users: More precise username validation

- Username should match [a-zA-Z0-9_.@-], can't start with '-'
- Use Python pwd module to retrieve all users instead of getent command.
- Checking, that a username already exists or is reservered, is case insensitive

Created usernames are now compatible with openldap and nslcd. Didn't change
urlpatterns in case of an invalid username is already created by the admin.

Closes #1773

Signed-off-by: Veiko Aasa <veiko17@disroot.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/users/forms.py

@@ -15,14 +15,17 @@
 # along with this program. If not, see <http://www.gnu.org/licenses/>.
 #
 
-import subprocess
+import pwd
+import re
 
 import plinth.forms
 from django import forms
 from django.contrib import auth, messages
 from django.contrib.auth.forms import SetPasswordForm, UserCreationForm
 from django.contrib.auth.models import Group, User
+from django.core import validators
 from django.core.exceptions import ValidationError
+from django.utils.deconstruct import deconstructible
 from django.utils.translation import ugettext as _
 from django.utils.translation import ugettext_lazy
 from plinth import actions, module_loader
@@ -46,35 +49,51 @@ def get_group_choices():
 
 class ValidNewUsernameCheckMixin(object):
     """Mixin to check if a username is valid for created new user."""
+
     def clean_username(self):
         """Check for username collisions with system users."""
         username = self.cleaned_data['username']
         if self.instance.username != username and \
                 not self.is_valid_new_username():
-            raise ValidationError(_('Username is taken or is reserved.'),
-                                  code='invalid')
+            raise ValidationError(
+                _('Username is taken or is reserved.'), code='invalid')
 
         return username
 
     def is_valid_new_username(self):
         """Check for username collisions with system users."""
         username = self.cleaned_data['username']
-        try:
-            subprocess.run(['getent', 'passwd', username],
-                           stdout=subprocess.DEVNULL, check=True)
-            # Exit code 0 means that the username is already in use.
+        existing_users = (a.pw_name.lower() for a in pwd.getpwall())
+        if username.lower() in existing_users:
             return False
-        except subprocess.CalledProcessError:
-            pass
 
         for module_name, module in module_loader.loaded_modules.items():
             for reserved_username in getattr(module, 'reserved_usernames', []):
-                if username == reserved_username:
+                if username.lower() == reserved_username.lower():
                     return False
 
         return True
 
 
+@deconstructible
+class UsernameValidator(validators.RegexValidator):
+    """Username validator.
+
+    Compared to django builtin ASCIIUsernameValidator, do not allow
+    '+' characters and no '-' character at the beginning.
+
+    """
+    regex = r'^[\w.@][\w.@-]+\Z'
+    message = ugettext_lazy('Enter a valid username.')
+    flags = re.ASCII
+
+
+USERNAME_FIELD = forms.CharField(
+    max_length=150, validators=[UsernameValidator()],
+    help_text='Required. 150 characters or fewer. English letters, \
+                digits and @/./-/_ only.')
+
+
 class CreateUserForm(ValidNewUsernameCheckMixin,
                      plinth.forms.LanguageSelectionFormMixin,
                      UserCreationForm):
@@ -82,6 +101,7 @@ class CreateUserForm(ValidNewUsernameCheckMixin,
 
     Include options to add user to groups.
     """
+    username = USERNAME_FIELD
     groups = forms.MultipleChoiceField(
         choices=get_group_choices(), label=ugettext_lazy('Permissions'),
         required=False, widget=forms.CheckboxSelectMultiple,
@@ -144,6 +164,7 @@ class CreateUserForm(ValidNewUsernameCheckMixin,
 class UserUpdateForm(ValidNewUsernameCheckMixin,
                      plinth.forms.LanguageSelectionFormMixin, forms.ModelForm):
     """When user info is changed, also updates LDAP user."""
+    username = USERNAME_FIELD
     ssh_keys = forms.CharField(
         label=ugettext_lazy('Authorized SSH Keys'), required=False,
         widget=forms.Textarea, help_text=ugettext_lazy(
@@ -294,12 +315,14 @@ class UserUpdateForm(ValidNewUsernameCheckMixin,
 
 class UserChangePasswordForm(SetPasswordForm):
     """Custom form that also updates password for LDAP users."""
+
     def __init__(self, request, *args, **kwargs):
         """Initialize the form with extra request argument."""
         self.request = request
         super(UserChangePasswordForm, self).__init__(*args, **kwargs)
-        self.fields['new_password1'].widget.attrs.update(
-            {'autofocus': 'autofocus'})
+        self.fields['new_password1'].widget.attrs.update({
+            'autofocus': 'autofocus'
+        })
 
     def save(self, commit=True):
         """Save the user model and change LDAP password as well."""
@@ -319,6 +342,8 @@ class UserChangePasswordForm(SetPasswordForm):
 
 class FirstBootForm(ValidNewUsernameCheckMixin, auth.forms.UserCreationForm):
     """User module first boot step: create a new admin user."""
+    username = USERNAME_FIELD
+
     def __init__(self, *args, **kwargs):
         self.request = kwargs.pop('request')
         super().__init__(*args, **kwargs)