Commit 0ab02ca8 authored by Li Zefan's avatar Li Zefan Committed by Tejun Heo
Browse files

cgroup: protect modifications to cgroup_idr with cgroup_mutex 



Setup cgroupfs like this:
  # mount -t cgroup -o cpuacct xxx /cgroup
  # mkdir /cgroup/sub1
  # mkdir /cgroup/sub2

Then run these two commands:
  # for ((; ;)) { mkdir /cgroup/sub1/tmp && rmdir /mnt/sub1/tmp; } &
  # for ((; ;)) { mkdir /cgroup/sub2/tmp && rmdir /mnt/sub2/tmp; } &

After seconds you may see this warning:

------------[ cut here ]------------
WARNING: CPU: 1 PID: 25243 at lib/idr.c:527 sub_remove+0x87/0x1b0()
idr_remove called for id=6 which is not allocated.
...
Call Trace:
 [<ffffffff8156063c>] dump_stack+0x7a/0x96
 [<ffffffff810591ac>] warn_slowpath_common+0x8c/0xc0
 [<ffffffff81059296>] warn_slowpath_fmt+0x46/0x50
 [<ffffffff81300aa7>] sub_remove+0x87/0x1b0
 [<ffffffff810f3f02>] ? css_killed_work_fn+0x32/0x1b0
 [<ffffffff81300bf5>] idr_remove+0x25/0xd0
 [<ffffffff810f2bab>] cgroup_destroy_css_killed+0x5b/0xc0
 [<ffffffff810f4000>] css_killed_work_fn+0x130/0x1b0
 [<ffffffff8107cdbc>] process_one_work+0x26c/0x550
 [<ffffffff8107eefe>] worker_thread+0x12e/0x3b0
 [<ffffffff81085f96>] kthread+0xe6/0xf0
 [<ffffffff81570bac>] ret_from_fork+0x7c/0xb0
---[ end trace 2d1577ec10cf80d0 ]---

It's because allocating/removing cgroup ID is not properly synchronized.

The bug was introduced when we converted cgroup_ida to cgroup_idr.
While synchronization is already done inside ida_simple_{get,remove}(),
users are responsible for concurrent calls to idr_{alloc,remove}().

tj: Refreshed on top of b58c8998 ("cgroup: fix error return from
cgroup_create()").

Fixes: 4e96ee8e ("cgroup: convert cgroup_ida to cgroup_idr")
Cc: <stable@vger.kernel.org> #3.12+
Reported-by: default avatarMichal Hocko <mhocko@suse.cz>
Signed-off-by: default avatarLi Zefan <lizefan@huawei.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent 48573a89

include/linux/cgroup.h

+2 −0
Original line number Diff line number Diff line
@@ -166,6 +166,8 @@ struct cgroup { 
	 *

	 * The ID of the root cgroup is always 0, and a new cgroup

	 * will be assigned with a smallest available ID.

	 *

	 * Allocating/Removing ID must be protected by cgroup_mutex.

	 */

	int id;

kernel/cgroup.c

+18 −16
Original line number Diff line number Diff line
@@ -886,7 +886,9 @@ static void cgroup_diput(struct dentry *dentry, struct inode *inode) 
		 * per-subsystem and moved to css->id so that lookups are

		 * successful until the target css is released.

		 */

		mutex_lock(&cgroup_mutex);

		idr_remove(&cgrp->root->cgroup_idr, cgrp->id);

		mutex_unlock(&cgroup_mutex);

		cgrp->id = -1;



		call_rcu(&cgrp->rcu_head, cgroup_free_rcu);
@@ -4167,16 +4169,6 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry, 
	}

	rcu_assign_pointer(cgrp->name, name);



	/*

	 * Temporarily set the pointer to NULL, so idr_find() won't return

	 * a half-baked cgroup.

	 */

	cgrp->id = idr_alloc(&root->cgroup_idr, NULL, 1, 0, GFP_KERNEL);

	if (cgrp->id < 0) {

		err = -ENOMEM;

		goto err_free_name;

	}



	/*

	 * Only live parents can have children.  Note that the liveliness

	 * check isn't strictly necessary because cgroup_mkdir() and
@@ -4186,7 +4178,17 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry, 
	 */

	if (!cgroup_lock_live_group(parent)) {

		err = -ENODEV;

		goto err_free_id;

		goto err_free_name;

	}



	/*

	 * Temporarily set the pointer to NULL, so idr_find() won't return

	 * a half-baked cgroup.

	 */

	cgrp->id = idr_alloc(&root->cgroup_idr, NULL, 1, 0, GFP_KERNEL);

	if (cgrp->id < 0) {

		err = -ENOMEM;

		goto err_unlock;

	}



	/* Grab a reference on the superblock so the hierarchy doesn't
@@ -4218,7 +4220,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry, 
	 */

	err = cgroup_create_file(dentry, S_IFDIR | mode, sb);

	if (err < 0)

		goto err_unlock;

		goto err_free_id;

	lockdep_assert_held(&dentry->d_inode->i_mutex);



	cgrp->serial_nr = cgroup_serial_nr_next++;
@@ -4254,12 +4256,12 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry, 


	return 0;



err_unlock:

	mutex_unlock(&cgroup_mutex);

	/* Release the reference count that we took on the superblock */

	deactivate_super(sb);

err_free_id:

	idr_remove(&root->cgroup_idr, cgrp->id);

	/* Release the reference count that we took on the superblock */

	deactivate_super(sb);

err_unlock:

	mutex_unlock(&cgroup_mutex);

err_free_name:

	kfree(rcu_dereference_raw(cgrp->name));

err_free_cgrp:

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