linux-kernel - [PATCH] memcg: Only free spare array when readers are done

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <001a113abaa499606605294b5b17@google.com>
Date:	Thu, 14 Jan 2016 14:33:52 +0100
From:	Martijn Coenen <maco@...gle.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Michal Hocko <mhocko@...nel.org>,
	Vladimir Davydov <vdavydov@...tuozzo.com>,
	cgroups@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] memcg: Only free spare array when readers are done

A spare array holding mem cgroup threshold events is kept around
to make sure we can always safely deregister an event and have an
array to store the new set of events in.

In the scenario where we're going from 1 to 0 registered events, the
pointer to the primary array containing 1 event is copied to the spare
slot, and then the spare slot is freed because no events are left.
However, it is freed before calling synchronize_rcu(), which means
readers may still be accessing threshold->primary after it is freed.

Fixed by only freeing after synchronize_rcu().

Signed-off-by: Martijn Coenen <maco@...gle.com>
---
  mm/memcontrol.c | 11 ++++++-----
  1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 14cb1db..73228b6 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3522,16 +3522,17 @@ static void  
__mem_cgroup_usage_unregister_event(struct mem_cgroup *memcg,
  swap_buffers:
  	/* Swap primary and spare array */
  	thresholds->spare = thresholds->primary;
-	/* If all events are unregistered, free the spare array */
-	if (!new) {
-		kfree(thresholds->spare);
-		thresholds->spare = NULL;
-	}

  	rcu_assign_pointer(thresholds->primary, new);

  	/* To be sure that nobody uses thresholds */
  	synchronize_rcu();
+
+	/* If all events are unregistered, free the spare array */
+	if (!new) {
+		kfree(thresholds->spare);
+		thresholds->spare = NULL;
+	}
  unlock:
  	mutex_unlock(&memcg->thresholds_lock);
  }
-- 
2.6.0.rc2.230.g3dd15c0