Fix CVE-2018-1116: Trusting client-supplied UID

As part of CVE-2013-4288, the D-Bus clients were allowed (and
encouraged) to submit the UID of the subject of authorization checks
to avoid races against UID changes (notably using executables
set-UID to root).

However, that also allowed any client to submit an arbitrary UID, and
that could be used to bypass "can only ask about / affect the same UID"
checks in CheckAuthorization / RegisterAuthenticationAgent /
UnregisterAuthenticationAgent.  This allowed an attacker:

- With CheckAuthorization, to cause the registered authentication
  agent in victim's session to pop up a dialog, or to determine whether
  the victim currently has a temporary authorization to perform an
  operation.

  (In principle, the attacker can also determine whether JavaScript
  rules allow the victim process to perform an operation; however,
  usually rules base their decisions on information determined from
  the supplied UID, so the attacker usually won't learn anything new.)

- With RegisterAuthenticationAgent, to prevent the victim's
  authentication agent to work (for a specific victim process),
  or to learn about which operations requiring authorization
  the victim is attempting.

To fix this, expose internal _polkit_unix_process_get_owner() /
obsolete polkit_unix_process_get_owner() as a private
polkit_unix_process_get_racy_uid__() (being more explicit about the
dangers on relying on it), and use it in
polkit_backend_session_monitor_get_user_for_subject() to return
a boolean indicating whether the subject UID may be caller-chosen.

Then, in the permission checks that require the subject to be
equal to the caller, fail on caller-chosen UIDs (and continue
through the pre-existing code paths which allow root, or root-designated
server processes, to ask about arbitrary subjects.)

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

4 files changed, 95 insertions, 23 deletions

diff --git a/src/polkitbackend/polkitbackendinteractiveauthority.c b/src/polkitbackend/polkitbackendinteractiveauthority.c
index 1cd60d3..cb6fdab 100644
--- a/src/polkitbackend/polkitbackendinteractiveauthority.c
+++ b/src/polkitbackend/polkitbackendinteractiveauthority.c
@@ -575,7 +575,7 @@ log_result (PolkitBackendInteractiveAuthority    *authority,
   if (polkit_authorization_result_get_is_authorized (result))
     log_result_str = "ALLOWING";
 
-  user_of_subject = polkit_backend_session_monitor_get_user_for_subject (priv->session_monitor, subject, NULL);
+  user_of_subject = polkit_backend_session_monitor_get_user_for_subject (priv->session_monitor, subject, NULL, NULL);
 
   subject_str = polkit_subject_to_string (subject);
 
@@ -847,6 +847,7 @@ polkit_backend_interactive_authority_check_authorization (PolkitBackendAuthority
   gchar *subject_str;
   PolkitIdentity *user_of_caller;
   PolkitIdentity *user_of_subject;
+  gboolean user_of_subject_matches;
   gchar *user_of_caller_str;
   gchar *user_of_subject_str;
   PolkitAuthorizationResult *result;
@@ -892,7 +893,7 @@ polkit_backend_interactive_authority_check_authorization (PolkitBackendAuthority
            action_id);
 
   user_of_caller = polkit_backend_session_monitor_get_user_for_subject (priv->session_monitor,
-                                                                        caller,
+                                                                        caller, NULL,
                                                                         &error);
   if (error != NULL)
     {
@@ -907,7 +908,7 @@ polkit_backend_interactive_authority_check_authorization (PolkitBackendAuthority
   g_debug (" user of caller is %s", user_of_caller_str);
 
   user_of_subject = polkit_backend_session_monitor_get_user_for_subject (priv->session_monitor,
-                                                                         subject,
+                                                                         subject, &user_of_subject_matches,
                                                                          &error);
   if (error != NULL)
     {
@@ -937,7 +938,10 @@ polkit_backend_interactive_authority_check_authorization (PolkitBackendAuthority
    * We only allow this if, and only if,
    *
    *  - processes may check for another process owned by the *same* user but not
-   *    if details are passed (otherwise you'd be able to spoof the dialog)
+   *    if details are passed (otherwise you'd be able to spoof the dialog);
+   *    the caller supplies the user_of_subject value, so we additionally
+   *    require it to match at least at one point in time (via
+   *    user_of_subject_matches).
    *
    *  - processes running as uid 0 may check anything and pass any details
    *
@@ -945,7 +949,9 @@ polkit_backend_interactive_authority_check_authorization (PolkitBackendAuthority
    *    then any uid referenced by that annotation is also allowed to check
    *    to check anything and pass any details
    */
-  if (!polkit_identity_equal (user_of_caller, user_of_subject) || has_details)
+  if (!user_of_subject_matches
+      || !polkit_identity_equal (user_of_caller, user_of_subject)
+      || has_details)
     {
       if (!may_identity_check_authorization (interactive_authority, action_id, user_of_caller))
         {
@@ -1110,9 +1116,10 @@ check_authorization_sync (PolkitBackendAuthority         *authority,
       goto out;
     }
 
-  /* every subject has a user */
+  /* every subject has a user; this is supplied by the client, so we rely
+   * on the caller to validate its acceptability. */
   user_of_subject = polkit_backend_session_monitor_get_user_for_subject (priv->session_monitor,
-                                                                         subject,
+                                                                         subject, NULL,
                                                                          error);
   if (user_of_subject == NULL)
       goto out;
@@ -2480,6 +2487,7 @@ polkit_backend_interactive_authority_register_authentication_agent (PolkitBacken
   PolkitSubject *session_for_caller;
   PolkitIdentity *user_of_caller;
   PolkitIdentity *user_of_subject;
+  gboolean user_of_subject_matches;
   AuthenticationAgent *agent;
   gboolean ret;
   gchar *caller_cmdline;
@@ -2532,7 +2540,7 @@ polkit_backend_interactive_authority_register_authentication_agent (PolkitBacken
       goto out;
     }
 
-  user_of_caller = polkit_backend_session_monitor_get_user_for_subject (priv->session_monitor, caller, NULL);
+  user_of_caller = polkit_backend_session_monitor_get_user_for_subject (priv->session_monitor, caller, NULL, NULL);
   if (user_of_caller == NULL)
     {
       g_set_error (error,
@@ -2541,7 +2549,7 @@ polkit_backend_interactive_authority_register_authentication_agent (PolkitBacken
                    "Cannot determine user of caller");
       goto out;
     }
-  user_of_subject = polkit_backend_session_monitor_get_user_for_subject (priv->session_monitor, subject, NULL);
+  user_of_subject = polkit_backend_session_monitor_get_user_for_subject (priv->session_monitor, subject, &user_of_subject_matches, NULL);
   if (user_of_subject == NULL)
     {
       g_set_error (error,
@@ -2550,7 +2558,8 @@ polkit_backend_interactive_authority_register_authentication_agent (PolkitBacken
                    "Cannot determine user of subject");
       goto out;
     }
-  if (!polkit_identity_equal (user_of_caller, user_of_subject))
+  if (!user_of_subject_matches
+      || !polkit_identity_equal (user_of_caller, user_of_subject))
     {
       if (identity_is_root_user (user_of_caller))
         {
@@ -2643,6 +2652,7 @@ polkit_backend_interactive_authority_unregister_authentication_agent (PolkitBack
   PolkitSubject *session_for_caller;
   PolkitIdentity *user_of_caller;
   PolkitIdentity *user_of_subject;
+  gboolean user_of_subject_matches;
   AuthenticationAgent *agent;
   gboolean ret;
   gchar *scope_str;
@@ -2691,7 +2701,7 @@ polkit_backend_interactive_authority_unregister_authentication_agent (PolkitBack
       goto out;
     }
 
-  user_of_caller = polkit_backend_session_monitor_get_user_for_subject (priv->session_monitor, caller, NULL);
+  user_of_caller = polkit_backend_session_monitor_get_user_for_subject (priv->session_monitor, caller, NULL, NULL);
   if (user_of_caller == NULL)
     {
       g_set_error (error,
@@ -2700,7 +2710,7 @@ polkit_backend_interactive_authority_unregister_authentication_agent (PolkitBack
                    "Cannot determine user of caller");
       goto out;
     }
-  user_of_subject = polkit_backend_session_monitor_get_user_for_subject (priv->session_monitor, subject, NULL);
+  user_of_subject = polkit_backend_session_monitor_get_user_for_subject (priv->session_monitor, subject, &user_of_subject_matches, NULL);
   if (user_of_subject == NULL)
     {
       g_set_error (error,
@@ -2709,7 +2719,8 @@ polkit_backend_interactive_authority_unregister_authentication_agent (PolkitBack
                    "Cannot determine user of subject");
       goto out;
     }
-  if (!polkit_identity_equal (user_of_caller, user_of_subject))
+  if (!user_of_subject_matches
+      || !polkit_identity_equal (user_of_caller, user_of_subject))
     {
       if (identity_is_root_user (user_of_caller))
         {
@@ -2819,7 +2830,7 @@ polkit_backend_interactive_authority_authentication_agent_response (PolkitBacken
            identity_str);
 
   user_of_caller = polkit_backend_session_monitor_get_user_for_subject (priv->session_monitor,
-                                                                        caller,
+                                                                        caller, NULL,
                                                                         error);
   if (user_of_caller == NULL)
     goto out;
diff --git a/src/polkitbackend/polkitbackendsessionmonitor-systemd.c b/src/polkitbackend/polkitbackendsessionmonitor-systemd.c
index 2a6c739..b00cdbd 100644
--- a/src/polkitbackend/polkitbackendsessionmonitor-systemd.c
+++ b/src/polkitbackend/polkitbackendsessionmonitor-systemd.c
@@ -29,6 +29,7 @@
 #include <stdlib.h>
 
 #include <polkit/polkit.h>
+#include <polkit/polkitprivate.h>
 #include "polkitbackendsessionmonitor.h"
 
 /* <internal>
@@ -246,26 +247,40 @@ polkit_backend_session_monitor_get_sessions (PolkitBackendSessionMonitor *monito
  * polkit_backend_session_monitor_get_user:
  * @monitor: A #PolkitBackendSessionMonitor.
  * @subject: A #PolkitSubject.
+ * @result_matches: If not %NULL, set to indicate whether the return value matches current (RACY) state.
  * @error: Return location for error.
  *
  * Gets the user corresponding to @subject or %NULL if no user exists.
  *
+ * NOTE: For a #PolkitUnixProcess, the UID is read from @subject (which may
+ * come from e.g. a D-Bus client), so it may not correspond to the actual UID
+ * of the referenced process (at any point in time).  This is indicated by
+ * setting @result_matches to %FALSE; the caller may reject such subjects or
+ * require additional privileges. @result_matches == %TRUE only indicates that
+ * the UID matched the underlying process at ONE point in time, it may not match
+ * later.
+ *
  * Returns: %NULL if @error is set otherwise a #PolkitUnixUser that should be freed with g_object_unref().
  */
 PolkitIdentity *
 polkit_backend_session_monitor_get_user_for_subject (PolkitBackendSessionMonitor  *monitor,
                                                      PolkitSubject                *subject,
+                                                     gboolean                     *result_matches,
                                                      GError                      **error)
 {
   PolkitIdentity *ret;
-  guint32 uid;
+  gboolean matches;
 
   ret = NULL;
+  matches = FALSE;
 
   if (POLKIT_IS_UNIX_PROCESS (subject))
     {
-      uid = polkit_unix_process_get_uid (POLKIT_UNIX_PROCESS (subject));
-      if ((gint) uid == -1)
+      gint subject_uid, current_uid;
+      GError *local_error;
+
+      subject_uid = polkit_unix_process_get_uid (POLKIT_UNIX_PROCESS (subject));
+      if (subject_uid == -1)
         {
           g_set_error (error,
                        POLKIT_ERROR,
@@ -273,14 +288,24 @@ polkit_backend_session_monitor_get_user_for_subject (PolkitBackendSessionMonitor
                        "Unix process subject does not have uid set");
           goto out;
         }
-      ret = polkit_unix_user_new (uid);
+      local_error = NULL;
+      current_uid = polkit_unix_process_get_racy_uid__ (POLKIT_UNIX_PROCESS (subject), &local_error);
+      if (local_error != NULL)
+	{
+	  g_propagate_error (error, local_error);
+	  goto out;
+	}
+      ret = polkit_unix_user_new (subject_uid);
+      matches = (subject_uid == current_uid);
     }
   else if (POLKIT_IS_SYSTEM_BUS_NAME (subject))
     {
       ret = (PolkitIdentity*)polkit_system_bus_name_get_user_sync (POLKIT_SYSTEM_BUS_NAME (subject), NULL, error);
+      matches = TRUE;
     }
   else if (POLKIT_IS_UNIX_SESSION (subject))
     {
+      uid_t uid;
 
       if (sd_session_get_uid (polkit_unix_session_get_session_id (POLKIT_UNIX_SESSION (subject)), &uid) < 0)
         {
@@ -292,9 +317,14 @@ polkit_backend_session_monitor_get_user_for_subject (PolkitBackendSessionMonitor
         }
 
       ret = polkit_unix_user_new (uid);
+      matches = TRUE;
     }
 
  out:
+  if (result_matches != NULL)
+    {
+      *result_matches = matches;
+    }
   return ret;
 }
 
diff --git a/src/polkitbackend/polkitbackendsessionmonitor.c b/src/polkitbackend/polkitbackendsessionmonitor.c
index e1a9ab3..ed30755 100644
--- a/src/polkitbackend/polkitbackendsessionmonitor.c
+++ b/src/polkitbackend/polkitbackendsessionmonitor.c
@@ -27,6 +27,7 @@
 #include <glib/gstdio.h>
 
 #include <polkit/polkit.h>
+#include <polkit/polkitprivate.h>
 #include "polkitbackendsessionmonitor.h"
 
 #define CKDB_PATH "/var/run/ConsoleKit/database"
@@ -273,28 +274,40 @@ polkit_backend_session_monitor_get_sessions (PolkitBackendSessionMonitor *monito
  * polkit_backend_session_monitor_get_user:
  * @monitor: A #PolkitBackendSessionMonitor.
  * @subject: A #PolkitSubject.
+ * @result_matches: If not %NULL, set to indicate whether the return value matches current (RACY) state.
  * @error: Return location for error.
  *
  * Gets the user corresponding to @subject or %NULL if no user exists.
  *
+ * NOTE: For a #PolkitUnixProcess, the UID is read from @subject (which may
+ * come from e.g. a D-Bus client), so it may not correspond to the actual UID
+ * of the referenced process (at any point in time).  This is indicated by
+ * setting @result_matches to %FALSE; the caller may reject such subjects or
+ * require additional privileges. @result_matches == %TRUE only indicates that
+ * the UID matched the underlying process at ONE point in time, it may not match
+ * later.
+ *
  * Returns: %NULL if @error is set otherwise a #PolkitUnixUser that should be freed with g_object_unref().
  */
 PolkitIdentity *
 polkit_backend_session_monitor_get_user_for_subject (PolkitBackendSessionMonitor  *monitor,
                                                      PolkitSubject                *subject,
+                                                     gboolean                     *result_matches,
                                                      GError                      **error)
 {
   PolkitIdentity *ret;
+  gboolean matches;
   GError *local_error;
-  gchar *group;
-  guint32 uid;
 
   ret = NULL;
+  matches = FALSE;
 
   if (POLKIT_IS_UNIX_PROCESS (subject))
     {
-      uid = polkit_unix_process_get_uid (POLKIT_UNIX_PROCESS (subject));
-      if ((gint) uid == -1)
+      gint subject_uid, current_uid;
+
+      subject_uid = polkit_unix_process_get_uid (POLKIT_UNIX_PROCESS (subject));
+      if (subject_uid == -1)
         {
           g_set_error (error,
                        POLKIT_ERROR,
@@ -302,14 +315,26 @@ polkit_backend_session_monitor_get_user_for_subject (PolkitBackendSessionMonitor
                        "Unix process subject does not have uid set");
           goto out;
         }
-      ret = polkit_unix_user_new (uid);
+      local_error = NULL;
+      current_uid = polkit_unix_process_get_racy_uid__ (POLKIT_UNIX_PROCESS (subject), &local_error);
+      if (local_error != NULL)
+	{
+	  g_propagate_error (error, local_error);
+	  goto out;
+	}
+      ret = polkit_unix_user_new (subject_uid);
+      matches = (subject_uid == current_uid);
     }
   else if (POLKIT_IS_SYSTEM_BUS_NAME (subject))
     {
       ret = (PolkitIdentity*)polkit_system_bus_name_get_user_sync (POLKIT_SYSTEM_BUS_NAME (subject), NULL, error);
+      matches = TRUE;
     }
   else if (POLKIT_IS_UNIX_SESSION (subject))
     {
+      gint uid;
+      gchar *group;
+
       if (!ensure_database (monitor, error))
         {
           g_prefix_error (error, "Error getting user for session: Error ensuring CK database at " CKDB_PATH ": ");
@@ -328,9 +353,14 @@ polkit_backend_session_monitor_get_user_for_subject (PolkitBackendSessionMonitor
       g_free (group);
 
       ret = polkit_unix_user_new (uid);
+      matches = TRUE;
     }
 
  out:
+  if (result_matches != NULL)
+    {
+      *result_matches = matches;
+    }
   return ret;
 }
 
diff --git a/src/polkitbackend/polkitbackendsessionmonitor.h b/src/polkitbackend/polkitbackendsessionmonitor.h
index 8f8a2ca..3972326 100644
--- a/src/polkitbackend/polkitbackendsessionmonitor.h
+++ b/src/polkitbackend/polkitbackendsessionmonitor.h
@@ -47,6 +47,7 @@ GList                       *polkit_backend_session_monitor_get_sessions (Polkit
 
 PolkitIdentity              *polkit_backend_session_monitor_get_user_for_subject (PolkitBackendSessionMonitor *monitor,
                                                                                   PolkitSubject               *subject,
+                                                                                  gboolean                    *result_matches,
                                                                                   GError                     **error);
 
 PolkitSubject               *polkit_backend_session_monitor_get_session_for_subject (PolkitBackendSessionMonitor *monitor,