vfs: Don't create inodes with a uid or gid unknown to the vfs

It is expected that filesystems can not represent uids and gids from
outside of their user namespace.  Keep things simple by not even
trying to create filesystem nodes with non-sense uids and gids.

Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

fs/namei.c

@@ -2814,16 +2814,22 @@ static int may_delete(struct inode *dir, struct dentry *victim, bool isdir)
  *  1. We can't do it if child already exists (open has special treatment for
  *     this case, but since we are inlined it's OK)
  *  2. We can't do it if dir is read-only (done in permission())
- *  3. We should have write and exec permissions on dir
- *  4. We can't do it if dir is immutable (done in permission())
+ *  3. We can't do it if the fs can't represent the fsuid or fsgid.
+ *  4. We should have write and exec permissions on dir
+ *  5. We can't do it if dir is immutable (done in permission())
  */
 static inline int may_create(struct inode *dir, struct dentry *child)
 {
+	struct user_namespace *s_user_ns;
 	audit_inode_child(dir, child, AUDIT_TYPE_CHILD_CREATE);
 	if (child->d_inode)
 		return -EEXIST;
 	if (IS_DEADDIR(dir))
 		return -ENOENT;
+	s_user_ns = dir->i_sb->s_user_ns;
+	if (!kuid_has_mapping(s_user_ns, current_fsuid()) ||
+	    !kgid_has_mapping(s_user_ns, current_fsgid()))
+		return -EOVERFLOW;
 	return inode_permission(dir, MAY_WRITE | MAY_EXEC);
 }