[PATCH 15/15] sysfs: user namespaces: fix bug with clone(CLONE_NEWUSER) with fairsched

[Eric W. Biederman]

Mark the /sys/kernel/uids directory to be tagged so that processes in
different user namespaces can remount /sys and see their own uid
listings.

Without this patch, having CONFIG_FAIR_SCHED=y makes user namespaces
unusable, because when you
  clone(CLONE_NEWUSER)
it will auto-create the root userid and try to create
/sys/kernel/uids/0.  Since that already exists from the parent user
namespace, the create fails, and the clone misleadingly ends up
returning -ENOMEM.

This patch fixes the issue by allowing each user namespace to remount
/sys, and having /sys filter the /sys/kernel/uid/ entries by user
namespace.

Changelong:
v2 - Reworked for the updated sysfs api

Signed-off-by: Serge Hallyn <serue@xxxxxxxxxx>
Signed-off-by: Benjamin Thery <benjamin.thery@xxxxxxxx>
Signed-off-by: Daniel Lezcano <dlezcano@xxxxxxxxxx>
Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
---
 include/linux/sched.h   |    1 +
 include/linux/sysfs.h   |    1 +
 kernel/user.c           |   22 ++++++++++++++++++++++
 kernel/user_namespace.c |    1 +
 4 files changed, 25 insertions(+), 0 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index c5d3f84..d2be6a5 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -598,6 +598,7 @@ struct user_struct {
 	/* Hash table maintenance information */
 	struct hlist_node uidhash_node;
 	uid_t uid;
+	struct user_namespace *user_ns;
 
 #ifdef CONFIG_USER_SCHED
 	struct task_group *tg;
diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
index 1ed31bb..ecb942c 100644
--- a/include/linux/sysfs.h
+++ b/include/linux/sysfs.h
@@ -81,6 +81,7 @@ struct sysfs_ops {
 enum sysfs_tag_type {
 	SYSFS_TAG_TYPE_NONE = 0,
 	SYSFS_TAG_TYPE_NETNS,
+	SYSFS_TAG_TYPE_USERNS,
 	SYSFS_TAG_TYPES
 };
 
diff --git a/kernel/user.c b/kernel/user.c
index 865ecf5..ca29fbc 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -53,6 +53,7 @@ struct user_struct root_user = {
 	.files		= ATOMIC_INIT(0),
 	.sigpending	= ATOMIC_INIT(0),
 	.locked_shm     = 0,
+	.user_ns	= &init_user_ns,
 #ifdef CONFIG_USER_SCHED
 	.tg		= &init_task_group,
 #endif
@@ -230,16 +231,33 @@ static struct attribute *uids_attributes[] = {
 	NULL
 };
 
+static const void *uids_mount_tag(void)
+{
+	return current->nsproxy->user_ns;
+}
+
+static struct sysfs_tag_type_operations uids_tag_type_operations = {
+	.mount_tag = uids_mount_tag,
+};
+
 /* the lifetime of user_struct is not managed by the core (now) */
 static void uids_release(struct kobject *kobj)
 {
 	return;
 }
 
+static const void *uids_sysfs_tag(struct kobject *kobj)
+{
+	struct user_struct *up;
+	up = container_of(kobj, struct user_struct, kobj);
+	return up->user_ns;
+}
+
 static struct kobj_type uids_ktype = {
 	.sysfs_ops = &kobj_sysfs_ops,
 	.default_attrs = uids_attributes,
 	.release = uids_release,
+	.sysfs_tag = uids_sysfs_tag,
 };
 
 /* create /sys/kernel/uids/<uid>/cpu_share file for this user */
@@ -272,6 +290,9 @@ int __init uids_sysfs_init(void)
 	if (!uids_kset)
 		return -ENOMEM;
 
+	sysfs_register_tag_type(SYSFS_TAG_TYPE_USERNS, &uids_tag_type_operations);
+	sysfs_make_tagged_dir(&uids_kset->kobj, SYSFS_TAG_TYPE_USERNS);
+
 	return uids_user_create(&root_user);
 }
 
@@ -405,6 +426,7 @@ struct user_struct *alloc_uid(struct user_namespace *ns, uid_t uid)
 
 		new->uid = uid;
 		atomic_set(&new->__count, 1);
+		new->user_ns = ns;
 
 		if (sched_create_user(new) < 0)
 			goto out_free_user;
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index a9ab059..f67bbe0 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -71,6 +71,7 @@ void free_user_ns(struct kref *kref)
 	struct user_namespace *ns;
 
 	ns = container_of(kref, struct user_namespace, kref);
+	sysfs_exit_tag(SYSFS_TAG_TYPE_USERNS, ns);
 	release_uids(ns);
 	kfree(ns);
 }
-- 
1.5.3.rc6.17.g1911

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