users: Avoid username conflicts with system users

When creating or renaming a user, check if the new username is in use by
any system user.

plinth/modules/first_boot/forms.py

@@ -22,6 +22,7 @@ Forms for first boot module.
 import json
 import logging
 import requests
+import subprocess
 
 from django import forms
 from django.contrib import auth
@@ -46,6 +47,18 @@ class State1Form(auth.forms.UserCreationForm):
         self.request = kwargs.pop('request')
         super().__init__(*args, **kwargs)
 
+    def clean(self):
+        """Check for username collisions with system users."""
+        username = self.cleaned_data['username']
+        try:
+            subprocess.run(['getent', 'passwd', username], check=True)
+            # Exit code 0 means that the username is already in use.
+            raise ValidationError(_('Username is reserved'))
+        except subprocess.CalledProcessError:
+            pass
+
+        return super().clean()
+
     def save(self, commit=True):
         """Create and log the user in."""
         user = super().save(commit=commit)

plinth/modules/users/forms.py

@@ -15,10 +15,13 @@
 # along with this program. If not, see <http://www.gnu.org/licenses/>.
 #
 
+import subprocess
+
 from django import forms
 from django.contrib import messages
 from django.contrib.auth.models import User, Group
 from django.contrib.auth.forms import UserCreationForm, SetPasswordForm
+from django.core.exceptions import ValidationError
 from django.utils.translation import ugettext as _, ugettext_lazy
 
 from plinth import actions
@@ -55,6 +58,18 @@ class CreateUserForm(UserCreationForm):
         self.request = request
         super(CreateUserForm, self).__init__(*args, **kwargs)
 
+    def clean(self):
+        """Check for username collisions with system users."""
+        username = self.cleaned_data['username']
+        try:
+            subprocess.run(['getent', 'passwd', username], check=True)
+            # Exit code 0 means that the username is already in use.
+            raise ValidationError(_('Username is reserved'))
+        except subprocess.CalledProcessError:
+            pass
+
+        return super().clean()
+
     def save(self, commit=True):
         """Save the user model and create LDAP user if required."""
         user = super(CreateUserForm, self).save(commit)
@@ -116,6 +131,18 @@ class UserUpdateForm(forms.ModelForm):
         self.username = username
         super(UserUpdateForm, self).__init__(*args, **kwargs)
 
+    def clean(self):
+        """Check for username collisions with system users."""
+        username = self.cleaned_data['username']
+        try:
+            subprocess.run(['getent', 'passwd', username], check=True)
+            # Exit code 0 means that the username is already in use.
+            raise ValidationError(_('Username is reserved'))
+        except subprocess.CalledProcessError:
+            pass
+
+        return super().clean()
+
     def save(self, commit=True):
         """Update LDAP user name and groups after saving user model."""
         user = super(UserUpdateForm, self).save(commit)