[PATCH 3/3] cgroup: implement "nsdelegate" mount option

[Tejun Heo]

Currently, cgroup only supports delegation to !root users and cgroup
namespaces don't get any special treatments.  This limits the
usefulness of cgroup namespaces as they by themselves can't be safe
delegation boundaries.  A process inside a cgroup can change the
resource control knobs of the parent in the namespace root and may
move processes in and out of the namespace if cgroups outside its
namespace are visible somehow.

This patch adds a new mount option "nsdelegate" which makes cgroup
namespaces delegation boundaries.  If set, cgroup behaves as if write
permission based delegation took place at namespace boundaries -
writes to the resource control knobs from the namespace root are
denied and migration crossing the namespace boundary aren't allowed
from inside the namespace.

This allows cgroup namespace to function as a delegation boundary by
itself.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Aravind Anbudurai <aru7@xxxxxx>
Cc: Serge Hallyn <serge@xxxxxxxxxx>
Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
---
 Documentation/cgroup-v2.txt |   60 +++++++++++++++++++--------
 include/linux/cgroup-defs.h |    9 ++++
 kernel/cgroup/cgroup.c      |   96 +++++++++++++++++++++++++++++++++++++++++---
 3 files changed, 142 insertions(+), 23 deletions(-)

--- a/Documentation/cgroup-v2.txt
+++ b/Documentation/cgroup-v2.txt
@@ -149,6 +149,15 @@ during boot, before manual intervention
 and experimenting easier, the kernel parameter cgroup_no_v1= allows
 disabling controllers in v1 and make them always available in v2.
 
+cgroup v2 currently supports the following mount options.
+
+  nsdelegate
+
+	Consider cgroup namespaces as delegation boundaries.  This
+	option is system wide and can only be set on mount or modified
+	through remount from the init namespace.  Please refer to the
+	Delegation section for details.
+
 
 2-2. Organizing Processes
 
@@ -308,19 +317,27 @@ file.
 
 2-5-1. Model of Delegation
 
-A cgroup can be delegated to a less privileged user by granting write
-access of the directory and its "cgroup.procs" and
-"cgroup.subtree_control" files to the user.  Note that resource
-control interface files in a given directory control the distribution
-of the parent's resources and thus must not be delegated along with
-the directory.
-
-Once delegated, the user can build sub-hierarchy under the directory,
-organize processes as it sees fit and further distribute the resources
-it received from the parent.  The limits and other settings of all
-resource controllers are hierarchical and regardless of what happens
-in the delegated sub-hierarchy, nothing can escape the resource
-restrictions imposed by the parent.
+A cgroup can be delegated in two ways.  First, to a less privileged
+user by granting write access of the directory and its "cgroup.procs"
+and "cgroup.subtree_control" files to the user.  Second, if the
+"nsdelegate" mount option is set, automatically to a cgroup namespace
+on namespace creation.
+
+Because the resource control interface files in a given directory
+control the distribution of the parent's resources, the delegatee
+shouldn't be allowed to write to them.  For the first method, this is
+achieved by not granting access to these files.  For the second, the
+kernel rejects writes to all files other than "cgroup.procs" and
+"cgroup.subtree_control" on a namespace root from inside the
+namespace.
+
+The end results are equivalent for both delegation types.  Once
+delegated, the user can build sub-hierarchy under the directory,
+organize processes inside it as it sees fit and further distribute the
+resources it received from the parent.  The limits and other settings
+of all resource controllers are hierarchical and regardless of what
+happens in the delegated sub-hierarchy, nothing can escape the
+resource restrictions imposed by the parent.
 
 Currently, cgroup doesn't impose any restrictions on the number of
 cgroups in or nesting depth of a delegated sub-hierarchy; however,
@@ -330,10 +347,12 @@ this may be limited explicitly in the fu
 2-5-2. Delegation Containment
 
 A delegated sub-hierarchy is contained in the sense that processes
-can't be moved into or out of the sub-hierarchy by the delegatee.  For
-a process with a non-root euid to migrate a target process into a
-cgroup by writing its PID to the "cgroup.procs" file, the following
-conditions must be met.
+can't be moved into or out of the sub-hierarchy by the delegatee.
+
+For delegations to a less privileged user, this is achieved by
+requiring the following conditions for a process with a non-root euid
+to migrate a target process into a cgroup by writing its PID to the
+"cgroup.procs" file.
 
 - The writer must have write access to the "cgroup.procs" file.
 
@@ -360,6 +379,11 @@ destination cgroup C00 is above the poin
 not have write access to its "cgroup.procs" files and thus the write
 will be denied with -EACCES.
 
+For delegations to namespaces, containment is achieved by requiring
+that both the source and destination cgroups are reachable from the
+namespace of the process which is attempting the migration.  If either
+is not reachable, the migration is rejected with -ENOENT.
+
 
 2-6. Guidelines
 
@@ -1414,7 +1438,7 @@ D. Deprecated v1 Core Features
 
 - Multiple hierarchies including named ones are not supported.
 
-- All mount options and remounting are not supported.
+- All v1 mount options are not supported.
 
 - The "tasks" file is removed and "cgroup.procs" is not sorted.
 
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -67,12 +67,21 @@ enum {
 enum {
 	CGRP_ROOT_NOPREFIX	= (1 << 1), /* mounted subsystems have no named prefix */
 	CGRP_ROOT_XATTR		= (1 << 2), /* supports extended attributes */
+
+	/*
+	 * Consider namespaces as delegation boundaries.  If this flag is
+	 * set, controller specific interface files in a namespace root
+	 * aren't writeable from inside the namespace.
+	 */
+	CGRP_ROOT_NS_DELEGATE	= (1 << 3),
 };
 
 /* cftype->flags */
 enum {
 	CFTYPE_ONLY_ON_ROOT	= (1 << 0),	/* only create on root cgrp */
 	CFTYPE_NOT_ON_ROOT	= (1 << 1),	/* don't create on root cgrp */
+	CFTYPE_NS_DELEGATABLE	= (1 << 2),	/* writeable beyond delegation boundaries */
+
 	CFTYPE_NO_PREFIX	= (1 << 3),	/* (DON'T USE FOR NEW FILES) no subsys prefix */
 	CFTYPE_WORLD_WRITABLE	= (1 << 4),	/* (DON'T USE FOR NEW FILES) S_IWUGO */
 
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1547,10 +1547,64 @@ int cgroup_show_path(struct seq_file *sf
 	return len;
 }
 
+static int parse_cgroup_root_flags(char *data, unsigned int *root_flags)
+{
+	char *token;
+
+	*root_flags = 0;
+
+	if (!data)
+		return 0;
+
+	while ((token = strsep(&data, ",")) != NULL) {
+		if (!strcmp(token, "nsdelegate")) {
+			*root_flags |= CGRP_ROOT_NS_DELEGATE;
+			continue;
+		}
+
+		pr_err("cgroup2: unknown option \"%s\"\n", token);
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+static void apply_cgroup_root_flags(unsigned int root_flags)
+{
+	unsigned int new_flags = cgrp_dfl_root.flags;
+
+	if (root_flags & CGRP_ROOT_NS_DELEGATE)
+		new_flags |= CGRP_ROOT_NS_DELEGATE;
+	else
+		new_flags &= ~CGRP_ROOT_NS_DELEGATE;
+
+	if (current->nsproxy->cgroup_ns != &init_cgroup_ns) {
+		if (cgrp_dfl_root.flags != new_flags)
+			pr_warning("cgroup2: can't apply mount flags outside init namespace\n");
+		return;
+	}
+
+	cgrp_dfl_root.flags = new_flags;
+}
+
+static int cgroup_show_options(struct seq_file *seq, struct kernfs_root *kf_root)
+{
+	if (cgrp_dfl_root.flags & CGRP_ROOT_NS_DELEGATE)
+		seq_puts(seq, ",nsdelegate");
+	return 0;
+}
+
 static int cgroup_remount(struct kernfs_root *kf_root, int *flags, char *data)
 {
-	pr_err("remount is not allowed\n");
-	return -EINVAL;
+	unsigned int root_flags;
+	int ret;
+
+	ret = parse_cgroup_root_flags(data, &root_flags);
+	if (ret)
+		return ret;
+
+	apply_cgroup_root_flags(root_flags);
+	return 0;
 }
 
 /*
@@ -1790,6 +1844,7 @@ static struct dentry *cgroup_mount(struc
 {
 	struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
 	struct dentry *dentry;
+	int ret;
 
 	get_cgroup_ns(ns);
 
@@ -1807,16 +1862,21 @@ static struct dentry *cgroup_mount(struc
 		cgroup_enable_task_cg_lists();
 
 	if (fs_type == &cgroup2_fs_type) {
-		if (data) {
-			pr_err("cgroup2: unknown option \"%s\"\n", (char *)data);
+		unsigned int root_flags;
+
+		ret = parse_cgroup_root_flags(data, &root_flags);
+		if (ret) {
 			put_cgroup_ns(ns);
-			return ERR_PTR(-EINVAL);
+			return ERR_PTR(ret);
 		}
+
 		cgrp_dfl_visible = true;
 		cgroup_get_live(&cgrp_dfl_root.cgrp);
 
 		dentry = cgroup_do_mount(&cgroup2_fs_type, flags, &cgrp_dfl_root,
 					 CGROUP2_SUPER_MAGIC, ns);
+		if (!IS_ERR(dentry))
+			apply_cgroup_root_flags(root_flags);
 	} else {
 		dentry = cgroup1_mount(&cgroup_fs_type, flags, data,
 				       CGROUP_SUPER_MAGIC, ns);
@@ -2364,6 +2424,8 @@ static int cgroup_procs_write_permission
 					 struct kernfs_open_file *of)
 {
 	struct super_block *sb = of->file->f_path.dentry->d_sb;
+	struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
+	struct cgroup *root_cgrp = ns->root_cset->dfl_cgrp;
 	struct cgroup *src_cgrp, *com_cgrp;
 	struct inode *inode;
 	int ret;
@@ -2407,6 +2469,15 @@ static int cgroup_procs_write_permission
 	if (ret)
 		return ret;
 
+	/*
+	 * If namespaces are delegation boundaries, %current must be able
+	 * to see both source and destination cgroups from its namespace.
+	 */
+	if ((cgrp_dfl_root.flags & CGRP_ROOT_NS_DELEGATE) &&
+	    (!cgroup_is_descendant(src_cgrp, root_cgrp) ||
+	     !cgroup_is_descendant(dst_cgrp, root_cgrp)))
+		return -ENOENT;
+
 	return 0;
 }
 
@@ -2971,11 +3042,23 @@ static void cgroup_file_release(struct k
 static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf,
 				 size_t nbytes, loff_t off)
 {
+	struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
 	struct cgroup *cgrp = of->kn->parent->priv;
 	struct cftype *cft = of->kn->priv;
 	struct cgroup_subsys_state *css;
 	int ret;
 
+	/*
+	 * If namespaces are delegation boundaries, disallow writes to
+	 * files in an non-init namespace root from inside the namespace
+	 * except for the files explicitly marked delegatable -
+	 * cgroup.procs and cgroup.subtree_control.
+	 */
+	if ((cgrp->root->flags & CGRP_ROOT_NS_DELEGATE) &&
+	    !(cft->flags & CFTYPE_NS_DELEGATABLE) &&
+	    ns != &init_cgroup_ns && ns->root_cset->dfl_cgrp == cgrp)
+		return -EPERM;
+
 	if (cft->write)
 		return cft->write(of, buf, nbytes, off);
 
@@ -3809,6 +3892,7 @@ static int cgroup_procs_show(struct seq_
 static struct cftype cgroup_base_files[] = {
 	{
 		.name = "cgroup.procs",
+		.flags = CFTYPE_NS_DELEGATABLE,
 		.file_offset = offsetof(struct cgroup, procs_file),
 		.release = cgroup_procs_release,
 		.seq_start = cgroup_procs_start,
@@ -3822,6 +3906,7 @@ static struct cftype cgroup_base_files[]
 	},
 	{
 		.name = "cgroup.subtree_control",
+		.flags = CFTYPE_NS_DELEGATABLE,
 		.seq_show = cgroup_subtree_control_show,
 		.write = cgroup_subtree_control_write,
 	},
@@ -4410,6 +4495,7 @@ int cgroup_rmdir(struct kernfs_node *kn)
 }
 
 static struct kernfs_syscall_ops cgroup_kf_syscall_ops = {
+	.show_options		= cgroup_show_options,
 	.remount_fs		= cgroup_remount,
 	.mkdir			= cgroup_mkdir,
 	.rmdir			= cgroup_rmdir,