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

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@gmail.com
Cc: stable@vger.kernel.org
Fixes: c8045810554f9 ("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@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index ddb7e7f5fe8d8316b73f1c163bd7a3c894735e5a..5b6ee4aadc268f425d0159596621d9721f5a5719 100644 (file)
--- 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"