linux-kernel - Re: [PATCH 2/2] block: fix use after free for bd_holder

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Date:   Thu, 3 Nov 2022 09:12:11 +0100
From:   Christoph Hellwig <hch@....de>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     hch@....de, axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yukuai3@...wei.com,
        yi.zhang@...wei.com
Subject: Re: [PATCH 2/2] block: fix use after free for bd_holder_dir

On Thu, Nov 03, 2022 at 10:55:41AM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@...wei.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().

Looks good with some minor stilistic nipicks:

> +	if (!disk_live(bdev->bd_disk)) {
> +		mutex_unlock(&bdev->bd_disk->open_mutex);
> +		return -ENODEV;
> +	}

This can use a goto out_unlock;

>  		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);

And I think a goto out_put_holder and out_free_holder would clean this up
nicely.

>  	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);

.. with this then jumping straight to out_unlock.


We should repost a series with my first 7 patches and your two.  I can do
that, but it might take some time as I just got through (minor) knee
surgery and am still at the hospital, so if you have spare cycles feel
free to do it.