tracefs: Restrict tracefs when the kernel is locked down

Tracefs may release more information about the kernel than desirable, so
restrict it when the kernel is locked down in confidentiality mode by
preventing open().

(Fixed by Ben Hutchings to avoid a null dereference in
default_file_open())

Signed-off-by: Matthew Garrett <mjg59@google.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: James Morris <jmorris@namei.org>

security/lockdown/lockdown.c

@@ -36,6 +36,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
 	[LOCKDOWN_KPROBES] = "use of kprobes",
 	[LOCKDOWN_BPF_READ] = "use of bpf to read kernel RAM",
 	[LOCKDOWN_PERF] = "unsafe use of perf",
+	[LOCKDOWN_TRACEFS] = "use of tracefs",
 	[LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
 };