[RFC PATCH v2 2/2] block: fix blk_get_backing_dev_info() crash, use bdev->bd_queue

[Dan Williams]

The ->bd_queue member of struct block_device was added in commit
87192a2a49c4 ("vfs: cache request_queue in struct block_device") in
v3.3. However, blk_get_backing_dev_info() has been using
bdev_get_queue() and grabbing the request_queue through the gendisk
since before the git era.

At final __blkdev_put() time ->bd_disk is cleared while ->bd_queue is
not. The queue remains valid until the final put of the parent disk.

The following crash signature results from blk_get_backing_dev_info()
trying to lookup the queue through ->bd_disk after the final put of the
block device. Simply switch bdev_get_queue() to use ->bd_queue directly
which is guaranteed to still be valid since the request_queue is alive
as long as the inode corresponding to the bdev has not been destroyed.

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000568
 IP: blk_get_backing_dev_info+0x10/0x20
 [..]
 Call Trace:
  __inode_attach_wb+0x3a7/0x5d0
  __filemap_fdatawrite_range+0xf8/0x100
  filemap_write_and_wait+0x40/0x90
  fsync_bdev+0x54/0x60
  ? bdget_disk+0x30/0x40
  invalidate_partition+0x24/0x50
  del_gendisk+0xfa/0x230

Cc: Jan Kara <jack@xxxxxxx>
Cc: Jens Axboe <axboe@xxxxxx>
Cc: Jeff Moyer <jmoyer@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Wei Fang <fangwei1@xxxxxxxxxx>
Cc: Rabin Vincent <rabinv@xxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 block/blk-core.c       |    4 ++--
 include/linux/blkdev.h |    6 +++++-
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index e8713137b846..cfd6731dfed7 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -108,8 +108,8 @@ void blk_queue_congestion_threshold(struct request_queue *q)
  * @bdev:	device
  *
  * Locates the passed device's request queue and returns the address of its
- * backing_dev_info.  This function can only be called if @bdev is opened
- * and the return value is never NULL.
+ * backing_dev_info.  This function can be called until the final iput()
+ * of the bdev inode.
  */
 struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev)
 {
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index c47c358ba052..fd332da0fc38 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -841,7 +841,11 @@ bool blk_poll(struct request_queue *q, blk_qc_t cookie);
 
 static inline struct request_queue *bdev_get_queue(struct block_device *bdev)
 {
-	return bdev->bd_disk->queue;	/* this is never NULL */
+	/*
+	 * ->bd_queue is valid as long as there is a reference against
+	 *  the bdev inode.
+	 */
+	return bdev->bd_queue;
 }
 
 /*