[S390] dasd: fix race between open and offline

The dasd_open function uses the private_data pointer of the gendisk to
find the dasd_block structure that matches the gendisk. When a DASD
device is set offline, we set the private_data pointer of the gendisk
to NULL and later remove the dasd_block structure, but there is still
a small race window, in which dasd_open could first read a pointer
from the private_data field and then try to use it, after the structure
has already been freed.
To close this race window, we will store a pointer to the dasd_devmap
structure of the base device in the private_data field. The devmap
entries are not deleted, and we already have proper locking and
reference counting in place, so that we can safely get from a devmap
pointer to the dasd_device and dasd_block structures of the device.

Signed-off-by: Stefan Weinhuber <wein@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 22 insertions, 18 deletions

diff --git a/drivers/s390/block/dasd.c b/drivers/s390/block/dasd.c
index 4d2df2f76ea0..475e603fc584 100644
--- a/drivers/s390/block/dasd.c
+++ b/drivers/s390/block/dasd.c
@@ -2314,15 +2314,14 @@ static void dasd_flush_request_queue(struct dasd_block *block)
 
 static int dasd_open(struct block_device *bdev, fmode_t mode)
 {
-	struct dasd_block *block = bdev->bd_disk->private_data;
 	struct dasd_device *base;
 	int rc;
 
-	if (!block)
+	base = dasd_device_from_gendisk(bdev->bd_disk);
+	if (!base)
 		return -ENODEV;
 
-	base = block->base;
-	atomic_inc(&block->open_count);
+	atomic_inc(&base->block->open_count);
 	if (test_bit(DASD_FLAG_OFFLINE, &base->flags)) {
 		rc = -ENODEV;
 		goto unlock;
@@ -2355,21 +2354,28 @@ static int dasd_open(struct block_device *bdev, fmode_t mode)
 		goto out;
 	}
 
+	dasd_put_device(base);
 	return 0;
 
 out:
 	module_put(base->discipline->owner);
 unlock:
-	atomic_dec(&block->open_count);
+	atomic_dec(&base->block->open_count);
+	dasd_put_device(base);
 	return rc;
 }
 
 static int dasd_release(struct gendisk *disk, fmode_t mode)
 {
-	struct dasd_block *block = disk->private_data;
+	struct dasd_device *base;
 
-	atomic_dec(&block->open_count);
-	module_put(block->base->discipline->owner);
+	base = dasd_device_from_gendisk(disk);
+	if (!base)
+		return -ENODEV;
+
+	atomic_dec(&base->block->open_count);
+	module_put(base->discipline->owner);
+	dasd_put_device(base);
 	return 0;
 }
 
@@ -2378,20 +2384,20 @@ static int dasd_release(struct gendisk *disk, fmode_t mode)
  */
 static int dasd_getgeo(struct block_device *bdev, struct hd_geometry *geo)
 {
-	struct dasd_block *block;
 	struct dasd_device *base;
 
-	block = bdev->bd_disk->private_data;
-	if (!block)
+	base = dasd_device_from_gendisk(bdev->bd_disk);
+	if (!base)
 		return -ENODEV;
-	base = block->base;
 
 	if (!base->discipline ||
-	    !base->discipline->fill_geometry)
+	    !base->discipline->fill_geometry) {
+		dasd_put_device(base);
 		return -EINVAL;
-
-	base->discipline->fill_geometry(block, geo);
-	geo->start = get_start_sect(bdev) >> block->s2b_shift;
+	}
+	base->discipline->fill_geometry(base->block, geo);
+	geo->start = get_start_sect(bdev) >> base->block->s2b_shift;
+	dasd_put_device(base);
 	return 0;
 }
 
@@ -2528,7 +2534,6 @@ void dasd_generic_remove(struct ccw_device *cdev)
 	dasd_set_target_state(device, DASD_STATE_NEW);
 	/* dasd_delete_device destroys the device reference. */
 	block = device->block;
-	device->block = NULL;
 	dasd_delete_device(device);
 	/*
 	 * life cycle of block is bound to device, so delete it after
@@ -2650,7 +2655,6 @@ int dasd_generic_set_offline(struct ccw_device *cdev)
 	dasd_set_target_state(device, DASD_STATE_NEW);
 	/* dasd_delete_device destroys the device reference. */
 	block = device->block;
-	device->block = NULL;
 	dasd_delete_device(device);
 	/*
 	 * life cycle of block is bound to device, so delete it after