Re: [PATCH V5 6/9] blk-mq: always free hctx after request queue is freed

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

On Fri, Apr 12, 2019 at 01:06:07PM +0200, Hannes Reinecke wrote:
> On 4/12/19 5:30 AM, Ming Lei wrote:
> > In normal queue cleanup path, hctx is released after request queue
> > is freed, see blk_mq_release().
> > 
> > However, in __blk_mq_update_nr_hw_queues(), hctx may be freed because
> > of hw queues shrinking. This way is easy to cause use-after-free,
> > because: one implicit rule is that it is safe to call almost all block
> > layer APIs if the request queue is alive; and one hctx may be retrieved
> > by one API, then the hctx can be freed by blk_mq_update_nr_hw_queues();
> > finally use-after-free is triggered.
> > 
> > Fixes this issue by always freeing hctx after releasing request queue.
> > If some hctxs are removed in blk_mq_update_nr_hw_queues(), introduce
> > a per-queue list to hold them, then try to resuse these hctxs if numa
> > node is matched.
> > 
> > Cc: Dongli Zhang <dongli.zhang@oracle.com>
> > Cc: James Smart <james.smart@broadcom.com>
> > Cc: Bart Van Assche <bart.vanassche@wdc.com>
> > Cc: linux-scsi@vger.kernel.org,
> > Cc: Martin K . Petersen <martin.petersen@oracle.com>,
> > Cc: Christoph Hellwig <hch@lst.de>,
> > Cc: James E . J . Bottomley <jejb@linux.vnet.ibm.com>,
> > Cc: jianchao wang <jianchao.w.wang@oracle.com>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >   block/blk-mq.c         | 40 +++++++++++++++++++++++++++-------------
> >   include/linux/blk-mq.h |  2 ++
> >   include/linux/blkdev.h |  7 +++++++
> >   3 files changed, 36 insertions(+), 13 deletions(-)
> > 
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 71996fe494eb..886fbb678617 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -2260,6 +2260,10 @@ static void blk_mq_exit_hctx(struct request_queue *q,
> >   		set->ops->exit_hctx(hctx, hctx_idx);
> >   	blk_mq_remove_cpuhp(hctx);
> > +
> > +	spin_lock(&q->dead_hctx_lock);
> > +	list_add(&hctx->hctx_list, &q->dead_hctx_list);
> > +	spin_unlock(&q->dead_hctx_lock);
> >   }
> >   static void blk_mq_exit_hw_queues(struct request_queue *q,
> > @@ -2660,15 +2664,13 @@ static int blk_mq_alloc_ctxs(struct request_queue *q)
> >    */
> >   void blk_mq_release(struct request_queue *q)
> >   {
> > -	struct blk_mq_hw_ctx *hctx;
> > -	unsigned int i;
> > +	struct blk_mq_hw_ctx *hctx, *next;
> >   	cancel_delayed_work_sync(&q->requeue_work);
> > -	/* hctx kobj stays in hctx */
> > -	queue_for_each_hw_ctx(q, hctx, i) {
> > -		if (!hctx)
> > -			continue;
> > +	/* all hctx are in .dead_hctx_list now */
> > +	list_for_each_entry_safe(hctx, next, &q->dead_hctx_list, hctx_list) {
> > +		list_del_init(&hctx->hctx_list);
> >   		kobject_put(&hctx->kobj);
> >   	}
> > @@ -2735,9 +2737,22 @@ static struct blk_mq_hw_ctx *blk_mq_alloc_and_init_hctx(
> >   		struct blk_mq_tag_set *set, struct request_queue *q,
> >   		int hctx_idx, int node)
> >   {
> > -	struct blk_mq_hw_ctx *hctx;
> > +	struct blk_mq_hw_ctx *hctx = NULL, *tmp;
> > +
> > +	/* reuse dead hctx first */
> > +	spin_lock(&q->dead_hctx_lock);
> > +	list_for_each_entry(tmp, &q->dead_hctx_list, hctx_list) {
> > +		if (tmp->numa_node == node) {
> > +			hctx = tmp;
> > +			break;
> > +		}
> > +	}
> > +	if (hctx)
> > +		list_del_init(&hctx->hctx_list);
> > +	spin_unlock(&q->dead_hctx_lock);
> > -	hctx = blk_mq_alloc_hctx(q, set, hctx_idx, node);
> > +	if (!hctx)
> > +		hctx = blk_mq_alloc_hctx(q, set, hctx_idx, node);
> >   	if (!hctx)
> >   		goto fail;
> > @@ -2775,10 +2790,8 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
> >   		hctx = blk_mq_alloc_and_init_hctx(set, q, i, node);
> >   		if (hctx) {
> > -			if (hctxs[i]) {
> > +			if (hctxs[i])
> >   				blk_mq_exit_hctx(q, set, hctxs[i], i);
> > -				kobject_put(&hctxs[i]->kobj);
> > -			}
> >   			hctxs[i] = hctx;
> >   		} else {
> >   			if (hctxs[i])
> > @@ -2809,9 +2822,7 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
> >   			if (hctx->tags)
> >   				blk_mq_free_map_and_requests(set, j);
> >   			blk_mq_exit_hctx(q, set, hctx, j);
> > -			kobject_put(&hctx->kobj);
> >   			hctxs[j] = NULL;
> > -
> >   		}
> >   	}
> >   	mutex_unlock(&q->sysfs_lock);
> > @@ -2854,6 +2865,9 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
> >   	if (!q->queue_hw_ctx)
> >   		goto err_sys_init;
> > +	INIT_LIST_HEAD(&q->dead_hctx_list);
> > +	spin_lock_init(&q->dead_hctx_lock);
> > +
> >   	blk_mq_realloc_hw_ctxs(set, q);
> >   	if (!q->nr_hw_queues)
> >   		goto err_hctxs;
> > diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> > index cb2aa7ecafff..a44c3f95dcc1 100644
> > --- a/include/linux/blk-mq.h
> > +++ b/include/linux/blk-mq.h
> > @@ -70,6 +70,8 @@ struct blk_mq_hw_ctx {
> >   	struct dentry		*sched_debugfs_dir;
> >   #endif
> > +	struct list_head	hctx_list;
> > +
> >   	/* Must be the last member - see also blk_mq_hw_ctx_size(). */
> >   	struct srcu_struct	srcu[0];
> >   };
> > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> > index 4b85dc066264..1325f941f0be 100644
> > --- a/include/linux/blkdev.h
> > +++ b/include/linux/blkdev.h
> > @@ -535,6 +535,13 @@ struct request_queue {
> >   	struct mutex		sysfs_lock;
> > +	/*
> > +	 * for reusing dead hctx instance in case of updating
> > +	 * nr_hw_queues
> > +	 */
> > +	struct list_head	dead_hctx_list;
> > +	spinlock_t		dead_hctx_lock;
> > +
> >   	atomic_t		mq_freeze_depth;
> >   #if defined(CONFIG_BLK_DEV_BSG)
> > 
> I actually had been looking into this, too, but couldn't convince myself
> that the code really is a problem.
> Did you see this happening in real life?

User-after-free on hctx can be triggered if the hctx is freed
in blk_mq_update_nr_hw_queues().

This patch aligns to normal cleanup queue model, and will avoid this
kind of issue.

Updating nr_hw_queues is only triggered on very specific situation, so
in practice, it isn't easy to trigger.

However, if we play CPU hotplug & timeout test on nvme-loop, it should
have been easy to trigger, but nvme-loop's reset/timeout handler is simply
broken. The same test can be done on nvme-pci too, just needs real
hardware and environment and takes time...


Thanks,
Ming