Protect mountpoint from relative paths and duplicate pathsep

Use os.path.realpath() to clean up any path manipulations

Signed-off-by: Ryan Harper <ryan.harper@canonical.com>

subiquity/models/filesystem.py

@@ -16,6 +16,7 @@
 import json
 import logging
 import re
+from os.path import realpath
 
 from .blockdev import Blockdev, Raiddev, Bcachedev, sort_actions
 import math
@@ -470,6 +471,9 @@ class FilesystemModel(ModelPolicy):
         if not mountpoint.startswith('/'):
             raise ValueError('Does not start with /')
 
+        # remove redundent // and ..
+        mountpoint = realpath(mountpoint)
+
         # /usr/include/linux/limits.h:PATH_MAX
         if len(mountpoint) > 4095:
             raise ValueError('Path exceeds PATH_MAX')