Commit 3bc0bb36 authored Jan 09, 2020 by Michal Koutný Committed by Tejun Heo Jan 15, 2020

cgroup: Prevent double killing of css when enabling threaded cgroup



The test_cgcore_no_internal_process_constraint_on_threads selftest when
running with subsystem controlling noise triggers two warnings:

> [  597.443115] WARNING: CPU: 1 PID: 28167 at kernel/cgroup/cgroup.c:3131 cgroup_apply_control_enable+0xe0/0x3f0
> [  597.443413] WARNING: CPU: 1 PID: 28167 at kernel/cgroup/cgroup.c:3177 cgroup_apply_control_disable+0xa6/0x160

Both stem from a call to cgroup_type_write. The first warning was also
triggered by syzkaller.

When we're switching cgroup to threaded mode shortly after a subsystem
was disabled on it, we can see the respective subsystem css dying there.

The warning in cgroup_apply_control_enable is harmless in this case
since we're not adding new subsys anyway.
The warning in cgroup_apply_control_disable indicates an attempt to kill
css of recently disabled subsystem repeatedly.

The commit prevents these situations by making cgroup_type_write wait
for all dying csses to go away before re-applying subtree controls.
When at it, the locations of WARN_ON_ONCE calls are moved so that
warning is triggered only when we are about to misuse the dying css.

Reported-by:  <syzbot+5493b2a54d31d6aea629@syzkaller.appspotmail.com>
Reported-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

parent 75ea91cd

kernel/cgroup/cgroup.c

+6 −5

Original line number	Diff line number	Diff line
		@@ -3055,8 +3055,6 @@ static int cgroup_apply_control_enable(struct cgroup *cgrp)
		for_each_subsys(ss, ssid) {
		struct cgroup_subsys_state *css = cgroup_css(dsct, ss);

		WARN_ON_ONCE(css && percpu_ref_is_dying(&css->refcnt));

		if (!(cgroup_ss_mask(dsct) & (1 << ss->id)))
		continue;

		@@ -3066,6 +3064,8 @@ static int cgroup_apply_control_enable(struct cgroup *cgrp)
		return PTR_ERR(css);
		}

		WARN_ON_ONCE(percpu_ref_is_dying(&css->refcnt));

		if (css_visible(css)) {
		ret = css_populate_dir(css);
		if (ret)
		@@ -3101,11 +3101,11 @@ static void cgroup_apply_control_disable(struct cgroup *cgrp)
		for_each_subsys(ss, ssid) {
		struct cgroup_subsys_state *css = cgroup_css(dsct, ss);

		WARN_ON_ONCE(css && percpu_ref_is_dying(&css->refcnt));

		if (!css)
		continue;

		WARN_ON_ONCE(percpu_ref_is_dying(&css->refcnt));

		if (css->parent &&
		!(cgroup_ss_mask(dsct) & (1 << ss->id))) {
		kill_css(css);
		@@ -3392,7 +3392,8 @@ static ssize_t cgroup_type_write(struct kernfs_open_file of, char buf,
		if (strcmp(strstrip(buf), "threaded"))
		return -EINVAL;

		cgrp = cgroup_kn_lock_live(of->kn, false);
		/* drain dying csses before we re-apply (threaded) subtree control */
		cgrp = cgroup_kn_lock_live(of->kn, true);
		if (!cgrp)
		return -ENOENT;

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