nsfs: Add an ioctl() to return owner UID of a userns

I'd like to write code that discovers the user namespace hierarchy on a
running system, and also shows who owns the various user namespaces.
Currently, there is no way of getting the owner UID of a user namespace.
Therefore, this patch adds a new NS_GET_CREATOR_UID ioctl() that fetches
the UID (as seen in the user namespace of the caller) of the creator of
the user namespace referred to by the specified file descriptor.

If the supplied file descriptor does not refer to a user namespace,
the operation fails with the error EINVAL. If the owner UID does
not have a mapping in the caller's user namespace return the
overflow UID as that appears easier to deal with in practice
in user-space applications.

-- EWB Changed the handling of unmapped UIDs from -EOVERFLOW
   back to the overflow uid.  Per conversation with
   Michael Kerrisk after examining his test code.
Acked-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Michael Kerrisk <mtk-manpages@gmail.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

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;
 	}

include/uapi/linux/nsfs.h

@@ -12,5 +12,7 @@
 /* Returns the type of namespace (CLONE_NEW* value) referred to by
    file descriptor */
 #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 */