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

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

On Fri, Apr 12, 2019 at 01:03:35PM +0200, Hannes Reinecke wrote:
> 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.

This patch fixes the following generic issue in block layer:

1) usually any block layer API may be called safely when the request
queue's refcount is held

2) when blk_cleanup_queue() is run, no use-after-free oops can be
triggered on the code path in 1)

> 
> 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.

This is one race between nvme reset and scan, and especially the
refcount of ns queue isn't held before calling nvme_stop_queues()
and nvme_start_queues().

The above situation is one NVMe specific issue and doesn't belong to what
this patch tries to address.

> 
> 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:

Looks the above 'Otherwise' shouldn't be needed, as I explained.

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

Thanks!

-- 
Ming