[PATCH -nect RFC v2 2/2] block: fix uaf for bd_holder_dir

[Yu Kuai]

Lifecycle of bd_holder_dir is problematic:

t1:			t2:

// get bdev of lower disk
blkdev_get_by_dev
			// remove lower disk
			del_gendisk
			 // initial reference is released, and
			 // bd_holder_dir can be freed
			 kobject_put
// uaf is triggered
bd_link_disk_holder

Fix the problem by protecting bd_holder_dir by open_mutex.

Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 block/genhd.c           |  6 +++++-
 block/holder.c          | 19 +++++++++++++------
 block/partitions/core.c |  5 ++++-
 3 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/block/genhd.c b/block/genhd.c
index 53f9c8b2690a..b3d04e79e854 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -499,6 +499,8 @@ int __must_check device_add_disk(struct device *parent, struct gendisk *disk,
 		ret = -ENOMEM;
 		goto out_del_integrity;
 	}
+	disk->part0->holder_dir_ref = 1;
+
 	disk->slave_dir = kobject_create_and_add("slaves", &ddev->kobj);
 	if (!disk->slave_dir) {
 		ret = -ENOMEM;
@@ -557,6 +559,8 @@ int __must_check device_add_disk(struct device *parent, struct gendisk *disk,
 	kobject_put(disk->slave_dir);
 out_put_holder_dir:
 	kobject_put(disk->part0->bd_holder_dir);
+	disk->part0->bd_holder_dir = NULL;
+	disk->part0->holder_dir_ref = 0;
 out_del_integrity:
 	blk_integrity_del(disk);
 out_del_block_link:
@@ -649,7 +653,7 @@ void del_gendisk(struct gendisk *disk)
 
 	blk_unregister_queue(disk);
 
-	kobject_put(disk->part0->bd_holder_dir);
+	bd_put_holder_dir(disk->part0);
 	kobject_put(disk->slave_dir);
 
 	part_stat_set_all(disk->part0, 0);
diff --git a/block/holder.c b/block/holder.c
index 5283bc804cc1..2e28b472ba1a 100644
--- a/block/holder.c
+++ b/block/holder.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0-only
 #include <linux/blkdev.h>
 #include <linux/slab.h>
+#include "blk.h"
 
 struct bd_holder_disk {
 	struct list_head	list;
@@ -85,9 +86,19 @@ int bd_link_disk_holder(struct block_device *bdev, struct gendisk *disk)
 		goto out_unlock;
 	}
 
+	/*
+	 * 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.
+	 */
+	if (!bd_try_get_holder_dir(bdev)) {
+		ret = -ENODEV;
+		goto out_unlock;
+	}
+
 	holder = kzalloc(sizeof(*holder), GFP_KERNEL);
 	if (!holder) {
 		ret = -ENOMEM;
+		bd_put_holder_dir(bdev);
 		goto out_unlock;
 	}
 
@@ -98,16 +109,12 @@ int bd_link_disk_holder(struct block_device *bdev, struct gendisk *disk)
 		ret = __link_disk_holder(bdev, disk);
 		if (ret) {
 			kfree(holder);
+			bd_put_holder_dir(bdev);
 			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);
@@ -141,7 +148,7 @@ void bd_unlink_disk_holder(struct block_device *bdev, struct gendisk *disk)
 	if (!WARN_ON_ONCE(holder == NULL) && !--holder->refcnt) {
 		if (disk->slave_dir)
 			__unlink_disk_holder(bdev, disk);
-		kobject_put(bdev->bd_holder_dir);
+		bd_put_holder_dir(bdev);
 		list_del_init(&holder->list);
 		kfree(holder);
 	}
diff --git a/block/partitions/core.c b/block/partitions/core.c
index b8112f52d388..39a14f7c308e 100644
--- a/block/partitions/core.c
+++ b/block/partitions/core.c
@@ -279,7 +279,7 @@ static void delete_partition(struct block_device *part)
 	__invalidate_device(part, true);
 
 	xa_erase(&part->bd_disk->part_tbl, part->bd_partno);
-	kobject_put(part->bd_holder_dir);
+	bd_put_holder_dir(part);
 	device_del(&part->bd_device);
 
 	/*
@@ -390,6 +390,7 @@ static struct block_device *add_partition(struct gendisk *disk, int partno,
 	bdev->bd_holder_dir = kobject_create_and_add("holders", &pdev->kobj);
 	if (!bdev->bd_holder_dir)
 		goto out_del;
+	bdev->holder_dir_ref = 1;
 
 	dev_set_uevent_suppress(pdev, 0);
 	if (flags & ADDPART_FLAG_WHOLEDISK) {
@@ -411,6 +412,8 @@ static struct block_device *add_partition(struct gendisk *disk, int partno,
 
 out_del:
 	kobject_put(bdev->bd_holder_dir);
+	bdev->bd_holder_dir = NULL;
+	bdev->holder_dir_ref = 0;
 	device_del(pdev);
 out_put:
 	put_device(pdev);
-- 
2.31.1