run: Convert all environment variables into bwrap arguments

This avoids some of them being filtered out by a setuid bwrap. It also
means that if they came from an untrusted source, they cannot be used
to inject arbitrary code into a non-setuid bwrap via mechanisms like
LD_PRELOAD.

Because they get bundled into a memfd or temporary file, they do not
actually appear in argv, ensuring that they remain inaccessible to
processes running under a different uid (which is important if their
values are tokens or other secrets).

Signed-off-by: Simon McVittie <smcv@collabora.com>
Part-of: https://github.com/flatpak/flatpak/security/advisories/GHSA-4ppf-fxf6-vxg2

common/flatpak-bwrap-private.h

@@ -43,6 +43,8 @@ void          flatpak_bwrap_unset_env (FlatpakBwrap *bwrap,
                                        const char   *variable);
 void          flatpak_bwrap_add_arg (FlatpakBwrap *bwrap,
                                      const char   *arg);
+void          flatpak_bwrap_take_arg (FlatpakBwrap *bwrap,
+                                      char         *arg);
 void          flatpak_bwrap_add_noinherit_fd (FlatpakBwrap *bwrap,
                                               int           fd);
 void          flatpak_bwrap_add_fd (FlatpakBwrap *bwrap,
@@ -73,6 +75,7 @@ void          flatpak_bwrap_add_bind_arg (FlatpakBwrap *bwrap,
                                           const char   *type,
                                           const char   *src,
                                           const char   *dest);
+void          flatpak_bwrap_envp_to_args (FlatpakBwrap *bwrap);
 gboolean      flatpak_bwrap_bundle_args (FlatpakBwrap *bwrap,
                                          int           start,
                                          int           end,

common/flatpak-bwrap.c

@@ -109,6 +109,18 @@ flatpak_bwrap_add_arg (FlatpakBwrap *bwrap, const char *arg)
   g_ptr_array_add (bwrap->argv, g_strdup (arg));
 }
 
+/*
+ * flatpak_bwrap_take_arg:
+ * @arg: (transfer full): Take ownership of this argument
+ *
+ * Add @arg to @bwrap's argv, taking ownership of the pointer.
+ */
+void
+flatpak_bwrap_take_arg (FlatpakBwrap *bwrap, char *arg)
+{
+  g_ptr_array_add (bwrap->argv, arg);
+}
+
 void
 flatpak_bwrap_finish (FlatpakBwrap *bwrap)
 {
@@ -274,6 +286,37 @@ flatpak_bwrap_add_bind_arg (FlatpakBwrap *bwrap,
     }
 }
 
+/*
+ * Convert bwrap->envp into a series of --setenv arguments for bwrap(1),
+ * assumed to be applied to an empty environment. Reset envp to be an
+ * empty environment.
+ */
+void
+flatpak_bwrap_envp_to_args (FlatpakBwrap *bwrap)
+{
+  gsize i;
+
+  for (i = 0; bwrap->envp[i] != NULL; i++)
+    {
+      char *key_val = bwrap->envp[i];
+      char *eq = strchr (key_val, '=');
+
+      if (eq)
+        {
+          flatpak_bwrap_add_arg (bwrap, "--setenv");
+          flatpak_bwrap_take_arg (bwrap, g_strndup (key_val, eq - key_val));
+          flatpak_bwrap_add_arg (bwrap, eq + 1);
+        }
+      else
+        {
+          g_warn_if_reached ();
+        }
+    }
+
+  g_strfreev (g_steal_pointer (&bwrap->envp));
+  bwrap->envp = g_strdupv (flatpak_bwrap_empty_env);
+}
+
 gboolean
 flatpak_bwrap_bundle_args (FlatpakBwrap *bwrap,
                            int           start,

common/flatpak-run.c

@@ -1463,24 +1463,6 @@ flatpak_run_add_environment_args (FlatpakBwrap    *bwrap,
   flatpak_run_add_system_dbus_args (bwrap, proxy_arg_bwrap, context, flags);
   flatpak_run_add_a11y_dbus_args (bwrap, proxy_arg_bwrap, context, flags);
 
-  if (g_environ_getenv (bwrap->envp, "LD_LIBRARY_PATH") != NULL)
-    {
-      /* LD_LIBRARY_PATH is overridden for setuid helper, so pass it as cmdline arg */
-      flatpak_bwrap_add_args (bwrap,
-                              "--setenv", "LD_LIBRARY_PATH", g_environ_getenv (bwrap->envp, "LD_LIBRARY_PATH"),
-                              NULL);
-      flatpak_bwrap_unset_env (bwrap, "LD_LIBRARY_PATH");
-    }
-
-  if (g_environ_getenv (bwrap->envp, "TMPDIR") != NULL)
-    {
-      /* TMPDIR is overridden for setuid helper, so pass it as cmdline arg */
-      flatpak_bwrap_add_args (bwrap,
-                              "--setenv", "TMPDIR", g_environ_getenv (bwrap->envp, "TMPDIR"),
-                              NULL);
-      flatpak_bwrap_unset_env (bwrap, "TMPDIR");
-    }
-
   /* Must run this before spawning the dbus proxy, to ensure it
      ends up in the app cgroup */
   if (!flatpak_run_in_transient_unit (app_id, &my_error))
@@ -4068,6 +4050,8 @@ flatpak_run_app (FlatpakDecomposed *app_ref,
       command = default_command;
     }
 
+  flatpak_bwrap_envp_to_args (bwrap);
+
   if (!flatpak_bwrap_bundle_args (bwrap, 1, -1, FALSE, error))
     return FALSE;
 
@@ -4098,6 +4082,12 @@ flatpak_run_app (FlatpakDecomposed *app_ref,
       /* We use LEAVE_DESCRIPTORS_OPEN to work around dead-lock, see flatpak_close_fds_workaround */
       spawn_flags |= G_SPAWN_LEAVE_DESCRIPTORS_OPEN;
 
+      /* flatpak_bwrap_envp_to_args() moved the environment variables to
+       * be set into --setenv instructions in argv, so the environment
+       * in which the bwrap command runs must be empty. */
+      g_assert (bwrap->envp != NULL);
+      g_assert (bwrap->envp[0] == NULL);
+
       if (!g_spawn_async (NULL,
                           (char **) bwrap->argv->pdata,
                           bwrap->envp,
@@ -4125,6 +4115,12 @@ flatpak_run_app (FlatpakDecomposed *app_ref,
        * we do want to allow inheriting fds into flatpak run. */
       flatpak_bwrap_child_setup (bwrap->fds, FALSE);
 
+      /* flatpak_bwrap_envp_to_args() moved the environment variables to
+       * be set into --setenv instructions in argv, so the environment
+       * in which the bwrap command runs must be empty. */
+      g_assert (bwrap->envp != NULL);
+      g_assert (bwrap->envp[0] == NULL);
+
       if (execvpe (flatpak_get_bwrap (), (char **) bwrap->argv->pdata, bwrap->envp) == -1)
         {
           g_set_error_literal (error, G_IO_ERROR, g_io_error_from_errno (errno),