Re: [PATCH V5 3/9] blk-mq: free hw queue's resource in hctx's release handler

[Hannes Reinecke <hare@suse.de>]

On 4/12/19 5:30 AM, Ming Lei wrote:
> Once blk_cleanup_queue() returns, tags shouldn't be used any more,
> because blk_mq_free_tag_set() may be called. Commit 45a9c9d909b2
> ("blk-mq: Fix a use-after-free") fixes this issue exactly.
> 
> However, that commit introduces another issue. Before 45a9c9d909b2,
> we are allowed to run queue during cleaning up queue if the queue's
> kobj refcount is held. After that commit, queue can't be run during
> queue cleaning up, otherwise oops can be triggered easily because
> some fields of hctx are freed by blk_mq_free_queue() in blk_cleanup_queue().
> 
> We have invented ways for addressing this kind of issue before, such as:
> 
> 	8dc765d438f1 ("SCSI: fix queue cleanup race before queue initialization is done")
> 	c2856ae2f315 ("blk-mq: quiesce queue before freeing queue")
> 
> But still can't cover all cases, recently James reports another such
> kind of issue:
> 
> 	https://marc.info/?l=linux-scsi&m=155389088124782&w=2
> 
> This issue can be quite hard to address by previous way, given
> scsi_run_queue() may run requeues for other LUNs.
> 
> Fixes the above issue by freeing hctx's resources in its release handler, and this
> way is safe becasue tags isn't needed for freeing such hctx resource.
> 
> This approach follows typical design pattern wrt. kobject's release handler.
> 
> 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>
> Reported-by: James Smart <james.smart@broadcom.com>
> Fixes: 45a9c9d909b2 ("blk-mq: Fix a use-after-free")
> Cc: stable@vger.kernel.org
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>   block/blk-core.c     | 2 +-
>   block/blk-mq-sysfs.c | 6 ++++++
>   block/blk-mq.c       | 8 ++------
>   block/blk-mq.h       | 2 +-
>   4 files changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 6583d67f3e34..20298aa5a77c 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -374,7 +374,7 @@ void blk_cleanup_queue(struct request_queue *q)
>   	blk_exit_queue(q);
>   
>   	if (queue_is_mq(q))
> -		blk_mq_free_queue(q);
> +		blk_mq_exit_queue(q);
>   
>   	percpu_ref_exit(&q->q_usage_counter);
>   
> diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
> index 3f9c3f4ac44c..4040e62c3737 100644
> --- a/block/blk-mq-sysfs.c
> +++ b/block/blk-mq-sysfs.c
> @@ -10,6 +10,7 @@
>   #include <linux/smp.h>
>   
>   #include <linux/blk-mq.h>
> +#include "blk.h"
>   #include "blk-mq.h"
>   #include "blk-mq-tag.h"
>   
> @@ -33,6 +34,11 @@ static void blk_mq_hw_sysfs_release(struct kobject *kobj)
>   {
>   	struct blk_mq_hw_ctx *hctx = container_of(kobj, struct blk_mq_hw_ctx,
>   						  kobj);
> +
> +	if (hctx->flags & BLK_MQ_F_BLOCKING)
> +		cleanup_srcu_struct(hctx->srcu);
> +	blk_free_flush_queue(hctx->fq);
> +	sbitmap_free(&hctx->ctx_map);
>   	free_cpumask_var(hctx->cpumask);
>   	kfree(hctx->ctxs);
>   	kfree(hctx);
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index b512ba0cb359..afc9912e2e42 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2259,12 +2259,7 @@ static void blk_mq_exit_hctx(struct request_queue *q,
>   	if (set->ops->exit_hctx)
>   		set->ops->exit_hctx(hctx, hctx_idx);
>   
> -	if (hctx->flags & BLK_MQ_F_BLOCKING)
> -		cleanup_srcu_struct(hctx->srcu);
> -
>   	blk_mq_remove_cpuhp(hctx);
> -	blk_free_flush_queue(hctx->fq);
> -	sbitmap_free(&hctx->ctx_map);
>   }
>   
>   static void blk_mq_exit_hw_queues(struct request_queue *q,
> @@ -2899,7 +2894,8 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
>   }
>   EXPORT_SYMBOL(blk_mq_init_allocated_queue);
>   
> -void blk_mq_free_queue(struct request_queue *q)
> +/* tags can _not_ be used after returning from blk_mq_exit_queue */
> +void blk_mq_exit_queue(struct request_queue *q)
>   {
>   	struct blk_mq_tag_set	*set = q->tag_set;
>   
> diff --git a/block/blk-mq.h b/block/blk-mq.h
> index d704fc7766f4..c421e3a16e36 100644
> --- a/block/blk-mq.h
> +++ b/block/blk-mq.h
> @@ -37,7 +37,7 @@ struct blk_mq_ctx {
>   	struct kobject		kobj;
>   } ____cacheline_aligned_in_smp;
>   
> -void blk_mq_free_queue(struct request_queue *q);
> +void blk_mq_exit_queue(struct request_queue *q);
>   int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr);
>   void blk_mq_wake_waiters(struct request_queue *q);
>   bool blk_mq_dispatch_rq_list(struct request_queue *, struct list_head *, bool);
> 
Hmm. While this is a good point, I'm somehow not convinced that it fixes 
the mentioned problem.

For the sbitmap_any_set() case, the problem appears to be the racy 
interaction between nvme_scan_queues() and nvme_reset_ctrl().
Both can (and do) run in parallel, and as nvme_scan_queue() is not able 
to update the nvme namespace list race-free (nvme_ns_remove() will kill 
the namespace before removing it from the queue) we'll hit issues where 
nvme_start_queues() is called on namespaces which are halfway through 
nvme_ns_remove(), causing all sorts of issues.

See "[PATCHv2 RFC] nvme: use nvme_set_queue_dying() during namespace 
rescanning" for my attempt to fix this.
That patch is actually not complete; there is another one under testing 
currently to not remove namespaces at all during nvme_reset_ctrl(), but 
results are not in yet.

Otherwise:

Reviewed-by: Hannes Reinecke <hare@suse.com>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare@suse.de			               +49 911 74053 688
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