2 files changed, 16 insertions, 3 deletions

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 5d534763c662..1656843e87d2 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -7,6 +7,7 @@
 #include <linux/seq_file.h>
 #include <linux/user_namespace.h>
 #include <linux/nsfs.h>
+#include <linux/uaccess.h>
 
 static struct vfsmount *nsfs_mnt;
 
@@ -163,7 +164,10 @@ int open_related_ns(struct ns_common *ns,
 static long ns_ioctl(struct file *filp, unsigned int ioctl,
                         unsigned long arg)
 {
+        struct user_namespace *user_ns;
         struct ns_common *ns = get_proc_ns(file_inode(filp));
+        uid_t __user *argp;
+        uid_t uid;
 
         switch (ioctl) {
         case NS_GET_USERNS:
@@ -174,6 +178,13 @@ static long ns_ioctl(struct file *filp, unsigned int ioctl,
                 return open_related_ns(ns, ns->ops->get_parent);
         case NS_GET_NSTYPE:
                 return ns->ops->type;
+        case NS_GET_OWNER_UID:
+                if (ns->ops->type != CLONE_NEWUSER)
+                        return -EINVAL;
+                user_ns = container_of(ns, struct user_namespace, ns);
+                argp = (uid_t __user *) arg;
+                uid = from_kuid_munged(current_user_ns(), user_ns->owner);
+                return put_user(uid, argp);
         default:
                 return -ENOTTY;
         }
diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h
index 2b48df11056a..1a3ca79f466b 100644
--- a/include/uapi/linux/nsfs.h
+++ b/include/uapi/linux/nsfs.h
@@ -6,11 +6,13 @@
 #define NSIO    0xb7
 
 /* Returns a file descriptor that refers to an owning user namespace */
-#define NS_GET_USERNS   _IO(NSIO, 0x1)
+#define NS_GET_USERNS           _IO(NSIO, 0x1)
 /* Returns a file descriptor that refers to a parent namespace */
-#define NS_GET_PARENT   _IO(NSIO, 0x2)
+#define NS_GET_PARENT           _IO(NSIO, 0x2)
 /* Returns the type of namespace (CLONE_NEW* value) referred to by
    file descriptor */
-#define NS_GET_NSTYPE   _IO(NSIO, 0x3)
+#define NS_GET_NSTYPE           _IO(NSIO, 0x3)
+/* Get owner UID (in the caller's user namespace) for a user namespace */
+#define NS_GET_OWNER_UID        _IO(NSIO, 0x4)
 
 #endif /* __LINUX_NSFS_H */