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

[Ming Lei <ming.lei@redhat.com>]

On Tue, Dec 31, 2019 at 10:55:47PM +0800, Hou Tao wrote:
> Hi,
> 
> On 2019/12/31 19:09, Yufen Yu wrote:
> > When delete partition executes concurrently with IOs issue,
> > it may cause use-after-free on part in disk_map_sector_rcu()
> > as following:
> snip
> 
> > 
> > diff --git a/block/genhd.c b/block/genhd.c
> > index ff6268970ddc..39fa8999905f 100644
> > --- a/block/genhd.c
> > +++ b/block/genhd.c
> > @@ -293,7 +293,23 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
> >  		part = rcu_dereference(ptbl->part[i]);
> >  
> >  		if (part && sector_in_part(part, sector)) {
> snip
> 
> >  			rcu_assign_pointer(ptbl->last_lookup, part);
> > +			part = rcu_dereference(ptbl->part[i]);
> > +			if (part == NULL) {
> > +				rcu_assign_pointer(ptbl->last_lookup, NULL);
> > +				break;
> > +			}
> >  			return part;
> >  		}
> >  	}
> 
> Not ensure whether the re-read can handle the following case or not:
> 
> process A                                 process B                          process C
> 
> disk_map_sector_rcu():                    delete_partition():               disk_map_sector_rcu():
> 
> rcu_read_lock
> 
>   // need to iterate partition table
>   part[i] != NULL   (1)                   part[i] = NULL (2)
>                                           smp_mb()
>                                           last_lookup = NULL (3)
>                                           call_rcu()  (4)
>     last_lookup = part[i] (5)
> 
> 
>                                                                              rcu_read_lock()
>                                                                              read last_lookup return part[i] (6)
>                                                                              sector_in_part() is OK (7)
>                                                                              return part[i] (8)
> 
>   part[i] == NULL (9)
>       last_lookup = NULL (10)
>   rcu_read_unlock() (11)
>                                            one RCU grace period completes
>                                            __delete_partition() (12)
>                                            free hd_partition (13)
>                                                                              // use-after-free
>                                                                              hd_struct_try_get(part[i])  (14)
> 
> * the number in the parenthesis is the sequence of events.
> 
> Maybe RCU experts can shed some light on this problem, so cc +paulmck@kernel.org, +joel@joelfernandes.org and +RCU maillist.
> 
> If the above case is possible, maybe we can fix the problem by pinning last_lookup through increasing its ref-count
> (the following patch is only compile tested):
> 
> diff --git a/block/genhd.c b/block/genhd.c
> index 6e8543ca6912..179e0056fae1 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -279,7 +279,14 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
>  		part = rcu_dereference(ptbl->part[i]);
> 
>  		if (part && sector_in_part(part, sector)) {
> -			rcu_assign_pointer(ptbl->last_lookup, part);
> +			struct hd_struct *old;
> +
> +			if (!hd_struct_try_get(part))
> +				break;
> +
> +			old = xchg(&ptbl->last_lookup, part);
> +			if (old)
> +				hd_struct_put(old);
>  			return part;
>  		}
>  	}
> @@ -1231,7 +1238,11 @@ static void disk_replace_part_tbl(struct gendisk *disk,
>  	rcu_assign_pointer(disk->part_tbl, new_ptbl);
> 
>  	if (old_ptbl) {
> -		rcu_assign_pointer(old_ptbl->last_lookup, NULL);
> +		struct hd_struct *part;
> +
> +		part = xchg(&old_ptbl->last_lookup, NULL);
> +		if (part)
> +			hd_struct_put(part);
>  		kfree_rcu(old_ptbl, rcu_head);
>  	}
>  }
> diff --git a/block/partition-generic.c b/block/partition-generic.c
> index 98d60a59b843..441c1c591c04 100644
> --- a/block/partition-generic.c
> +++ b/block/partition-generic.c
> @@ -285,7 +285,8 @@ void delete_partition(struct gendisk *disk, int partno)
>  		return;
> 
>  	rcu_assign_pointer(ptbl->part[partno], NULL);
> -	rcu_assign_pointer(ptbl->last_lookup, NULL);
> +	if (cmpxchg(&ptbl->last_lookup, part, NULL) == part)
> +		hd_struct_put(part);
>  	kobject_put(part->holder_dir);
>  	device_del(part_to_dev(part));

IMO this approach looks good.

Given partition is actually protected by percpu-refcount now, I guess the
RCU annotation for referencing ->part[partno] and ->last_lookup may not
be necessary, together with the part->rcu_work.


Thanks,
Ming