Don't expose /proc when running apply_extra

As shown by CVE-2019-5736, it is sometimes possible for the sandbox
app to access outside files using /proc/self/exe. This is not
typically an issue for flatpak as the sandbox runs as the user which
has no permissions to e.g. modify the host files.

However, when installing apps using extra-data into the system repo
we *do* actually run a sandbox as root. So, in this case we disable mounting
/proc in the sandbox, which will neuter attacks like this.

common/flatpak-common-types-private.h

@@ -46,6 +46,7 @@ typedef enum {
   FLATPAK_RUN_FLAG_BLUETOOTH          = (1 << 16),
   FLATPAK_RUN_FLAG_CANBUS            = (1 << 17),
   FLATPAK_RUN_FLAG_DO_NOT_REAP        = (1 << 18),
+  FLATPAK_RUN_FLAG_NO_PROC            = (1 << 19),
 } FlatpakRunFlags;
 
 typedef struct FlatpakDir          FlatpakDir;

common/flatpak-dir.c

@@ -6786,7 +6786,7 @@ apply_extra_data (FlatpakDir   *self,
                           NULL);
 
   if (!flatpak_run_setup_base_argv (bwrap, runtime_files, NULL, runtime_ref_parts[2],
-                                    FLATPAK_RUN_FLAG_NO_SESSION_HELPER,
+                                    FLATPAK_RUN_FLAG_NO_SESSION_HELPER | FLATPAK_RUN_FLAG_NO_PROC,
                                     error))
     return FALSE;
 

common/flatpak-run.c

@@ -2606,9 +2606,13 @@ flatpak_run_setup_base_argv (FlatpakBwrap   *bwrap,
     "# Disable user pkcs11 config, because the host modules don't work in the runtime\n"
     "user-config: none\n";
 
+  if ((flags & FLATPAK_RUN_FLAG_NO_PROC) == 0)
+    flatpak_bwrap_add_args (bwrap,
+                            "--proc", "/proc",
+                            NULL);
+
   flatpak_bwrap_add_args (bwrap,
                           "--unshare-pid",
-                          "--proc", "/proc",
                           "--dir", "/tmp",
                           "--dir", "/var/tmp",
                           "--dir", "/run/host",