Re: [syzbot] KASAN: use-after-free Read in disk_part_iter_next (2)

[Bart Van Assche <bvanassche@acm.org>]

On 3/22/21 12:18 AM, Christoph Hellwig wrote:
> I've been running the reproducer on a KASAN enable VM for about
> 15 minutes now, but haven't been able to reproduce it.
> 
> Is there a way to inject this proposed fix into the syzbot queue?
> 
> diff --git a/block/partitions/core.c b/block/partitions/core.c
> index 1a7558917c47d6..f5d5872b89d57e 100644
> --- a/block/partitions/core.c
> +++ b/block/partitions/core.c
> @@ -288,15 +288,12 @@ struct device_type part_type = {
>   void delete_partition(struct block_device *part)
>   {
>   	xa_erase(&part->bd_disk->part_tbl, part->bd_partno);
> -	kobject_put(part->bd_holder_dir);
> -	device_del(&part->bd_device);
> -
> -	/*
> -	 * Remove the block device from the inode hash, so that it cannot be
> -	 * looked up any more even when openers still hold references.
> -	 */
>   	remove_inode_hash(part->bd_inode);
>   
> +	synchronize_rcu();
> +
> +	kobject_put(part->bd_holder_dir);
> +	device_del(&part->bd_device);
>   	put_device(&part->bd_device);
>   }

Hi Christoph,

disk_part_iter_next() calls bdgrab() before returning a pointer to a 
certain partition. 'part' is only freed if its reference count drops to 
zero. The function that frees the partition information, 
bdev_free_inode(), is invoked via call_rcu(). bdgrab() fails if the 
refcount of a partition is zero. Does that mean that it is not necessary 
to call synchronize_rcu() between xa_erase() and put_device()?

Thanks,

Bart.