[PATCH 28/91] cfq-iosched: fix locking around ioc->ioc_data assignment

[Willy Tarreau]

2.6.27-longterm review patch.  If anyone has any objections, please let us know.

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

commit ab4bd22d3cce6977dc039664cc2d052e3147d662 upstream.

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.

Signed-off-by: Jens Axboe <jaxboe@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
---
 block/cfq-iosched.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

Index: longterm-2.6.27/block/cfq-iosched.c
===================================================================
--- longterm-2.6.27.orig/block/cfq-iosched.c	2012-02-05 22:34:34.185915429 +0100
+++ longterm-2.6.27/block/cfq-iosched.c	2012-02-05 22:34:38.850917663 +0100
@@ -1286,8 +1286,11 @@
 	cic->dead_key = (unsigned long) cic->key;
 	cic->key = NULL;
 
-	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[ASYNC]) {
 		cfq_exit_cfqq(cfqd, cic->cfqq[ASYNC]);


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