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Subject[PATCH 4.14 51/89] cgroup: Prevent double killing of css when enabling threaded cgroup
Date
From: Michal Koutný <mkoutny@suse.com>

commit 3bc0bb36fa30e95ca829e9cf480e1ef7f7638333 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/cgroup/cgroup.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)

--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -2884,8 +2884,6 @@ static int cgroup_apply_control_enable(s
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;

@@ -2895,6 +2893,8 @@ static int cgroup_apply_control_enable(s
return PTR_ERR(css);
}

+ WARN_ON_ONCE(percpu_ref_is_dying(&css->refcnt));
+
if (css_visible(css)) {
ret = css_populate_dir(css);
if (ret)
@@ -2930,11 +2930,11 @@ static void cgroup_apply_control_disable
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);
@@ -3221,7 +3221,8 @@ static ssize_t cgroup_type_write(struct
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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