[PATCH 5/7] cgroup: mount cgroupns-root when inside non-init cgroupns

serge.hallyn
2015-12-07T23:07:46Z

From: Aditya Kali <adityakali@google.com>

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.
In order to support this, a new kernfs api is added to lookup the
dentry for the cgroupns-root.

Changelog:
20151116 - Don't allow user namespaces to bind new subsystems
20151118 - postpone the FS_USERNS_MOUNT flag until the
last patch, until we can convince ourselves it
is safe.
20151207 - Switch to walking up the kernfs path from kn root.

Signed-off-by: Aditya Kali <adityakali@google.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
---
fs/kernfs/mount.c | 74 ++++++++++++++++++++++++++++++++++++++++++++++++
include/linux/kernfs.h | 2 ++
kernel/cgroup.c | 39 ++++++++++++++++++++++++-
3 files changed, 114 insertions(+), 1 deletion(-)

diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index 8eaf417..9219444 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -14,6 +14,7 @@
#include <linux/magic.h>
#include <linux/slab.h>
#include <linux/pagemap.h>
+#include <linux/namei.h>

#include "kernfs-internal.h"

@@ -62,6 +63,79 @@ struct kernfs_root *kernfs_root_from_sb(struct super_block *sb)
return NULL;
}

+/*
+ * find the next ancestor in the path down to @child, where @parent was the
+ * parent whose child we want to find.
+ *
+ * Say the path is /a/b/c/d. @child is d, @parent is NULL. We return the root
+ * node. If @parent is b, then we return the node for c.
+ * Passing in d as @parent is not ok.
+ */
+static struct kernfs_node *
+find_next_ancestor(struct kernfs_node *child, struct kernfs_node *parent)
+{
+ if (child == parent) {
+ pr_crit_once("BUG in find_next_ancestor: called with parent == child");
+ return NULL;
+ }
+
+ while (child->parent != parent) {
+ if (!child->parent)
+ return NULL;
+ child = child->parent;
+ }
+
+ return child;
+}
+
+/**
+ * kernfs_obtain_root - get a dentry for the given kernfs_node
+ * @sb: the kernfs super_block
+ * @kn: kernfs_node for which a dentry is needed
+ *
+ * This can be used by callers which want to mount only a part of the kernfs
+ * as root of the filesystem.
+ */
+struct dentry *kernfs_obtain_root(struct super_block *sb,
+ struct kernfs_node *kn)
+{
+ struct dentry *dentry;
+ struct kernfs_node *knparent = NULL;
+
+ BUG_ON(sb->s_op != &kernfs_sops);
+
+ dentry = dget(sb->s_root);
+ if (!kn->parent) // this is the root
+ return dentry;
+
+ knparent = find_next_ancestor(kn, NULL);
+ if (!knparent) {
+ pr_crit("BUG: find_next_ancestor for root dentry returned NULL\n");
+ return ERR_PTR(-EINVAL);
+ }
+
+ do {
+ struct dentry *dtmp;
+ struct kernfs_node *kntmp;
+
+ if (kn == knparent)
+ return dentry;
+ kntmp = find_next_ancestor(kn, knparent);
+ if (!kntmp) {
+ pr_crit("BUG: find_next_ancestor returned NULL for node\n");
+ return ERR_PTR(-EINVAL);
+ }
+ dtmp = lookup_one_len(kntmp->name, dentry, strlen(kntmp->name));
+ dput(dentry);
+ if (IS_ERR(dtmp))
+ return dtmp;
+ knparent = kntmp;
+ dentry = dtmp;
+ } while (1);
+
+ // notreached
+}
+
static int kernfs_fill_super(struct super_block *sb, unsigned long magic)
{
struct kernfs_super_info *info = kernfs_info(sb);
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index d025ebd..1903777 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -284,6 +284,8 @@ struct kernfs_node *kernfs_node_from_dentry(struct dentry *dentry);
struct kernfs_root *kernfs_root_from_sb(struct super_block *sb);
struct inode *kernfs_get_inode(struct super_block *sb, struct kernfs_node *kn);

+struct dentry *kernfs_obtain_root(struct super_block *sb,
+ struct kernfs_node *kn);
struct kernfs_root *kernfs_create_root(struct kernfs_syscall_ops *scops,
unsigned int flags, void *priv);
void kernfs_destroy_root(struct kernfs_root *root);
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index a5ab74d..09cd718 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2011,6 +2011,15 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
int ret;
int i;
bool new_sb;
+ struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
+
+ 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
@@ -2127,6 +2136,11 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
goto out_unlock;
}

+ if (!opts.none && !capable(CAP_SYS_ADMIN)) {
+ ret = -EPERM;
+ goto out_unlock;
+ }
+
root = kzalloc(sizeof(*root), GFP_KERNEL);
if (!root) {
ret = -ENOMEM;
@@ -2145,12 +2159,34 @@ 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);
+
+ if (!IS_ERR(dentry)) {
+ /*
+ * In non-init cgroup namespace, instead of root cgroup's
+ * dentry, we return the dentry corresponding to the
+ * cgroupns->root_cgrp.
+ */
+ if (ns != &init_cgroup_ns) {
+ struct dentry *nsdentry;
+ struct cgroup *cgrp;
+
+ cgrp = cset_cgroup_from_root(ns->root_cset, root);
+ nsdentry = kernfs_obtain_root(dentry->d_sb,
+ cgrp->kn);
+ dput(dentry);
+ dentry = nsdentry;
+ }
+ }
+
if (IS_ERR(dentry) || !new_sb)
cgroup_put(&root->cgrp);

@@ -2163,6 +2199,7 @@ out_mount:
deactivate_super(pinned_sb);
}

+ put_cgroup_ns(ns);
return dentry;
}

--
1.7.9.5