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authorLi Zefan <lizefan@huawei.com>2014-09-04 14:43:07 +0800
committerTejun Heo <tj@kernel.org>2014-09-05 01:36:18 +0900
commita4189487da1b4f8260c6006b9dc47c3c4107a5ae (patch)
treef58a1f62c4f9282659b5f84bf53888ed9ffab0ab /kernel
parentfa8137be6ba632041e725e4623258ba27a2cf9be (diff)
cgroup: delay the clearing of cgrp->kn->priv
Run these two scripts concurrently: for ((; ;)) { mkdir /cgroup/sub rmdir /cgroup/sub } for ((; ;)) { echo $$ > /cgroup/sub/cgroup.procs echo $$ > /cgroup/cgroup.procs } A kernel bug will be triggered: BUG: unable to handle kernel NULL pointer dereference at 00000038 IP: [<c10bbd69>] cgroup_put+0x9/0x80 ... Call Trace: [<c10bbe19>] cgroup_kn_unlock+0x39/0x50 [<c10bbe91>] cgroup_kn_lock_live+0x61/0x70 [<c10be3c1>] __cgroup_procs_write.isra.26+0x51/0x230 [<c10be5b2>] cgroup_tasks_write+0x12/0x20 [<c10bb7b0>] cgroup_file_write+0x40/0x130 [<c11aee71>] kernfs_fop_write+0xd1/0x160 [<c1148e58>] vfs_write+0x98/0x1e0 [<c114934d>] SyS_write+0x4d/0xa0 [<c16f656b>] sysenter_do_call+0x12/0x12 We clear cgrp->kn->priv in the end of cgroup_rmdir(), but another concurrent thread can access kn->priv after the clearing. We should move the clearing to css_release_work_fn(). At that time no one is holding reference to the cgroup and no one can gain a new reference to access it. v2: - move RCU_INIT_POINTER() into the else block. (Tejun) - remove the cgroup_parent() check. (Tejun) - update the comment in css_tryget_online_from_dir(). Cc: <stable@vger.kernel.org> # 3.15+ Reported-by: Toralf Förster <toralf.foerster@gmx.de> Signed-off-by: Zefan Li <lizefan@huawei.com> Signed-off-by: Tejun Heo <tj@kernel.org>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/cgroup.c21
1 files changed, 10 insertions, 11 deletions
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 50b94113f4f7..bf30076664ca 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4396,6 +4396,15 @@ static void css_release_work_fn(struct work_struct *work)
/* cgroup release path */
cgroup_idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
cgrp->id = -1;
+
+ /*
+ * There are two control paths which try to determine
+ * cgroup from dentry without going through kernfs -
+ * cgroupstats_build() and css_tryget_online_from_dir().
+ * Those are supported by RCU protecting clearing of
+ * cgrp->kn->priv backpointer.
+ */
+ RCU_INIT_POINTER(*(void __rcu __force **)&cgrp->kn->priv, NULL);
}
mutex_unlock(&cgroup_mutex);
@@ -4834,16 +4843,6 @@ static int cgroup_rmdir(struct kernfs_node *kn)
cgroup_kn_unlock(kn);
- /*
- * There are two control paths which try to determine cgroup from
- * dentry without going through kernfs - cgroupstats_build() and
- * css_tryget_online_from_dir(). Those are supported by RCU
- * protecting clearing of cgrp->kn->priv backpointer, which should
- * happen after all files under it have been removed.
- */
- if (!ret)
- RCU_INIT_POINTER(*(void __rcu __force **)&kn->priv, NULL);
-
cgroup_put(cgrp);
return ret;
}
@@ -5430,7 +5429,7 @@ struct cgroup_subsys_state *css_tryget_online_from_dir(struct dentry *dentry,
/*
* This path doesn't originate from kernfs and @kn could already
* have been or be removed at any point. @kn->priv is RCU
- * protected for this access. See cgroup_rmdir() for details.
+ * protected for this access. See css_release_work_fn() for details.
*/
cgrp = rcu_dereference(kn->priv);
if (cgrp)