Description: Fix file existence disclosures (CVE-2020-16128)
 Move set_euid_egid into aptdaemon.utils and use it when setting
 Terminal and DebConfSocket properties, to ensure our checks only
 have access to the same objects the user has.
 .
 Rewrite the checks so that they avoid extra os.access lookups and
 have correct error handling for os.open() in the terminal, and
 os.stat() in the debconf case.
Author: Julian Andres Klode <juliank@ubuntu.com>
Bug-Ubuntu: https://bugs.launchpad.net/bugs/1899513
--- a/aptdaemon/core.py
+++ b/aptdaemon/core.py
@@ -60,7 +60,7 @@ from . import enums
 from defer import inline_callbacks, return_value, Deferred
 from defer.utils import dbus_deferred_method
 from . import policykit1
-from .utils import split_package_id
+from .utils import split_package_id, set_euid_egid
 from .worker import DummyWorker
 from .worker.aptworker import (AptWorker,
                                trans_only_installs_pkgs_from_high_trust_repos)
@@ -1107,24 +1107,28 @@ class Transaction(DBusObject):
         """
         if self.status != enums.STATUS_SETTING_UP:
             raise errors.TransactionAlreadyRunning()
-        if not os.access(ttyname, os.W_OK):
-            raise errors.AptDaemonError("Pty device does not exist: "
-                                        "%s" % ttyname)
-        if not os.stat(ttyname)[4] == self.uid:
-            raise errors.AptDaemonError("Pty device '%s' has to be owned by"
-                                        "the owner of the transaction "
-                                        "(uid %s) " % (ttyname, self.uid))
-        if os.path.dirname(ttyname) != "/dev/pts":
-            raise errors.AptDaemonError("%s isn't a tty" % ttyname)
-        try:
-            slave_fd = os.open(ttyname, os.O_RDWR | os.O_NOCTTY)
-            if os.isatty(slave_fd):
-                self.terminal = dbus.String(ttyname)
-                self.PropertyChanged("Terminal", self.terminal)
-            else:
+        with set_euid_egid(self.uid, self.gid):
+            if os.path.dirname(ttyname) != "/dev/pts":
                 raise errors.AptDaemonError("%s isn't a tty" % ttyname)
-        finally:
-            os.close(slave_fd)
+
+            slave_fd = None
+            try:
+                slave_fd = os.open(ttyname, os.O_RDWR | os.O_NOCTTY)
+            except Exception:
+                raise errors.AptDaemonError("Could not open %s" % ttyname)
+            else:
+                if os.fstat(slave_fd).st_uid != self.uid:
+                    raise errors.AptDaemonError("Pty device '%s' has to be owned by"
+                                                "the owner of the transaction "
+                                                "(uid %s) " % (ttyname, self.uid))
+                if os.isatty(slave_fd):
+                    self.terminal = dbus.String(ttyname)
+                    self.PropertyChanged("Terminal", self.terminal)
+                else:
+                    raise errors.AptDaemonError("%s isn't a tty" % ttyname)
+            finally:
+                if slave_fd is not None:
+                    os.close(slave_fd)
 
     def _set_debconf(self, debconf_socket):
         """Set the socket of the debconf proxy.
@@ -1140,13 +1144,17 @@ class Transaction(DBusObject):
         """
         if self.status != enums.STATUS_SETTING_UP:
             raise errors.TransactionAlreadyRunning()
-        if not os.access(debconf_socket, os.W_OK):
-            raise errors.AptDaemonError("socket does not exist: "
-                                        "%s" % debconf_socket)
-        if not os.stat(debconf_socket)[4] == self.uid:
-            raise errors.AptDaemonError("socket '%s' has to be owned by the "
-                                        "owner of the "
-                                        "transaction" % debconf_socket)
+        with set_euid_egid(self.uid, self.gid):
+            try:
+                stat = os.stat(debconf_socket)
+            except Exception:
+                raise errors.AptDaemonError("socket status could not be read: "
+                                            "%s" % debconf_socket)
+            else:
+                if stat.st_uid != self.uid:
+                    raise errors.AptDaemonError("socket '%s' has to be owned by the "
+                                                "owner of the "
+                                                "transaction" % debconf_socket)
         self.debconf = dbus.String(debconf_socket)
         self.PropertyChanged("DebconfSocket", self.debconf)
 
--- a/aptdaemon/worker/aptworker.py
+++ b/aptdaemon/worker/aptworker.py
@@ -58,6 +58,7 @@ from . import BaseWorker
 from ..enums import *
 from ..errors import *
 from .. import lock
+from ..utils import set_euid_egid
 from ..progress import (
     DaemonOpenProgress,
     DaemonInstallProgress,
@@ -90,25 +91,6 @@ USE_HTTP="yes"
 """
 
 
-@contextlib.contextmanager
-def set_euid_egid(uid, gid):
-    # no need to drop privs
-    if os.getuid() != 0 and os.getgid() != 0:
-        yield
-        return
-    # temporary drop privs
-    os.setegid(gid)
-    old_groups = os.getgroups()
-    os.setgroups([gid])
-    os.seteuid(uid)
-    try:
-        yield
-    finally:
-        os.seteuid(os.getuid())
-        os.setegid(os.getgid())
-        os.setgroups(old_groups)
-
-
 def trans_only_installs_pkgs_from_high_trust_repos(trans,
                                                    whitelist=set()):
     """Return True if this transaction only touches packages in the
--- a/aptdaemon/utils.py
+++ b/aptdaemon/utils.py
@@ -24,7 +24,9 @@ __author__ = "Sebastian Heinlein <devel@
 
 __all__ = ("deprecated", "IsoCodes")
 
+import os
 import sys
+import contextlib
 import gettext
 import functools
 import warnings
@@ -58,6 +60,25 @@ def deprecated(func):
     return new_func
 
 
+@contextlib.contextmanager
+def set_euid_egid(uid, gid):
+    # no need to drop privs
+    if os.getuid() != 0 and os.getgid() != 0:
+        yield
+        return
+    # temporary drop privs
+    os.setegid(gid)
+    old_groups = os.getgroups()
+    os.setgroups([gid])
+    os.seteuid(uid)
+    try:
+        yield
+    finally:
+        os.seteuid(os.getuid())
+        os.setegid(os.getgid())
+        os.setgroups(old_groups)
+
+
 class IsoCodes(object):
 
     """Provides access to the iso-codes language, script and country