Commit b7d6c303 authored by Ming Lei's avatar Ming Lei Committed by Jens Axboe
Browse files

block: fix use-after-free on cached last_lookup partition 

delete_partition() clears the cached last_lookup partition. However the
.last_lookup cache may be overwritten by one IO path after it is cleared
from delete_partition(). Then another IO path may use the cached deleting
partition after hd_struct_free() is called, then use-after-free is triggered
on the cached partition.

Fixes the issue by the following approach:

1) always get the partition's refcount via hd_struct_try_get() before
setting .last_lookup

2) move clearing .last_lookup from delete_partition() to hd_struct_free()
which is the release handle of the partition's percpu-refcount, so that no
IO path can cache deleteing partition via .last_lookup.

It is one candidate approach of Yufen's patch[1] which adds overhead
in fast path by indirect lookup which may introduce one extra cacheline
in IO path. Also this patch relies on percpu-refcount's protection, and
it is easier to understand and verify.

[1] https://lore.kernel.org/linux-block/20200109013551.GB9655@ming.t460p/T/#t



Reported-by: default avatarYufen Yu <yuyufen@huawei.com>
Signed-off-by: default avatarMing Lei <ming.lei@redhat.com>
Reviewed-by: default avatarChristoph Hellwig <hch@infradead.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Hou Tao <houtao1@huawei.com>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent aa880ad6

block/blk-core.c

+0 −12
Original line number Diff line number Diff line
@@ -1362,18 +1362,6 @@ 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;

	}

block/genhd.c

+12 −3
Original line number Diff line number Diff line
@@ -344,11 +344,12 @@ static inline int sector_in_part(struct hd_struct *part, sector_t sector) 
 * primarily used for stats accounting.

 *

 * CONTEXT:

 * RCU read locked.  The returned partition pointer is valid only

 * while preemption is disabled.

 * RCU read locked.  The returned partition pointer is always valid

 * because its refcount is grabbed.

 *

 * RETURNS:

 * Found partition on success, part0 is returned if no partition matches

 * or the matched partition is being deleted.

 */

struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)

{
@@ -359,17 +360,25 @@ 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)) {

			/*

			 * only live partition can be cached for lookup,

			 * so use-after-free on cached & deleting partition

			 * can be avoided

			 */

			if (!hd_struct_try_get(part))

				break;

			rcu_assign_pointer(ptbl->last_lookup, part);

			return part;

		}

	}

	hd_struct_get(&disk->part0);

	return &disk->part0;

}

block/partitions/core.c

+11 −1
Original line number Diff line number Diff line
@@ -288,6 +288,12 @@ static void hd_struct_free_work(struct work_struct *work) 
static void hd_struct_free(struct percpu_ref *ref)

{

	struct hd_struct *part = container_of(ref, struct hd_struct, ref);

	struct gendisk *disk = part_to_disk(part);

	struct disk_part_tbl *ptbl =

		rcu_dereference_protected(disk->part_tbl, 1);



	rcu_assign_pointer(ptbl->last_lookup, NULL);

	put_device(disk_to_dev(disk));



	INIT_RCU_WORK(&part->rcu_work, hd_struct_free_work);

	queue_rcu_work(system_wq, &part->rcu_work);
@@ -309,8 +315,12 @@ void delete_partition(struct gendisk *disk, struct hd_struct *part) 
	struct disk_part_tbl *ptbl =

		rcu_dereference_protected(disk->part_tbl, 1);



	/*

	 * ->part_tbl is referenced in this part's release handler, so

	 *  we have to hold the disk device

	 */

	get_device(disk_to_dev(part_to_disk(part)));

	rcu_assign_pointer(ptbl->part[part->partno], NULL);

	rcu_assign_pointer(ptbl->last_lookup, NULL);

	kobject_put(part->holder_dir);

	device_del(part_to_dev(part));

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