Re: blk-mq crash under KVM in multiqueue block code (with virtio-blk and ext4)

[Jens Axboe]

On 2014-09-17 07:52, Ming Lei wrote:
> On Wed, 17 Sep 2014 14:00:34 +0200
> David Hildenbrand <dahi@xxxxxxxxxxxxxxxxxx> wrote:
>
> > > > > > Does anyone have an idea?
> > > > > > The request itself is completely filled with cc
> > > > >
> > > > > That is very weird, the 'rq' is got from hctx->tags,  and rq should be
> > > > > valid, and rq->q shouldn't have been changed even though it was
> > > > > double free or double allocation.
> > > > >
> > > > > > I am currently asking myself if blk_mq_map_request should protect against softirq here but I cant say for sure,as I have never looked into that code before.
> > > > >
> > > > > No, it needn't the protection.
> > > > >
> > > > > Thanks,
> > > >
> > > > --
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> >
> > Digging through the code, I think I found a possible cause:
> >
> > tags->rqs[..] is not initialized with zeroes (via alloc_pages_node in
> > blk-mq.c:blk_mq_init_rq_map()).
>
> Yes, it may cause problem when the request is allocated at the 1st time,
> and timeout handler may comes just after the allocation and before its
> initialization, then oops triggered because of garbage data in the request.
>
> --
>  From ffd0824b7b686074c2d5d70bc4e6bba3ba56a30c Mon Sep 17 00:00:00 2001
> From: Ming Lei <ming.lei@xxxxxxxxxxxxx>
> Date: Wed, 17 Sep 2014 21:00:34 +0800
> Subject: [PATCH] blk-mq: initialize request before the 1st allocation
>
> Otherwise the request can be accessed from timeout handler
> just after its 1st allocation from tag pool and before
> initialization in blk_mq_rq_ctx_init(), so cause oops since
> the request is filled up with garbage data.
>
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>
> ---
>   block/blk-mq.c |   10 ++++++++++
>   1 file changed, 10 insertions(+)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 4aac826..d24673f 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -514,6 +514,10 @@ struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag)
>   {
>   	struct request *rq = tags->rqs[tag];
>
> +	/* uninitialized request */
> +	if (!rq->q || rq->tag == -1)
> +		return rq;
> +
>   	if (!is_flush_request(rq, tag))
>   		return rq;
>
> @@ -1401,6 +1405,12 @@ static struct blk_mq_tags *blk_mq_init_rq_map(struct blk_mq_tag_set *set,
>   		left -= to_do * rq_size;
>   		for (j = 0; j < to_do; j++) {
>   			tags->rqs[i] = p;
> +
> +			/* Avoiding early access from timeout handler */
> +			tags->rqs[i]->tag = -1;
> +			tags->rqs[i]->q = NULL;
> +			tags->rqs[i]->cmd_flags = 0;
> +
>   			if (set->ops->init_request) {
>   				if (set->ops->init_request(set->driver_data,
>   						tags->rqs[i], hctx_idx, i,

Another way would be to ensure that the timeout handler doesn't touch 
hw_ctx or tag_sets that aren't fully initialized yet. But I think this 
is safer/cleaner.

--
Jens Axboe

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