block: fix error unwinding in device_add_disk

One device_add is called disk->ev will be freed by disk_release, so we
should free it twice.  Fix this by allocating disk->ev after device_add
so that the extra local unwinding can be removed entirely.

Based on an earlier patch from Tetsuo Handa.
Reported-by: syzbot <syzbot+28a66a9fbc621c939000@syzkaller.appspotmail.com>
Tested-by: syzbot <syzbot+28a66a9fbc621c939000@syzkaller.appspotmail.com>
Fixes: 83cbce95 ("block: add error handling for device_add_disk / add_disk")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211221161851.788424-1-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/genhd.c

@@ -442,10 +442,6 @@ int __must_check device_add_disk(struct device *parent, struct gendisk *disk,
 		disk->first_minor = ret;
 	}
 
-	ret = disk_alloc_events(disk);
-	if (ret)
-		goto out_free_ext_minor;
-
 	/* delay uevents, until we scanned partition table */
 	dev_set_uevent_suppress(ddev, 1);
 
@@ -456,7 +452,12 @@ int __must_check device_add_disk(struct device *parent, struct gendisk *disk,
 		ddev->devt = MKDEV(disk->major, disk->first_minor);
 	ret = device_add(ddev);
 	if (ret)
-		goto out_disk_release_events;
+		goto out_free_ext_minor;
+
+	ret = disk_alloc_events(disk);
+	if (ret)
+		goto out_device_del;
+
 	if (!sysfs_deprecated) {
 		ret = sysfs_create_link(block_depr, &ddev->kobj,
 					kobject_name(&ddev->kobj));
@@ -538,8 +539,6 @@ int __must_check device_add_disk(struct device *parent, struct gendisk *disk,
 		sysfs_remove_link(block_depr, dev_name(ddev));
 out_device_del:
 	device_del(ddev);
-out_disk_release_events:
-	disk_release_events(disk);
 out_free_ext_minor:
 	if (disk->major == BLOCK_EXT_MAJOR)
 		blk_free_ext_minor(disk->first_minor);