[PATCH 5.4 044/104] tracing: Do not set trace clock if tracefs lockdown is in effect

[Greg Kroah-Hartman]

From: Masami Ichikawa <masami256@xxxxxxxxx>

commit bf24daac8f2bd5b8affaec03c2be1d20bcdd6837 upstream.

When trace_clock option is not set and unstable clcok detected,
tracing_set_default_clock() sets trace_clock(ThinkPad A285 is one of
case). In that case, if lockdown is in effect, null pointer
dereference error happens in ring_buffer_set_clock().

Link: http://lkml.kernel.org/r/20200116131236.3866925-1-masami256@xxxxxxxxx

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 17911ff38aa58 ("tracing: Add locked_down checks to the open calls of files created for tracefs")
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1788488
Signed-off-by: Masami Ichikawa <masami256@xxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/trace/trace.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -9270,6 +9270,11 @@ __init static int tracing_set_default_cl
 {
 	/* sched_clock_stable() is determined in late_initcall */
 	if (!trace_boot_clock && !sched_clock_stable()) {
+		if (security_locked_down(LOCKDOWN_TRACEFS)) {
+			pr_warn("Can not set tracing clock due to lockdown\n");
+			return -EPERM;
+		}
+
 		printk(KERN_WARNING
 		       "Unstable clock detected, switching default tracing clock to \"global\"\n"
 		       "If you want to keep using the local clock, then add:\n"