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Subject[PATCH 2/2] block: fix use after free for bd_holder_dir
Date
From: Yu Kuai <yukuai3@huawei.com>

Currently, the caller of bd_link_disk_holer() get 'bdev' by
blkdev_get_by_dev(), which will look up 'bdev' by inode number 'dev'.
Howerver, it's possible that del_gendisk() can be called currently, and
'bd_holder_dir' can be freed before bd_link_disk_holer() access it, thus
use after free is triggered.

t1: t2:
bdev = blkdev_get_by_dev
del_gendisk
kobject_put(bd_holder_dir)
kobject_free()
bd_link_disk_holder

Fix the problem by checking disk is still live and grabbing a reference
to 'bd_holder_dir' first in bd_link_disk_holder().

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
---
block/holder.c | 23 ++++++++++++++++-------
1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/block/holder.c b/block/holder.c
index 5fc68238ce3a..1c6c5b132a92 100644
--- a/block/holder.c
+++ b/block/holder.c
@@ -78,19 +78,32 @@ int bd_link_disk_holder(struct block_device *bdev, struct gendisk *disk)
if (bdev->bd_disk == disk)
return -EINVAL;

- mutex_lock(&disk->open_mutex);
+ /*
+ * del_gendisk drops the initial reference to bd_holder_dir, so we
+ * need to keep our own here to allow for cleanup past that point.
+ */
+ mutex_lock(&bdev->bd_disk->open_mutex);
+ if (!disk_live(bdev->bd_disk)) {
+ mutex_unlock(&bdev->bd_disk->open_mutex);
+ return -ENODEV;
+ }

- WARN_ON_ONCE(!bdev->bd_holder);
+ kobject_get(bdev->bd_holder_dir);
+ mutex_unlock(&bdev->bd_disk->open_mutex);

+ mutex_lock(&disk->open_mutex);
+ WARN_ON_ONCE(!bdev->bd_holder);
holder = bd_find_holder_disk(bdev, disk);
if (holder) {
holder->refcnt++;
+ kobject_put(bdev->bd_holder_dir);
goto out_unlock;
}

holder = kzalloc(sizeof(*holder), GFP_KERNEL);
if (!holder) {
ret = -ENOMEM;
+ kobject_put(bdev->bd_holder_dir);
goto out_unlock;
}

@@ -101,16 +114,12 @@ int bd_link_disk_holder(struct block_device *bdev, struct gendisk *disk)
ret = __link_disk_holder(bdev, disk);
if (ret) {
kfree(holder);
+ kobject_put(bdev->bd_holder_dir);
goto out_unlock;
}
}

list_add(&holder->list, &disk->slave_bdevs);
- /*
- * del_gendisk drops the initial reference to bd_holder_dir, so we need
- * to keep our own here to allow for cleanup past that point.
- */
- kobject_get(bdev->bd_holder_dir);

out_unlock:
mutex_unlock(&disk->open_mutex);
--
2.31.1
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