gnome3.mutter: drop inheritable cap_sys_nice

In NixOS extra capabilities are provided through the ambient set which provides
real inheritability to user run processes [0].

We don't want gome-shell to spawn processes with cap_sys_nice however (apart
from the obvious this also breaks eg. flatpaks). So we drop inheritable when
starting to prevent further inheritance (the ambient set is only propagated
if inherit is set).

[0] 58319057b7

pkgs/desktops/gnome-3/core/mutter/default.nix

@@ -9,6 +9,7 @@
 , wrapGAppsHook
 , sysprof
 , desktop-file-utils
+, libcap_ng
 }:
 
 stdenv.mkDerivation rec {
@@ -31,6 +32,7 @@ stdenv.mkDerivation rec {
     # required for pkgconfig to detect mutter-clutter
     libXtst
     json-glib
+    libcap_ng
   ];
 
   nativeBuildInputs = [
@@ -55,6 +57,9 @@ stdenv.mkDerivation rec {
   ];
 
   patches = [
+    # Drop inheritable cap_sys_nice, to prevent the ambient set from leaking
+    # from mutter/gnome-shell, see https://github.com/NixOS/nixpkgs/issues/71381
+    ./drop-inheritable.patch
    # TODO: submit upstream
    ./0001-build-use-get_pkgconfig_variable-for-sysprof-dbusdir.patch
     (substituteAll {

pkgs/desktops/gnome-3/core/mutter/drop-inheritable.patch

@@ -0,0 +1,132 @@
+From e9c772e265b2293af031c79f4bbc99b5847dfe3c Mon Sep 17 00:00:00 2001
+From: =?UTF-8?q?Tor=20Hedin=20Br=C3=B8nner?= <torhedinbronner@gmail.com>
+Date: Sat, 19 Oct 2019 13:26:05 +0200
+Subject: [PATCH] drop inheritable
+
+Adapted from https://gitlab.gnome.org/GNOME/mutter/commit/c53c47ae123b03cc66044d2b846342123ecb3a01
+
+We only want to drop inheritable though, to prevent the ambient set leaking further than gnome-shell.
+
+---
+ config.h.meson    |  3 +++
+ meson.build       |  5 +++++
+ meson_options.txt |  6 ++++++
+ src/core/main.c   | 11 +++++++++++
+ src/meson.build   |  1 +
+ 5 files changed, 26 insertions(+)
+
+diff --git a/config.h.meson b/config.h.meson
+index 0bab71848..202fb7ed1 100644
+--- a/config.h.meson
++++ b/config.h.meson
+@@ -58,6 +58,9 @@
+ /* Xwayland applications allowed to issue keyboard grabs */
+ #mesondefine XWAYLAND_GRAB_DEFAULT_ACCESS_RULES
+ 
++/* Defined if libcap-ng is available */
++#mesondefine HAVE_LIBCAPNG
++
+ /* XKB base prefix */
+ #mesondefine XKB_BASE
+ 
+diff --git a/meson.build b/meson.build
+index 3322bd3b1..01c8020fa 100644
+--- a/meson.build
++++ b/meson.build
+@@ -35,6 +35,7 @@ libstartup_notification_req = '>= 0.7'
+ libcanberra_req = '>= 0.26'
+ libwacom_req = '>= 0.13'
+ atk_req = '>= 2.5.3'
++libcapng_req = '>= 0.7.9'
+ 
+ # optional version requirements
+ udev_req = '>= 228'
+@@ -125,6 +126,7 @@ xau_dep = dependency('xau')
+ ice_dep = dependency('ice')
+ atk_dep = dependency('atk', version: atk_req)
+ libcanberra_dep = dependency('libcanberra', version: libcanberra_req)
++libcapng_dep = dependency('libcap-ng', required: get_option('libcapng'))
+ 
+ # For now always require X11 support
+ have_x11 = true
+@@ -256,6 +258,7 @@ have_core_tests = false
+ have_cogl_tests = false
+ have_clutter_tests = false
+ have_installed_tests = false
++have_libcapng = libcapng_dep.found()
+ 
+ if have_tests
+   have_core_tests = get_option('core_tests')
+@@ -361,6 +364,7 @@ cdata.set('HAVE_LIBWACOM', have_libwacom)
+ cdata.set('HAVE_SM', have_sm)
+ cdata.set('HAVE_STARTUP_NOTIFICATION', have_startup_notification)
+ cdata.set('HAVE_INTROSPECTION', have_introspection)
++cdata.set('HAVE_LIBCAPNG', have_libcapng)
+ cdata.set('HAVE_PROFILER', have_profiler)
+ 
+ xkb_base = xkeyboard_config_dep.get_pkgconfig_variable('xkb_base')
+@@ -443,6 +447,7 @@ output = [
+   '        Startup notification..... ' + have_startup_notification.to_string(),
+   '        Introspection............ ' + have_introspection.to_string(),
+   '        Profiler................. ' + have_profiler.to_string(),
++  '        libcap-ng................ ' + have_libcapng.to_string(),
+   '',
+   '    Tests:',
+   '',
+diff --git a/meson_options.txt b/meson_options.txt
+index 73aa7adde..8bfaacd9a 100644
+--- a/meson_options.txt
++++ b/meson_options.txt
+@@ -152,3 +152,9 @@ option('xwayland_grab_default_access_rules',
+   value: 'gnome-boxes,remote-viewer,virt-viewer,virt-manager,vinagre,vncviewer,Xephyr',
+   description: 'Comma delimited list of applications ressources or class allowed to issue X11 grabs in Xwayland'
+ )
++
++option('libcapng',
++  type: 'feature',
++  value: 'auto',
++  description: 'Enable libcap-ng support'
++)
+diff --git a/src/core/main.c b/src/core/main.c
+index 7f4f666d2..b27968f13 100644
+--- a/src/core/main.c
++++ b/src/core/main.c
+@@ -66,6 +66,10 @@
+ #include <girepository.h>
+ #endif
+ 
++#ifdef HAVE_LIBCAPNG
++#include <cap-ng.h>
++#endif
++
+ #if defined(HAVE_NATIVE_BACKEND) && defined(HAVE_WAYLAND)
+ #include <systemd/sd-login.h>
+ #endif /* HAVE_WAYLAND && HAVE_NATIVE_BACKEND */
+@@ -673,6 +677,12 @@ meta_run (void)
+   if (!meta_display_open ())
+     meta_exit (META_EXIT_ERROR);
+ 
++#ifdef HAVE_LIBCAPNG
++  capng_clear(CAPNG_SELECT_BOTH);
++  capng_update(CAPNG_ADD, CAPNG_EFFECTIVE|CAPNG_PERMITTED, CAP_SYS_NICE);
++  capng_apply(CAPNG_SELECT_BOTH);
++#endif
++
+   g_main_loop_run (meta_main_loop);
+ 
+   meta_finalize ();
+diff --git a/src/meson.build b/src/meson.build
+index 90d80734f..a9fffa2c2 100644
+--- a/src/meson.build
++++ b/src/meson.build
+@@ -18,6 +18,7 @@ mutter_pkg_deps = [
+   glib_dep,
+   gsettings_desktop_schemas_dep,
+   gtk3_dep,
++  libcapng_dep,
+   pango_dep,
+ ]
+ 
+-- 
+2.23.0
+