blkcg: fix use-after-free in __blkg_release_rcu() by making blkcg_gq refcnt an atomic_t

Hello,

So, this patch should do.  Joe, Vivek, can one of you guys please
verify that the oops goes away with this patch?

Jens, the original thread can be read at

  http://thread.gmane.org/gmane.linux.kernel/1720729

The fix converts blkg->refcnt from int to atomic_t.  It does some
overhead but it should be minute compared to everything else which is
going on and the involved cacheline bouncing, so I think it's highly
unlikely to cause any noticeable difference.  Also, the refcnt in
question should be converted to a perpcu_ref for blk-mq anyway, so the
atomic_t is likely to go away pretty soon anyway.

Thanks.

------- 8< -------
__blkg_release_rcu() may be invoked after the associated request_queue
is released with a RCU grace period inbetween.  As such, the function
and callbacks invoked from it must not dereference the associated
request_queue.  This is clearly indicated in the comment above the
function.

Unfortunately, while trying to fix a different issue, 2a4fd070ee
("blkcg: move bulk of blkcg_gq release operations to the RCU
callback") ignored this and added [un]locking of @blkg->q->queue_lock
to __blkg_release_rcu().  This of course can cause oops as the
request_queue may be long gone by the time this code gets executed.

  general protection fault: 0000 [#1] SMP
  CPU: 21 PID: 30 Comm: rcuos/21 Not tainted 3.15.0 #1
  Hardware name: Stratus ftServer 6400/G7LAZ, BIOS BIOS Version 6.3:57 12/25/2013
  task: ffff880854021de0 ti: ffff88085403c000 task.ti: ffff88085403c000
  RIP: 0010:[<ffffffff8162e9e5>]  [<ffffffff8162e9e5>] _raw_spin_lock_irq+0x15/0x60
  RSP: 0018:ffff88085403fdf0  EFLAGS: 00010086
  RAX: 0000000000020000 RBX: 0000000000000010 RCX: 0000000000000000
  RDX: 000060ef80008248 RSI: 0000000000000286 RDI: 6b6b6b6b6b6b6b6b
  RBP: ffff88085403fdf0 R08: 0000000000000286 R09: 0000000000009f39
  R10: 0000000000020001 R11: 0000000000020001 R12: ffff88103c17a130
  R13: ffff88103c17a080 R14: 0000000000000000 R15: 0000000000000000
  FS:  0000000000000000(0000) GS:ffff88107fca0000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 00000000006e5ab8 CR3: 000000000193d000 CR4: 00000000000407e0
  Stack:
   ffff88085403fe18 ffffffff812cbfc2 ffff88103c17a130 0000000000000000
   ffff88103c17a130 ffff88085403fec0 ffffffff810d1d28 ffff880854021de0
   ffff880854021de0 ffff88107fcaec58 ffff88085403fe80 ffff88107fcaec30
  Call Trace:
   [<ffffffff812cbfc2>] __blkg_release_rcu+0x72/0x150
   [<ffffffff810d1d28>] rcu_nocb_kthread+0x1e8/0x300
   [<ffffffff81091d81>] kthread+0xe1/0x100
   [<ffffffff8163813c>] ret_from_fork+0x7c/0xb0
  Code: ff 47 04 48 8b 7d 08 be 00 02 00 00 e8 55 48 a4 ff 5d c3 0f 1f 00 66 66 66 66 90 55 48 89 e5
  +fa 66 66 90 66 66 90 b8 00 00 02 00 <f0> 0f c1 07 89 c2 c1 ea 10 66 39 c2 75 02 5d c3 83 e2 fe 0f
  +b7
  RIP  [<ffffffff8162e9e5>] _raw_spin_lock_irq+0x15/0x60
   RSP <ffff88085403fdf0>

The request_queue locking was added because blkcg_gq->refcnt is an int
protected with the queue lock and __blkg_release_rcu() needs to put
the parent.  Let's fix it by making blkcg_gq->refcnt an atomic_t and
dropping queue locking in the function.

Given the general heavy weight of the current request_queue and blkcg
operations, this is unlikely to cause any noticeable overhead.
Moreover, blkcg_gq->refcnt is likely to be converted to percpu_ref in
the near future, so whatever (most likely negligible) overhead it may
add is temporary.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Joe Lawrence <joe.lawrence@stratus.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Link: http://lkml.kernel.org/g/alpine.DEB.2.02.1406081816540.17948@jlaw-desktop.mno.stratus.com
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-cgroup.c

@@ -80,7 +80,7 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct request_queue *q,
 	blkg->q = q;
 	INIT_LIST_HEAD(&blkg->q_node);
 	blkg->blkcg = blkcg;
-	blkg->refcnt = 1;
+	atomic_set(&blkg->refcnt, 1);
 
 	/* root blkg uses @q->root_rl, init rl only for !root blkgs */
 	if (blkcg != &blkcg_root) {
@@ -399,11 +399,8 @@ void __blkg_release_rcu(struct rcu_head *rcu_head)
 
 	/* release the blkcg and parent blkg refs this blkg has been holding */
 	css_put(&blkg->blkcg->css);
-	if (blkg->parent) {
-		spin_lock_irq(blkg->q->queue_lock);
+	if (blkg->parent)
 		blkg_put(blkg->parent);
-		spin_unlock_irq(blkg->q->queue_lock);
-	}
 
 	blkg_free(blkg);
 }

block/blk-cgroup.h

@@ -18,6 +18,7 @@
 #include <linux/seq_file.h>
 #include <linux/radix-tree.h>
 #include <linux/blkdev.h>
+#include <linux/atomic.h>
 
 /* Max limits for throttle policy */
 #define THROTL_IOPS_MAX		UINT_MAX
@@ -104,7 +105,7 @@ struct blkcg_gq {
 	struct request_list		rl;
 
 	/* reference count */
-	int				refcnt;
+	atomic_t			refcnt;
 
 	/* is this blkg online? protected by both blkcg and q locks */
 	bool				online;
@@ -257,13 +258,12 @@ static inline int blkg_path(struct blkcg_gq *blkg, char *buf, int buflen)
  * blkg_get - get a blkg reference
  * @blkg: blkg to get
  *
- * The caller should be holding queue_lock and an existing reference.
+ * The caller should be holding an existing reference.
  */
 static inline void blkg_get(struct blkcg_gq *blkg)
 {
-	lockdep_assert_held(blkg->q->queue_lock);
-	WARN_ON_ONCE(!blkg->refcnt);
-	blkg->refcnt++;
+	WARN_ON_ONCE(atomic_read(&blkg->refcnt) <= 0);
+	atomic_inc(&blkg->refcnt);
 }
 
 void __blkg_release_rcu(struct rcu_head *rcu);
@@ -271,14 +271,11 @@ void __blkg_release_rcu(struct rcu_head *rcu);
 /**
  * blkg_put - put a blkg reference
  * @blkg: blkg to put
- *
- * The caller should be holding queue_lock.
  */
 static inline void blkg_put(struct blkcg_gq *blkg)
 {
-	lockdep_assert_held(blkg->q->queue_lock);
-	WARN_ON_ONCE(blkg->refcnt <= 0);
-	if (!--blkg->refcnt)
+	WARN_ON_ONCE(atomic_read(&blkg->refcnt) <= 0);
+	if (atomic_dec_and_test(&blkg->refcnt))
 		call_rcu(&blkg->rcu_head, __blkg_release_rcu);
 }