logind: check PolicyKit before allowing VT switch

Let's lock this down a bit. Effectively nothing much changes, since the
default PK policy will allow users on the VT to change VT. Only users
with no local VT session won't be able to switch VTs.

src/login/logind-dbus.c

@@ -1016,6 +1016,8 @@ static int method_activate_session(sd_bus_message *message, void *userdata, sd_b
         if (r < 0)
                 return r;
 
+        /* PolicyKit is done by bus_session_method_activate() */
+
         return bus_session_method_activate(message, session, error);
 }
 
@@ -1047,6 +1049,20 @@ static int method_activate_session_on_seat(sd_bus_message *message, void *userda
                 return sd_bus_error_setf(error, BUS_ERROR_SESSION_NOT_ON_SEAT,
                                          "Session %s not on seat %s", session_name, seat_name);
 
+        r = bus_verify_polkit_async(
+                        message,
+                        CAP_SYS_ADMIN,
+                        "org.freedesktop.login1.chvt",
+                        NULL,
+                        false,
+                        UID_INVALID,
+                        &m->polkit_registry,
+                        error);
+        if (r < 0)
+                return r;
+        if (r == 0)
+                return 1; /* Will call us back */
+
         r = session_activate(session);
         if (r < 0)
                 return r;

src/login/logind-seat-dbus.c

@@ -178,6 +178,20 @@ static int method_activate_session(sd_bus_message *message, void *userdata, sd_b
         if (session->seat != s)
                 return sd_bus_error_setf(error, BUS_ERROR_SESSION_NOT_ON_SEAT, "Session %s not on seat %s", name, s->id);
 
+        r = bus_verify_polkit_async(
+                        message,
+                        CAP_SYS_ADMIN,
+                        "org.freedesktop.login1.chvt",
+                        NULL,
+                        false,
+                        UID_INVALID,
+                        &s->manager->polkit_registry,
+                        error);
+        if (r < 0)
+                return r;
+        if (r == 0)
+                return 1; /* Will call us back */
+
         r = session_activate(session);
         if (r < 0)
                 return r;
@@ -198,7 +212,21 @@ static int method_switch_to(sd_bus_message *message, void *userdata, sd_bus_erro
                 return r;
 
         if (to <= 0)
-                return -EINVAL;
+                return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS, "Invalid virtual terminal");
+
+        r = bus_verify_polkit_async(
+                        message,
+                        CAP_SYS_ADMIN,
+                        "org.freedesktop.login1.chvt",
+                        NULL,
+                        false,
+                        UID_INVALID,
+                        &s->manager->polkit_registry,
+                        error);
+        if (r < 0)
+                return r;
+        if (r == 0)
+                return 1; /* Will call us back */
 
         r = seat_switch_to(s, to);
         if (r < 0)
@@ -214,6 +242,20 @@ static int method_switch_to_next(sd_bus_message *message, void *userdata, sd_bus
         assert(message);
         assert(s);
 
+        r = bus_verify_polkit_async(
+                        message,
+                        CAP_SYS_ADMIN,
+                        "org.freedesktop.login1.chvt",
+                        NULL,
+                        false,
+                        UID_INVALID,
+                        &s->manager->polkit_registry,
+                        error);
+        if (r < 0)
+                return r;
+        if (r == 0)
+                return 1; /* Will call us back */
+
         r = seat_switch_to_next(s);
         if (r < 0)
                 return r;
@@ -228,6 +270,20 @@ static int method_switch_to_previous(sd_bus_message *message, void *userdata, sd
         assert(message);
         assert(s);
 
+        r = bus_verify_polkit_async(
+                        message,
+                        CAP_SYS_ADMIN,
+                        "org.freedesktop.login1.chvt",
+                        NULL,
+                        false,
+                        UID_INVALID,
+                        &s->manager->polkit_registry,
+                        error);
+        if (r < 0)
+                return r;
+        if (r == 0)
+                return 1; /* Will call us back */
+
         r = seat_switch_to_previous(s);
         if (r < 0)
                 return r;

src/login/logind-session-dbus.c

@@ -191,6 +191,20 @@ int bus_session_method_activate(sd_bus_message *message, void *userdata, sd_bus_
         assert(message);
         assert(s);
 
+        r = bus_verify_polkit_async(
+                        message,
+                        CAP_SYS_ADMIN,
+                        "org.freedesktop.login1.chvt",
+                        NULL,
+                        false,
+                        UID_INVALID,
+                        &s->manager->polkit_registry,
+                        error);
+        if (r < 0)
+                return r;
+        if (r == 0)
+                return 1; /* Will call us back */
+
         r = session_activate(s);
         if (r < 0)
                 return r;

src/login/org.freedesktop.login1.policy

@@ -391,4 +391,14 @@
                 </defaults>
         </action>
 
+        <action id="org.freedesktop.login1.chvt">
+                <description gettext-domain="systemd">Change Session</description>
+                <message gettext-domain="systemd">Authentication is required for changing the virtual terminal.</message>
+                <defaults>
+                        <allow_any>auth_admin_keep</allow_any>
+                        <allow_inactive>auth_admin_keep</allow_inactive>
+                        <allow_active>yes</allow_active>
+                </defaults>
+        </action>
+
 </policyconfig>