perf: Fix racy group access

While looking at some fuzzer output I noticed that we do not hold any
locks on leader->ctx and therefore the sibling_list iteration is
unsafe.

Acquire the relevant ctx->mutex before calling into the pmu specific
code.

Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Vince Weaver <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Sasha Levin <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: Peter Zijlstra

kernel/events/core.c

@@ -7036,12 +7036,23 @@ EXPORT_SYMBOL_GPL(perf_pmu_unregister);
 
 static int perf_try_init_event(struct pmu *pmu, struct perf_event *event)
 {
+	struct perf_event_context *ctx = NULL;
 	int ret;
 
 	if (!try_module_get(pmu->module))
 		return -ENODEV;
+
+	if (event->group_leader != event) {
+		ctx = perf_event_ctx_lock(event->group_leader);
+		BUG_ON(!ctx);
+	}
+
 	event->pmu = pmu;
 	ret = pmu->event_init(event);
+
+	if (ctx)
+		perf_event_ctx_unlock(event->group_leader, ctx);
+
 	if (ret)
 		module_put(pmu->module);