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

[Masami Ichikawa <masami256@gmail.com>]

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: https://bugzilla.redhat.com/show_bug.cgi?id=1788488
Signed-off-by: Masami Ichikawa <masami256@gmail.com>
---
 kernel/trace/trace.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index ddb7e7f5fe8d..5b6ee4aadc26 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -9420,6 +9420,11 @@ __init static int tracing_set_default_clock(void)
 {
 	/* 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"
-- 
2.24.1