cgroupns: Only allow creation of hierarchies in the initial cgroup namespace

ebiederm · htejun · commit 726a4994b05f · 2016-07-15T08:04:27.000-04:00
Unprivileged users can't use hierarchies if they create them as they do not have privilieges to the root directory. Which means the only thing a hiearchy created by an unprivileged user is good for is expanding the number of cgroup links in every css_set, which is a DOS attack. We could allow hierarchies to be created in namespaces in the initial user namespace. Unfortunately there is only a single namespace for the names of heirarchies, so that is likely to create more confusion than not. So do the simple thing and restrict hiearchy creation to the initial cgroup namespace. Cc: stable@vger.kernel.org Fixes: a79a908 ("cgroup: introduce cgroup namespaces") Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
@@ -2215,12 +2215,8 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 		goto out_unlock;
 	}
 
-	/*
-	 * We know this subsystem has not yet been bound.  Users in a non-init
-	 * user namespace may only mount hierarchies with no bound subsystems,
-	 * i.e. 'none,name=user1'
-	 */
-	if (!opts.none && !capable(CAP_SYS_ADMIN)) {
+	/* Hierarchies may only be created in the initial cgroup namespace. */
+	if (ns != &init_cgroup_ns) {
 		ret = -EPERM;
 		goto out_unlock;
 	}

Original file line number	Diff line number	Diff line change
`@@ -2215,12 +2215,8 @@ static struct dentry cgroup_mount(struct file_system_type fs_type,`
`2215`	`2215`	`goto out_unlock;`
`2216`	`2216`	`}`
`2217`	`2217`
`2218`		`- /*`
`2219`		`- * We know this subsystem has not yet been bound. Users in a non-init`
`2220`		`- * user namespace may only mount hierarchies with no bound subsystems,`
`2221`		`- * i.e. 'none,name=user1'`
`2222`		`- */`
`2223`		`- if (!opts.none && !capable(CAP_SYS_ADMIN)) {`
	`2218`	`+ /* Hierarchies may only be created in the initial cgroup namespace. */`
	`2219`	`+ if (ns != &init_cgroup_ns) {`
`2224`	`2220`	`ret = -EPERM;`
`2225`	`2221`	`goto out_unlock;`
`2226`	`2222`	`}`