_cset;

	get_css_set(cset);
	child->nsproxy->cgroup_ns->root_cset = cset;
	put_css_set(rcset);
}

Thanks!
Christian