Patch "sched/cgroup: Move sched_online_group() back into css_online() to fix crash" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

sched/cgroup: Move sched_online_group() back into css_online() to fix crash

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
sched-cgroup-move-sched_online_group-back-into-css_online-to-fix-crash.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Konstantin Khlebnikov <[email protected]>
Date: Wed, 8 Feb 2017 14:27:27 +0300
Subject: sched/cgroup: Move sched_online_group() back into css_online() to fix crash

From: Konstantin Khlebnikov <[email protected]>

commit 96b777452d8881480fd5be50112f791c17db4b6b upstream.

Commit:

2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init")

.. moved sched_online_group() from css_online() to css_alloc(). It exposes half-baked task group into global lists before initializing generic cgroup stuff.

LTP testcase (third in cgroup_regression_test) written for testing similar race in kernels 2.6.26-2.6.28 easily triggers this oops:

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
  IP: kernfs_path_from_node_locked+0x260/0x320
  CPU: 1 PID: 30346 Comm: cat Not tainted 4.10.0-rc5-test #4
  Call Trace:
  ? kernfs_path_from_node+0x4f/0x60
  kernfs_path_from_node+0x3e/0x60
  print_rt_rq+0x44/0x2b0
  print_rt_stats+0x7a/0xd0
  print_cpu+0x2fc/0xe80
  ? __might_sleep+0x4a/0x80
  sched_debug_show+0x17/0x30
  seq_read+0xf2/0x3b0
  proc_reg_read+0x42/0x70
  __vfs_read+0x28/0x130
  ? security_file_permission+0x9b/0xc0
  ? rw_verify_area+0x4e/0xb0
  vfs_read+0xa5/0x170
  SyS_read+0x46/0xa0
  entry_SYSCALL_64_fastpath+0x1e/0xad

Here the task group is already linked into the global RCU-protected 'task_groups' list, but the css->cgroup pointer is still NULL.

This patch reverts this chunk and moves online back to css_online().

---
 kernel/sched/core.c |   14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

@@ -8253,11 +8253,20 @@ cpu_cgroup_css_alloc(struct cgroup_subsy
 	if (IS_ERR(tg))
 		return ERR_PTR(-ENOMEM);
 
-	sched_online_group(tg, parent);
-
 	return &tg->css;
 }
 
+/* Expose task group only after completing cgroup initialization */
+static int cpu_cgroup_css_online(struct cgroup_subsys_state *css)
+{
+	struct task_group *tg = css_tg(css);
+	struct task_group *parent = css_tg(css->parent);
+
+	if (parent)
+		sched_online_group(tg, parent);
+	return 0;
+}
+
 static void cpu_cgroup_css_released(struct cgroup_subsys_state *css)
 {
 	struct task_group *tg = css_tg(css);

@@ -8632,6 +8641,7 @@ static struct cftype cpu_files[] = {
 
 struct cgroup_subsys cpu_cgrp_subsys = {
 	.css_alloc	= cpu_cgroup_css_alloc,
+	.css_online	= cpu_cgroup_css_online,
 	.css_released	= cpu_cgroup_css_released,
 	.css_free	= cpu_cgroup_css_free,
 	.fork		= cpu_cgroup_fork,

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/sched-cgroup-move-sched_online_group-back-into-css_online-to-fix-crash.patch