From 9732adc5d6520238223df16630f1f8cad2269317 Mon Sep 17 00:00:00 2001 From: Zefan Li Date: Wed, 19 Apr 2017 10:15:59 +0800 Subject: cgroup: avoid attaching a cgroup root to two different superblocks, take 2 Commit bfb0b80db5f9 ("cgroup: avoid attaching a cgroup root to two different superblocks") is broken. Now we try to fix the race by delaying the initialization of cgroup root refcnt until a superblock has been allocated. Reported-by: Dmitry Vyukov Reported-by: Andrei Vagin Tested-by: Andrei Vagin Signed-off-by: Zefan Li Signed-off-by: Tejun Heo --- kernel/cgroup/cgroup-v1.c | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-) (limited to 'kernel/cgroup/cgroup-v1.c') diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c index c4a68c438fde..a478995e7ed3 100644 --- a/kernel/cgroup/cgroup-v1.c +++ b/kernel/cgroup/cgroup-v1.c @@ -1072,6 +1072,7 @@ struct dentry *cgroup1_mount(struct file_system_type *fs_type, int flags, struct cgroup_subsys *ss; struct dentry *dentry; int i, ret; + bool new_root = false; cgroup_lock_and_drain_offline(&cgrp_dfl_root.cgrp); @@ -1181,10 +1182,11 @@ struct dentry *cgroup1_mount(struct file_system_type *fs_type, int flags, ret = -ENOMEM; goto out_unlock; } + new_root = true; init_cgroup_root(root, &opts); - ret = cgroup_setup_root(root, opts.subsys_mask); + ret = cgroup_setup_root(root, opts.subsys_mask, PERCPU_REF_INIT_DEAD); if (ret) cgroup_free_root(root); @@ -1200,6 +1202,18 @@ out_free: dentry = cgroup_do_mount(&cgroup_fs_type, flags, root, CGROUP_SUPER_MAGIC, ns); + /* + * There's a race window after we release cgroup_mutex and before + * allocating a superblock. Make sure a concurrent process won't + * be able to re-use the root during this window by delaying the + * initialization of root refcnt. + */ + if (new_root) { + mutex_lock(&cgroup_mutex); + percpu_ref_reinit(&root->cgrp.self.refcnt); + mutex_unlock(&cgroup_mutex); + } + /* * If @pinned_sb, we're reusing an existing root and holding an * extra ref on its sb. Mount is complete. Put the extra ref. -- cgit v1.2.3-59-g8ed1b