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

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

On Fri, Jan 03, 2020 at 03:35:42PM +0800, Hou Tao wrote:
> Hi,
> 
> On 2020/1/3 12:18, Ming Lei wrote:
> > On Fri, Jan 03, 2020 at 11:06:25AM +0800, Hou Tao wrote:
> >> Hi,
> >>
> >> On 2020/1/2 9:23, Ming Lei wrote:
> >>> On Tue, Dec 31, 2019 at 10:55:47PM +0800, Hou Tao wrote:
> >>>> Hi,
> >>>>
> snip
> 
> >> We have got a seemingly better solution: caching the index of last_lookup in tbl->part[]
> >> instead of caching the pointer itself, so we can ensure the validity of returned pointer
> >> by ensuring it's not NULL in tbl->part[] as does when last_lookup is NULL or 0.
> > 
> > Thinking of the problem further, looks we don't need to hold ref for
> > .last_lookup.
> > 
> > What we need is to make sure the partition's ref is increased just
> > before assigning .last_lookup, so how about something like the following?
> > 
> The approach will work for the above case, but it will not work for the following case:
> 
> when blk_account_io_done() releases the last ref-counter of last_lookup and calls call_rcu(),
> and then a RCU read gets the to-be-freed hd-struct.
> 
> blk_account_io_done
>   rcu_read_lock()
>   // the last ref of last_lookup
>   hd_struct_put()
>     call_rcu
> 
>                               rcu_read_lock
>                               read last_lookup
> 
>     free()
>                               // use-after-free ?
>                               hd_struct_try_get

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));
 }
 
 static ssize_t whole_disk_show(struct device *dev,

Thanks,
Ming