linux-kernel - [PATCH 3.16 287/306] perf: Fix race in swevent hash

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Date:   Wed, 15 Feb 2017 22:41:41 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Ingo Molnar" <mingo@...nel.org>,
        "Stephane Eranian" <eranian@...gle.com>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Arnaldo Carvalho de Melo" <acme@...hat.com>,
        "Vince Weaver" <vincent.weaver@...ne.edu>,
        "Sasha Levin" <sasha.levin@...cle.com>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Jiri Olsa" <jolsa@...hat.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Frederic Weisbecker" <fweisbec@...il.com>
Subject: [PATCH 3.16 287/306] perf: Fix race in swevent hash

3.16.40-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Peter Zijlstra <peterz@...radead.org>

commit 12ca6ad2e3a896256f086497a7c7406a547ee373 upstream.

There's a race on CPU unplug where we free the swevent hash array
while it can still have events on. This will result in a
use-after-free which is BAD.

Simply do not free the hash array on unplug. This leaves the thing
around and no use-after-free takes place.

When the last swevent dies, we do a for_each_possible_cpu() iteration
anyway to clean these up, at which time we'll free it, so no leakage
will occur.

Reported-by: Sasha Levin <sasha.levin@...cle.com>
Tested-by: Sasha Levin <sasha.levin@...cle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Jiri Olsa <jolsa@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Stephane Eranian <eranian@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Vince Weaver <vincent.weaver@...ne.edu>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 kernel/events/core.c | 20 +-------------------
 1 file changed, 1 insertion(+), 19 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5595,9 +5595,6 @@ struct swevent_htable {
 
 	/* Recursion avoidance in each contexts */
 	int				recursion[PERF_NR_CONTEXTS];
-
-	/* Keeps track of cpu being initialized/exited */
-	bool				online;
 };
 
 static DEFINE_PER_CPU(struct swevent_htable, swevent_htable);
@@ -5844,14 +5841,8 @@ static int perf_swevent_add(struct perf_
 	hwc->state = !(flags & PERF_EF_START);
 
 	head = find_swevent_head(swhash, event);
-	if (!head) {
-		/*
-		 * We can race with cpu hotplug code. Do not
-		 * WARN if the cpu just got unplugged.
-		 */
-		WARN_ON_ONCE(swhash->online);
+	if (WARN_ON_ONCE(!head))
 		return -EINVAL;
-	}
 
 	hlist_add_head_rcu(&event->hlist_entry, head);
 
@@ -5918,7 +5909,6 @@ static int swevent_hlist_get_cpu(struct
 	int err = 0;
 
 	mutex_lock(&swhash->hlist_mutex);
-
 	if (!swevent_hlist_deref(swhash) && cpu_online(cpu)) {
 		struct swevent_hlist *hlist;
 
@@ -8050,7 +8040,6 @@ static void perf_event_init_cpu(int cpu)
 	struct swevent_htable *swhash = &per_cpu(swevent_htable, cpu);
 
 	mutex_lock(&swhash->hlist_mutex);
-	swhash->online = true;
 	if (swhash->hlist_refcount > 0) {
 		struct swevent_hlist *hlist;
 
@@ -8103,14 +8092,7 @@ static void perf_event_exit_cpu_context(
 
 static void perf_event_exit_cpu(int cpu)
 {
-	struct swevent_htable *swhash = &per_cpu(swevent_htable, cpu);
-
 	perf_event_exit_cpu_context(cpu);
-
-	mutex_lock(&swhash->hlist_mutex);
-	swhash->online = false;
-	swevent_hlist_release(swhash);
-	mutex_unlock(&swhash->hlist_mutex);
 }
 #else
 static inline void perf_event_exit_cpu(int cpu) { }