[tip:perf/core] perf: Fix racy group access

[tip-bot for Peter Zijlstra <tipbot@zytor.com>]

Commit-ID:  ccd41c86ad4d464d0ed4e48d80759ff85c2115b0
Gitweb:     http://git.kernel.org/tip/ccd41c86ad4d464d0ed4e48d80759ff85c2115b0
Author:     Peter Zijlstra <peterz@infradead.org>
AuthorDate: Wed, 25 Feb 2015 15:56:04 +0100
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 27 Mar 2015 09:49:45 +0100

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) <peterz@infradead.org>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Link: http://lkml.kernel.org/r/20150225151639.GL5029@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/events/core.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index b01dfb6..bb1a7c3 100644
--- a/kernel/events/core.c
+++ b/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);