cgroup: mount cgroupns-root when inside non-init cgroupns

This patch enables cgroup mounting inside userns when a process
as appropriate privileges. The cgroup filesystem mounted is
rooted at the cgroupns-root. Thus, in a container-setup, only
the hierarchy under the cgroupns-root is exposed inside the container.
This allows container management tools to run inside the containers
without depending on any global state.

Signed-off-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 47 insertions, 1 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index b086a461be23..24989022ff62 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1994,6 +1994,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 {
         bool is_v2 = fs_type == &cgroup2_fs_type;
         struct super_block *pinned_sb = NULL;
+        struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
         struct cgroup_subsys *ss;
         struct cgroup_root *root;
         struct cgroup_sb_opts opts;
@@ -2002,6 +2003,14 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
         int i;
         bool new_sb;
 
+        get_cgroup_ns(ns);
+
+        /* Check if the caller has permission to mount. */
+        if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN)) {
+                put_cgroup_ns(ns);
+                return ERR_PTR(-EPERM);
+        }
+
         /*
          * The first time anyone tries to mount a cgroup, enable the list
          * linking each css_set to its tasks and fix up all existing tasks.
@@ -2012,6 +2021,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
         if (is_v2) {
                 if (data) {
                         pr_err("cgroup2: unknown option \"%s\"\n", (char *)data);
+                        put_cgroup_ns(ns);
                         return ERR_PTR(-EINVAL);
                 }
                 cgrp_dfl_root_visible = true;
@@ -2117,6 +2127,16 @@ 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)) {
+                ret = -EPERM;
+                goto out_unlock;
+        }
+
         root = kzalloc(sizeof(*root), GFP_KERNEL);
         if (!root) {
                 ret = -ENOMEM;
@@ -2135,12 +2155,37 @@ out_free:
         kfree(opts.release_agent);
         kfree(opts.name);
 
-        if (ret)
+        if (ret) {
+                put_cgroup_ns(ns);
                 return ERR_PTR(ret);
+        }
 out_mount:
         dentry = kernfs_mount(fs_type, flags, root->kf_root,
                               is_v2 ? CGROUP2_SUPER_MAGIC : CGROUP_SUPER_MAGIC,
                               &new_sb);
+
+        /*
+         * In non-init cgroup namespace, instead of root cgroup's
+         * dentry, we return the dentry corresponding to the
+         * cgroupns->root_cgrp.
+         */
+        if (!IS_ERR(dentry) && ns != &init_cgroup_ns) {
+                struct dentry *nsdentry;
+                struct cgroup *cgrp;
+
+                mutex_lock(&cgroup_mutex);
+                spin_lock_bh(&css_set_lock);
+
+                cgrp = cset_cgroup_from_root(ns->root_cset, root);
+
+                spin_unlock_bh(&css_set_lock);
+                mutex_unlock(&cgroup_mutex);
+
+                nsdentry = kernfs_node_dentry(cgrp->kn, dentry->d_sb);
+                dput(dentry);
+                dentry = nsdentry;
+        }
+
         if (IS_ERR(dentry) || !new_sb)
                 cgroup_put(&root->cgrp);
 
@@ -2153,6 +2198,7 @@ out_mount:
                 deactivate_super(pinned_sb);
         }
 
+        put_cgroup_ns(ns);
         return dentry;
 }