Re: [PATCH] block: make sure last_lookup set as NULL after part deleted

[Yufen Yu <yuyufen@xxxxxxxxxx>]

Hi, Ming

On 2020/1/3 16:17, Ming Lei wrote:
> We may avoid that by clearing partition pointer after killing the
> partition, how about the following change?
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 089e890ab208..79599f5fd5b7 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1365,18 +1365,6 @@ void blk_account_io_start(struct request *rq, bool new_io)
>   		part_stat_inc(part, merges[rw]);
>   	} else {
>   		part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
> -		if (!hd_struct_try_get(part)) {
> -			/*
> -			 * The partition is already being removed,
> -			 * the request will be accounted on the disk only
> -			 *
> -			 * We take a reference on disk->part0 although that
> -			 * partition will never be deleted, so we can treat
> -			 * it as any other partition.
> -			 */
> -			part = &rq->rq_disk->part0;
> -			hd_struct_get(part);
> -		}
>   		part_inc_in_flight(rq->q, part, rw);
>   		rq->part = part;
>   	}
> diff --git a/block/genhd.c b/block/genhd.c
> index ff6268970ddc..e3dec90b1f43 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -286,17 +286,21 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
>   	ptbl = rcu_dereference(disk->part_tbl);
>   
>   	part = rcu_dereference(ptbl->last_lookup);
> -	if (part && sector_in_part(part, sector))
> +	if (part && sector_in_part(part, sector) && hd_struct_try_get(part))
>   		return part;
>   
>   	for (i = 1; i < ptbl->len; i++) {
>   		part = rcu_dereference(ptbl->part[i]);
>   
>   		if (part && sector_in_part(part, sector)) {
> +                       if (!hd_struct_try_get(part))
> +                               goto exit;
>   			rcu_assign_pointer(ptbl->last_lookup, part);
>   			return part;
>   		}
>   	}
> + exit:
> +	hd_struct_get(&disk->part0);
>   	return &disk->part0;
>   }
>   EXPORT_SYMBOL_GPL(disk_map_sector_rcu);
> diff --git a/block/partition-generic.c b/block/partition-generic.c
> index 1d20c9cf213f..9ef6c13d5650 100644
> --- a/block/partition-generic.c
> +++ b/block/partition-generic.c
> @@ -283,8 +283,8 @@ void delete_partition(struct gendisk *disk, int partno)
>   	if (!part)
>   		return;
>   
> -	rcu_assign_pointer(ptbl->part[partno], NULL);
> -	rcu_assign_pointer(ptbl->last_lookup, NULL);
> +	get_device(disk_to_dev(disk));
> +
>   	kobject_put(part->holder_dir);
>   	device_del(part_to_dev(part));
>   
> @@ -296,6 +296,15 @@ void delete_partition(struct gendisk *disk, int partno)
>   	 */
>   	blk_invalidate_devt(part_devt(part));
>   	hd_struct_kill(part);
> +
> +	/*
> +	 * clear partition pointers after this partition is killed, then
> +	 * IO path can't re-assign ->last_lookup any more
> +	 */
> +	rcu_assign_pointer(ptbl->part[partno], NULL);
> +	rcu_assign_pointer(ptbl->last_lookup, NULL);
> +
> +	put_device(disk_to_dev(disk));
>   }
>   

This change may cannot solve follow case:

disk_map_sector_rcu     delete_partition  disk_map_sector_rcu
hd_struct_try_get(part)
                        hd_struct_kill
                        last_lookup = NULL;
last_lookup = part


call_rcu
                                           read last_lookup

free()
                                           //use-after-free
                                           sector_in_part(part, sector)

There is an interval between getting part and setting last_lookup
in disk_map_sector_rcu(). If we kill the part and clear last_lookup
at that interval, last_lookup will be re-assign again, which can cause
use-after-free for readers after call_rcu.

Thanks
Yufen