Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace

Pull user namespace enhancements from Eric Biederman:
 "This is a course correction for the user namespace, so that we can
  reach an inexpensive, maintainable, and reasonably complete
  implementation.

  Highlights:
   - Config guards make it impossible to enable the user namespace and
     code that has not been converted to be user namespace safe.

   - Use of the new kuid_t type ensures the if you somehow get past the
     config guards the kernel will encounter type errors if you enable
     user namespaces and attempt to compile in code whose permission
     checks have not been updated to be user namespace safe.

   - All uids from child user namespaces are mapped into the initial
     user namespace before they are processed.  Removing the need to add
     an additional check to see if the user namespace of the compared
     uids remains the same.

   - With the user namespaces compiled out the performance is as good or
     better than it is today.

   - For most operations absolutely nothing changes performance or
     operationally with the user namespace enabled.

   - The worst case performance I could come up with was timing 1
     billion cache cold stat operations with the user namespace code
     enabled.  This went from 156s to 164s on my laptop (or 156ns to
     164ns per stat operation).

   - (uid_t)-1 and (gid_t)-1 are reserved as an internal error value.
     Most uid/gid setting system calls treat these value specially
     anyway so attempting to use -1 as a uid would likely cause
     entertaining failures in userspace.

   - If setuid is called with a uid that can not be mapped setuid fails.
     I have looked at sendmail, login, ssh and every other program I
     could think of that would call setuid and they all check for and
     handle the case where setuid fails.

   - If stat or a similar system call is called from a context in which
     we can not map a uid we lie and return overflowuid.  The LFS
     experience suggests not lying and returning an error code might be
     better, but the historical precedent with uids is different and I
     can not think of anything that would break by lying about a uid we
     can't map.

   - Capabilities are localized to the current user namespace making it
     safe to give the initial user in a user namespace all capabilities.

  My git tree covers all of the modifications needed to convert the core
  kernel and enough changes to make a system bootable to runlevel 1."

Fix up trivial conflicts due to nearby independent changes in fs/stat.c

* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace: (46 commits)
  userns:  Silence silly gcc warning.
  cred: use correct cred accessor with regards to rcu read lock
  userns: Convert the move_pages, and migrate_pages permission checks to use uid_eq
  userns: Convert cgroup permission checks to use uid_eq
  userns: Convert tmpfs to use kuid and kgid where appropriate
  userns: Convert sysfs to use kgid/kuid where appropriate
  userns: Convert sysctl permission checks to use kuid and kgids.
  userns: Convert proc to use kuid/kgid where appropriate
  userns: Convert ext4 to user kuid/kgid where appropriate
  userns: Convert ext3 to use kuid/kgid where appropriate
  userns: Convert ext2 to use kuid/kgid where appropriate.
  userns: Convert devpts to use kuid/kgid where appropriate
  userns: Convert binary formats to use kuid/kgid where appropriate
  userns: Add negative depends on entries to avoid building code that is userns unsafe
  userns: signal remove unnecessary map_cred_ns
  userns: Teach inode_capable to understand inodes whose uids map to other namespaces.
  userns: Fail exec for suid and sgid binaries with ids outside our user namespace.
  userns: Convert stat to return values mapped from kuids and kgids
  userns: Convert user specfied uids and gids in chown into kuids and kgid
  userns: Use uid_eq gid_eq helpers when comparing kuids and kgids in the vfs
  ...

1 files changed, 531 insertions, 64 deletions

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 3b906e98b1db..86602316422d 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -11,9 +11,20 @@
 #include <linux/user_namespace.h>
 #include <linux/highuid.h>
 #include <linux/cred.h>
+#include <linux/securebits.h>
+#include <linux/keyctl.h>
+#include <linux/key-type.h>
+#include <keys/user-type.h>
+#include <linux/seq_file.h>
+#include <linux/fs.h>
+#include <linux/uaccess.h>
+#include <linux/ctype.h>
 
 static struct kmem_cache *user_ns_cachep __read_mostly;
 
+static bool new_idmap_permitted(struct user_namespace *ns, int cap_setid,
+				struct uid_gid_map *map);
+
 /*
  * Create a new user namespace, deriving the creator from the user in the
  * passed credentials, and replacing that user with the new root user for the
@@ -24,109 +35,565 @@ static struct kmem_cache *user_ns_cachep __read_mostly;
  */
 int create_user_ns(struct cred *new)
 {
-	struct user_namespace *ns;
-	struct user_struct *root_user;
-	int n;
+	struct user_namespace *ns, *parent_ns = new->user_ns;
+	kuid_t owner = new->euid;
+	kgid_t group = new->egid;
+
+	/* The creator needs a mapping in the parent user namespace
+	 * or else we won't be able to reasonably tell userspace who
+	 * created a user_namespace.
+	 */
+	if (!kuid_has_mapping(parent_ns, owner) ||
+	    !kgid_has_mapping(parent_ns, group))
+		return -EPERM;
 
-	ns = kmem_cache_alloc(user_ns_cachep, GFP_KERNEL);
+	ns = kmem_cache_zalloc(user_ns_cachep, GFP_KERNEL);
 	if (!ns)
 		return -ENOMEM;
 
 	kref_init(&ns->kref);
+	ns->parent = parent_ns;
+	ns->owner = owner;
+	ns->group = group;
 
-	for (n = 0; n < UIDHASH_SZ; ++n)
-		INIT_HLIST_HEAD(ns->uidhash_table + n);
-
-	/* Alloc new root user.  */
-	root_user = alloc_uid(ns, 0);
-	if (!root_user) {
-		kmem_cache_free(user_ns_cachep, ns);
-		return -ENOMEM;
-	}
-
-	/* set the new root user in the credentials under preparation */
-	ns->creator = new->user;
-	new->user = root_user;
-	new->uid = new->euid = new->suid = new->fsuid = 0;
-	new->gid = new->egid = new->sgid = new->fsgid = 0;
-	put_group_info(new->group_info);
-	new->group_info = get_group_info(&init_groups);
+	/* Start with the same capabilities as init but useless for doing
+	 * anything as the capabilities are bound to the new user namespace.
+	 */
+	new->securebits = SECUREBITS_DEFAULT;
+	new->cap_inheritable = CAP_EMPTY_SET;
+	new->cap_permitted = CAP_FULL_SET;
+	new->cap_effective = CAP_FULL_SET;
+	new->cap_bset = CAP_FULL_SET;
 #ifdef CONFIG_KEYS
 	key_put(new->request_key_auth);
 	new->request_key_auth = NULL;
 #endif
 	/* tgcred will be cleared in our caller bc CLONE_THREAD won't be set */
 
-	/* root_user holds a reference to ns, our reference can be dropped */
-	put_user_ns(ns);
+	/* Leave the new->user_ns reference with the new user namespace. */
+	/* Leave the reference to our user_ns with the new cred. */
+	new->user_ns = ns;
 
 	return 0;
 }
 
-/*
- * Deferred destructor for a user namespace.  This is required because
- * free_user_ns() may be called with uidhash_lock held, but we need to call
- * back to free_uid() which will want to take the lock again.
- */
-static void free_user_ns_work(struct work_struct *work)
+void free_user_ns(struct kref *kref)
 {
-	struct user_namespace *ns =
-		container_of(work, struct user_namespace, destroyer);
-	free_uid(ns->creator);
+	struct user_namespace *parent, *ns =
+		container_of(kref, struct user_namespace, kref);
+
+	parent = ns->parent;
 	kmem_cache_free(user_ns_cachep, ns);
+	put_user_ns(parent);
 }
+EXPORT_SYMBOL(free_user_ns);
 
-void free_user_ns(struct kref *kref)
+static u32 map_id_range_down(struct uid_gid_map *map, u32 id, u32 count)
 {
-	struct user_namespace *ns =
-		container_of(kref, struct user_namespace, kref);
+	unsigned idx, extents;
+	u32 first, last, id2;
+
+	id2 = id + count - 1;
+
+	/* Find the matching extent */
+	extents = map->nr_extents;
+	smp_read_barrier_depends();
+	for (idx = 0; idx < extents; idx++) {
+		first = map->extent[idx].first;
+		last = first + map->extent[idx].count - 1;
+		if (id >= first && id <= last &&
+		    (id2 >= first && id2 <= last))
+			break;
+	}
+	/* Map the id or note failure */
+	if (idx < extents)
+		id = (id - first) + map->extent[idx].lower_first;
+	else
+		id = (u32) -1;
 
-	INIT_WORK(&ns->destroyer, free_user_ns_work);
-	schedule_work(&ns->destroyer);
+	return id;
 }
-EXPORT_SYMBOL(free_user_ns);
 
-uid_t user_ns_map_uid(struct user_namespace *to, const struct cred *cred, uid_t uid)
+static u32 map_id_down(struct uid_gid_map *map, u32 id)
 {
-	struct user_namespace *tmp;
+	unsigned idx, extents;
+	u32 first, last;
 
-	if (likely(to == cred->user->user_ns))
-		return uid;
+	/* Find the matching extent */
+	extents = map->nr_extents;
+	smp_read_barrier_depends();
+	for (idx = 0; idx < extents; idx++) {
+		first = map->extent[idx].first;
+		last = first + map->extent[idx].count - 1;
+		if (id >= first && id <= last)
+			break;
+	}
+	/* Map the id or note failure */
+	if (idx < extents)
+		id = (id - first) + map->extent[idx].lower_first;
+	else
+		id = (u32) -1;
 
+	return id;
+}
 
-	/* Is cred->user the creator of the target user_ns
-	 * or the creator of one of it's parents?
-	 */
-	for ( tmp = to; tmp != &init_user_ns;
-	      tmp = tmp->creator->user_ns ) {
-		if (cred->user == tmp->creator) {
-			return (uid_t)0;
-		}
+static u32 map_id_up(struct uid_gid_map *map, u32 id)
+{
+	unsigned idx, extents;
+	u32 first, last;
+
+	/* Find the matching extent */
+	extents = map->nr_extents;
+	smp_read_barrier_depends();
+	for (idx = 0; idx < extents; idx++) {
+		first = map->extent[idx].lower_first;
+		last = first + map->extent[idx].count - 1;
+		if (id >= first && id <= last)
+			break;
 	}
+	/* Map the id or note failure */
+	if (idx < extents)
+		id = (id - first) + map->extent[idx].first;
+	else
+		id = (u32) -1;
+
+	return id;
+}
+
+/**
+ *	make_kuid - Map a user-namespace uid pair into a kuid.
+ *	@ns:  User namespace that the uid is in
+ *	@uid: User identifier
+ *
+ *	Maps a user-namespace uid pair into a kernel internal kuid,
+ *	and returns that kuid.
+ *
+ *	When there is no mapping defined for the user-namespace uid
+ *	pair INVALID_UID is returned.  Callers are expected to test
+ *	for and handle handle INVALID_UID being returned.  INVALID_UID
+ *	may be tested for using uid_valid().
+ */
+kuid_t make_kuid(struct user_namespace *ns, uid_t uid)
+{
+	/* Map the uid to a global kernel uid */
+	return KUIDT_INIT(map_id_down(&ns->uid_map, uid));
+}
+EXPORT_SYMBOL(make_kuid);
+
+/**
+ *	from_kuid - Create a uid from a kuid user-namespace pair.
+ *	@targ: The user namespace we want a uid in.
+ *	@kuid: The kernel internal uid to start with.
+ *
+ *	Map @kuid into the user-namespace specified by @targ and
+ *	return the resulting uid.
+ *
+ *	There is always a mapping into the initial user_namespace.
+ *
+ *	If @kuid has no mapping in @targ (uid_t)-1 is returned.
+ */
+uid_t from_kuid(struct user_namespace *targ, kuid_t kuid)
+{
+	/* Map the uid from a global kernel uid */
+	return map_id_up(&targ->uid_map, __kuid_val(kuid));
+}
+EXPORT_SYMBOL(from_kuid);
 
-	/* No useful relationship so no mapping */
-	return overflowuid;
+/**
+ *	from_kuid_munged - Create a uid from a kuid user-namespace pair.
+ *	@targ: The user namespace we want a uid in.
+ *	@kuid: The kernel internal uid to start with.
+ *
+ *	Map @kuid into the user-namespace specified by @targ and
+ *	return the resulting uid.
+ *
+ *	There is always a mapping into the initial user_namespace.
+ *
+ *	Unlike from_kuid from_kuid_munged never fails and always
+ *	returns a valid uid.  This makes from_kuid_munged appropriate
+ *	for use in syscalls like stat and getuid where failing the
+ *	system call and failing to provide a valid uid are not an
+ *	options.
+ *
+ *	If @kuid has no mapping in @targ overflowuid is returned.
+ */
+uid_t from_kuid_munged(struct user_namespace *targ, kuid_t kuid)
+{
+	uid_t uid;
+	uid = from_kuid(targ, kuid);
+
+	if (uid == (uid_t) -1)
+		uid = overflowuid;
+	return uid;
 }
+EXPORT_SYMBOL(from_kuid_munged);
 
-gid_t user_ns_map_gid(struct user_namespace *to, const struct cred *cred, gid_t gid)
+/**
+ *	make_kgid - Map a user-namespace gid pair into a kgid.
+ *	@ns:  User namespace that the gid is in
+ *	@uid: group identifier
+ *
+ *	Maps a user-namespace gid pair into a kernel internal kgid,
+ *	and returns that kgid.
+ *
+ *	When there is no mapping defined for the user-namespace gid
+ *	pair INVALID_GID is returned.  Callers are expected to test
+ *	for and handle INVALID_GID being returned.  INVALID_GID may be
+ *	tested for using gid_valid().
+ */
+kgid_t make_kgid(struct user_namespace *ns, gid_t gid)
 {
-	struct user_namespace *tmp;
+	/* Map the gid to a global kernel gid */
+	return KGIDT_INIT(map_id_down(&ns->gid_map, gid));
+}
+EXPORT_SYMBOL(make_kgid);
 
-	if (likely(to == cred->user->user_ns))
-		return gid;
+/**
+ *	from_kgid - Create a gid from a kgid user-namespace pair.
+ *	@targ: The user namespace we want a gid in.
+ *	@kgid: The kernel internal gid to start with.
+ *
+ *	Map @kgid into the user-namespace specified by @targ and
+ *	return the resulting gid.
+ *
+ *	There is always a mapping into the initial user_namespace.
+ *
+ *	If @kgid has no mapping in @targ (gid_t)-1 is returned.
+ */
+gid_t from_kgid(struct user_namespace *targ, kgid_t kgid)
+{
+	/* Map the gid from a global kernel gid */
+	return map_id_up(&targ->gid_map, __kgid_val(kgid));
+}
+EXPORT_SYMBOL(from_kgid);
+
+/**
+ *	from_kgid_munged - Create a gid from a kgid user-namespace pair.
+ *	@targ: The user namespace we want a gid in.
+ *	@kgid: The kernel internal gid to start with.
+ *
+ *	Map @kgid into the user-namespace specified by @targ and
+ *	return the resulting gid.
+ *
+ *	There is always a mapping into the initial user_namespace.
+ *
+ *	Unlike from_kgid from_kgid_munged never fails and always
+ *	returns a valid gid.  This makes from_kgid_munged appropriate
+ *	for use in syscalls like stat and getgid where failing the
+ *	system call and failing to provide a valid gid are not options.
+ *
+ *	If @kgid has no mapping in @targ overflowgid is returned.
+ */
+gid_t from_kgid_munged(struct user_namespace *targ, kgid_t kgid)
+{
+	gid_t gid;
+	gid = from_kgid(targ, kgid);
 
-	/* Is cred->user the creator of the target user_ns
-	 * or the creator of one of it's parents?
+	if (gid == (gid_t) -1)
+		gid = overflowgid;
+	return gid;
+}
+EXPORT_SYMBOL(from_kgid_munged);
+
+static int uid_m_show(struct seq_file *seq, void *v)
+{
+	struct user_namespace *ns = seq->private;
+	struct uid_gid_extent *extent = v;
+	struct user_namespace *lower_ns;
+	uid_t lower;
+
+	lower_ns = current_user_ns();
+	if ((lower_ns == ns) && lower_ns->parent)
+		lower_ns = lower_ns->parent;
+
+	lower = from_kuid(lower_ns, KUIDT_INIT(extent->lower_first));
+
+	seq_printf(seq, "%10u %10u %10u\n",
+		extent->first,
+		lower,
+		extent->count);
+
+	return 0;
+}
+
+static int gid_m_show(struct seq_file *seq, void *v)
+{
+	struct user_namespace *ns = seq->private;
+	struct uid_gid_extent *extent = v;
+	struct user_namespace *lower_ns;
+	gid_t lower;
+
+	lower_ns = current_user_ns();
+	if ((lower_ns == ns) && lower_ns->parent)
+		lower_ns = lower_ns->parent;
+
+	lower = from_kgid(lower_ns, KGIDT_INIT(extent->lower_first));
+
+	seq_printf(seq, "%10u %10u %10u\n",
+		extent->first,
+		lower,
+		extent->count);
+
+	return 0;
+}
+
+static void *m_start(struct seq_file *seq, loff_t *ppos, struct uid_gid_map *map)
+{
+	struct uid_gid_extent *extent = NULL;
+	loff_t pos = *ppos;
+
+	if (pos < map->nr_extents)
+		extent = &map->extent[pos];
+
+	return extent;
+}
+
+static void *uid_m_start(struct seq_file *seq, loff_t *ppos)
+{
+	struct user_namespace *ns = seq->private;
+
+	return m_start(seq, ppos, &ns->uid_map);
+}
+
+static void *gid_m_start(struct seq_file *seq, loff_t *ppos)
+{
+	struct user_namespace *ns = seq->private;
+
+	return m_start(seq, ppos, &ns->gid_map);
+}
+
+static void *m_next(struct seq_file *seq, void *v, loff_t *pos)
+{
+	(*pos)++;
+	return seq->op->start(seq, pos);
+}
+
+static void m_stop(struct seq_file *seq, void *v)
+{
+	return;
+}
+
+struct seq_operations proc_uid_seq_operations = {
+	.start = uid_m_start,
+	.stop = m_stop,
+	.next = m_next,
+	.show = uid_m_show,
+};
+
+struct seq_operations proc_gid_seq_operations = {
+	.start = gid_m_start,
+	.stop = m_stop,
+	.next = m_next,
+	.show = gid_m_show,
+};
+
+static DEFINE_MUTEX(id_map_mutex);
+
+static ssize_t map_write(struct file *file, const char __user *buf,
+			 size_t count, loff_t *ppos,
+			 int cap_setid,
+			 struct uid_gid_map *map,
+			 struct uid_gid_map *parent_map)
+{
+	struct seq_file *seq = file->private_data;
+	struct user_namespace *ns = seq->private;
+	struct uid_gid_map new_map;
+	unsigned idx;
+	struct uid_gid_extent *extent, *last = NULL;
+	unsigned long page = 0;
+	char *kbuf, *pos, *next_line;
+	ssize_t ret = -EINVAL;
+
+	/*
+	 * The id_map_mutex serializes all writes to any given map.
+	 *
+	 * Any map is only ever written once.
+	 *
+	 * An id map fits within 1 cache line on most architectures.
+	 *
+	 * On read nothing needs to be done unless you are on an
+	 * architecture with a crazy cache coherency model like alpha.
+	 *
+	 * There is a one time data dependency between reading the
+	 * count of the extents and the values of the extents.  The
+	 * desired behavior is to see the values of the extents that
+	 * were written before the count of the extents.
+	 *
+	 * To achieve this smp_wmb() is used on guarantee the write
+	 * order and smp_read_barrier_depends() is guaranteed that we
+	 * don't have crazy architectures returning stale data.
+	 *
 	 */
-	for ( tmp = to; tmp != &init_user_ns;
-	      tmp = tmp->creator->user_ns ) {
-		if (cred->user == tmp->creator) {
-			return (gid_t)0;
+	mutex_lock(&id_map_mutex);
+
+	ret = -EPERM;
+	/* Only allow one successful write to the map */
+	if (map->nr_extents != 0)
+		goto out;
+
+	/* Require the appropriate privilege CAP_SETUID or CAP_SETGID
+	 * over the user namespace in order to set the id mapping.
+	 */
+	if (!ns_capable(ns, cap_setid))
+		goto out;
+
+	/* Get a buffer */
+	ret = -ENOMEM;
+	page = __get_free_page(GFP_TEMPORARY);
+	kbuf = (char *) page;
+	if (!page)
+		goto out;
+
+	/* Only allow <= page size writes at the beginning of the file */
+	ret = -EINVAL;
+	if ((*ppos != 0) || (count >= PAGE_SIZE))
+		goto out;
+
+	/* Slurp in the user data */
+	ret = -EFAULT;
+	if (copy_from_user(kbuf, buf, count))
+		goto out;
+	kbuf[count] = '\0';
+
+	/* Parse the user data */
+	ret = -EINVAL;
+	pos = kbuf;
+	new_map.nr_extents = 0;
+	for (;pos; pos = next_line) {
+		extent = &new_map.extent[new_map.nr_extents];
+
+		/* Find the end of line and ensure I don't look past it */
+		next_line = strchr(pos, '\n');
+		if (next_line) {
+			*next_line = '\0';
+			next_line++;
+			if (*next_line == '\0')
+				next_line = NULL;
 		}
+
+		pos = skip_spaces(pos);
+		extent->first = simple_strtoul(pos, &pos, 10);
+		if (!isspace(*pos))
+			goto out;
+
+		pos = skip_spaces(pos);
+		extent->lower_first = simple_strtoul(pos, &pos, 10);
+		if (!isspace(*pos))
+			goto out;
+
+		pos = skip_spaces(pos);
+		extent->count = simple_strtoul(pos, &pos, 10);
+		if (*pos && !isspace(*pos))
+			goto out;
+
+		/* Verify there is not trailing junk on the line */
+		pos = skip_spaces(pos);
+		if (*pos != '\0')
+			goto out;
+
+		/* Verify we have been given valid starting values */
+		if ((extent->first == (u32) -1) ||
+		    (extent->lower_first == (u32) -1 ))
+			goto out;
+
+		/* Verify count is not zero and does not cause the extent to wrap */
+		if ((extent->first + extent->count) <= extent->first)
+			goto out;
+		if ((extent->lower_first + extent->count) <= extent->lower_first)
+			goto out;
+
+		/* For now only accept extents that are strictly in order */
+		if (last &&
+		    (((last->first + last->count) > extent->first) ||
+		     ((last->lower_first + last->count) > extent->lower_first)))
+			goto out;
+
+		new_map.nr_extents++;
+		last = extent;
+
+		/* Fail if the file contains too many extents */
+		if ((new_map.nr_extents == UID_GID_MAP_MAX_EXTENTS) &&
+		    (next_line != NULL))
+			goto out;
+	}
+	/* Be very certaint the new map actually exists */
+	if (new_map.nr_extents == 0)
+		goto out;
+
+	ret = -EPERM;
+	/* Validate the user is allowed to use user id's mapped to. */
+	if (!new_idmap_permitted(ns, cap_setid, &new_map))
+		goto out;
+
+	/* Map the lower ids from the parent user namespace to the
+	 * kernel global id space.
+	 */
+	for (idx = 0; idx < new_map.nr_extents; idx++) {
+		u32 lower_first;
+		extent = &new_map.extent[idx];
+
+		lower_first = map_id_range_down(parent_map,
+						extent->lower_first,
+						extent->count);
+
+		/* Fail if we can not map the specified extent to
+		 * the kernel global id space.
+		 */
+		if (lower_first == (u32) -1)
+			goto out;
+
+		extent->lower_first = lower_first;
 	}
 
-	/* No useful relationship so no mapping */
-	return overflowgid;
+	/* Install the map */
+	memcpy(map->extent, new_map.extent,
+		new_map.nr_extents*sizeof(new_map.extent[0]));
+	smp_wmb();
+	map->nr_extents = new_map.nr_extents;
+
+	*ppos = count;
+	ret = count;
+out:
+	mutex_unlock(&id_map_mutex);
+	if (page)
+		free_page(page);
+	return ret;
+}
+
+ssize_t proc_uid_map_write(struct file *file, const char __user *buf, size_t size, loff_t *ppos)
+{
+	struct seq_file *seq = file->private_data;
+	struct user_namespace *ns = seq->private;
+
+	if (!ns->parent)
+		return -EPERM;
+
+	return map_write(file, buf, size, ppos, CAP_SETUID,
+			 &ns->uid_map, &ns->parent->uid_map);
+}
+
+ssize_t proc_gid_map_write(struct file *file, const char __user *buf, size_t size, loff_t *ppos)
+{
+	struct seq_file *seq = file->private_data;
+	struct user_namespace *ns = seq->private;
+
+	if (!ns->parent)
+		return -EPERM;
+
+	return map_write(file, buf, size, ppos, CAP_SETGID,
+			 &ns->gid_map, &ns->parent->gid_map);
+}
+
+static bool new_idmap_permitted(struct user_namespace *ns, int cap_setid,
+				struct uid_gid_map *new_map)
+{
+	/* Allow the specified ids if we have the appropriate capability
+	 * (CAP_SETUID or CAP_SETGID) over the parent user namespace.
+	 */
+	if (ns_capable(ns->parent, cap_setid))
+		return true;
+
+	return false;
 }
 
 static __init int user_namespaces_init(void)