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>

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 3c7b537bf9081f9f43b90c7cdcd9a95073f1c02b..545b8d4c829dd71d4e343707568b131ec18e64ed 100644 (file)
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -2772,8 +2772,11 @@ 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]);