never allow untrusted apps accessing debugfs_tracing

debugfs_tracing can only be accessed by tracing tools provided by the
platform.

Bug: 172028429
Test: boot with no relevant log showing up
Change-Id: I412dd51a1b268061c5a972488b8bc4a0ee456601
diff --git a/private/untrusted_app_all.te b/private/untrusted_app_all.te
index bc90450..23ee943 100644
--- a/private/untrusted_app_all.te
+++ b/private/untrusted_app_all.te
@@ -64,6 +64,9 @@
 neverallow untrusted_app_all trace_data_file:dir *;
 neverallow untrusted_app_all trace_data_file:file { no_w_file_perms open };
 
+# neverallow untrusted apps accessing debugfs_tracing
+neverallow untrusted_app_all debugfs_tracing:file no_rw_file_perms;
+
 # Allow to read staged apks.
 allow untrusted_app_all { apk_tmp_file apk_private_tmp_file }:file {read getattr};
 
@@ -146,6 +149,9 @@
 # Allow the renderscript compiler to be run.
 domain_auto_trans(untrusted_app_all, rs_exec, rs)
 
+# suppress denials caused by debugfs_tracing
+dontaudit untrusted_app_all debugfs_tracing:file rw_file_perms;
+
 # This is allowed for targetSdkVersion <= 25 but disallowed on newer versions.
 dontaudit untrusted_app_all net_dns_prop:file read;