cfq-iosched: fix locking around ioc->ioc_data assignment

Since we are modifying this RCU pointer, we need to hold
the lock protecting it around it.

This fixes a potential reuse and double free of a cfq
io_context structure. The bug has been in CFQ for a long
time, it hit very few people but those it did hit seemed
to see it a lot.

Tracked in RH bugzilla here:

https://bugzilla.redhat.com/show_bug.cgi?id=577968

Credit goes to Paul Bolle for figuring out that the issue
was around the one-hit ioc->ioc_data cache. Thanks to his
hard work the issue is now fixed.

Cc: stable@kernel.org
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index a2fb14bb1402..000719c484c3 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -2767,8 +2767,11 @@ static void __cfq_exit_single_io_context(struct cfq_data *cfqd,
         smp_wmb();
         cic->key = cfqd_dead_key(cfqd);
 
-        if (ioc->ioc_data == cic)
+        if (rcu_dereference(ioc->ioc_data) == cic) {
+                spin_lock(&ioc->lock);
                 rcu_assign_pointer(ioc->ioc_data, NULL);
+                spin_unlock(&ioc->lock);
+        }
 
         if (cic->cfqq[BLK_RW_ASYNC]) {
                 cfq_exit_cfqq(cfqd, cic->cfqq[BLK_RW_ASYNC]);