Re: [PATCH v7 3/5] blk-mq: Fix races between iterating over requests and freeing requests

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

On Tue, Apr 20, 2021 at 05:02:33PM -0700, Bart Van Assche wrote:
> When multiple request queues share a tag set and when switching the I/O
> scheduler for one of the request queues associated with that tag set, the
> following race can happen:
> * blk_mq_tagset_busy_iter() calls bt_tags_iter() and bt_tags_iter() assigns
>   a pointer to a scheduler request to the local variable 'rq'.
> * blk_mq_sched_free_requests() is called to free hctx->sched_tags.
> * blk_mq_tagset_busy_iter() dereferences 'rq' and triggers a use-after-free.
> 
> Fix this race as follows:
> * Use rcu_assign_pointer() and rcu_dereference() to access hctx->tags->rqs[].
>   The performance impact of the assignments added to the hot path is minimal
>   (about 1% according to one particular test).
> * Protect hctx->tags->rqs[] reads with an RCU read-side lock or with a
>   semaphore. Which mechanism is used depends on whether or not it is allowed
>   to sleep and also on whether or not the callback function may sleep.
> * Wait for all preexisting readers to finish before freeing scheduler
>   requests.
> 
> Another race is as follows:
> * blk_mq_sched_free_requests() is called to free hctx->sched_tags.
> * blk_mq_queue_tag_busy_iter() iterates over the same tag set but for another
>   request queue than the queue for which scheduler tags are being freed.
> * bt_iter() examines rq->q after *rq has been freed.
> 
> Fix this race by protecting the rq->q read in bt_iter() with an RCU read
> lock and by calling synchronize_rcu() before freeing scheduler tags.
> 
> Multiple users have reported use-after-free complaints similar to the
> following (from https://lore.kernel.org/linux-block/1545261885.185366.488.camel@acm.org/ ):
> 
> BUG: KASAN: use-after-free in bt_iter+0x86/0xf0
> Read of size 8 at addr ffff88803b335240 by task fio/21412
> 
> CPU: 0 PID: 21412 Comm: fio Tainted: G        W         4.20.0-rc6-dbg+ #3
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
> Call Trace:
>  dump_stack+0x86/0xca
>  print_address_description+0x71/0x239
>  kasan_report.cold.5+0x242/0x301
>  __asan_load8+0x54/0x90
>  bt_iter+0x86/0xf0
>  blk_mq_queue_tag_busy_iter+0x373/0x5e0
>  blk_mq_in_flight+0x96/0xb0
>  part_in_flight+0x40/0x140
>  part_round_stats+0x18e/0x370
>  blk_account_io_start+0x3d7/0x670
>  blk_mq_bio_to_request+0x19c/0x3a0
>  blk_mq_make_request+0x7a9/0xcb0
>  generic_make_request+0x41d/0x960
>  submit_bio+0x9b/0x250
>  do_blockdev_direct_IO+0x435c/0x4c70
>  __blockdev_direct_IO+0x79/0x88
>  ext4_direct_IO+0x46c/0xc00
>  generic_file_direct_write+0x119/0x210
>  __generic_file_write_iter+0x11c/0x280
>  ext4_file_write_iter+0x1b8/0x6f0
>  aio_write+0x204/0x310
>  io_submit_one+0x9d3/0xe80
>  __x64_sys_io_submit+0x115/0x340
>  do_syscall_64+0x71/0x210
> 
> Reviewed-by: Khazhismel Kumykov <khazhy@google.com>
> Tested-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Martin K. Petersen <martin.petersen@oracle.com>
> Cc: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
> Cc: Ming Lei <ming.lei@redhat.com>
> Cc: Hannes Reinecke <hare@suse.de>
> Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> Cc: John Garry <john.garry@huawei.com>
> Cc: Khazhy Kumykov <khazhy@google.com>
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
>  block/blk-core.c   | 34 ++++++++++++++++++++++++++++++-
>  block/blk-mq-tag.c | 51 ++++++++++++++++++++++++++++++++++++++++------
>  block/blk-mq-tag.h |  4 +++-
>  block/blk-mq.c     | 21 +++++++++++++++----
>  block/blk-mq.h     |  1 +
>  block/blk.h        |  2 ++
>  block/elevator.c   |  1 +
>  7 files changed, 102 insertions(+), 12 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 9bcdae93f6d4..ca7f833e25a8 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -279,6 +279,36 @@ void blk_dump_rq_flags(struct request *rq, char *msg)
>  }
>  EXPORT_SYMBOL(blk_dump_rq_flags);
>  
> +/**
> + * blk_mq_wait_for_tag_iter - wait for preexisting tag iteration functions to finish
> + * @set: Tag set to wait on.
> + *
> + * Waits for preexisting calls of blk_mq_all_tag_iter(),
> + * blk_mq_tagset_busy_iter() and also for their atomic variants to finish
> + * accessing hctx->tags->rqs[]. New readers may start while this function is
> + * in progress or after this function has returned. Use this function to make
> + * sure that hctx->tags->rqs[] changes have become globally visible.
> + *
> + * Waits for preexisting blk_mq_queue_tag_busy_iter(q, fn, priv) calls to
> + * finish accessing requests associated with other request queues than 'q'.
> + */
> +void blk_mq_wait_for_tag_iter(struct blk_mq_tag_set *set)
> +{
> +	struct blk_mq_tags *tags;
> +	int i;
> +
> +	if (set->tags) {
> +		for (i = 0; i < set->nr_hw_queues; i++) {
> +			tags = set->tags[i];
> +			if (!tags)
> +				continue;
> +			down_write(&tags->iter_rwsem);
> +			up_write(&tags->iter_rwsem);
> +		}
> +	}
> +	synchronize_rcu();
> +}
> +
>  /**
>   * blk_sync_queue - cancel any pending callbacks on a queue
>   * @q: the queue
> @@ -412,8 +442,10 @@ void blk_cleanup_queue(struct request_queue *q)
>  	 * it is safe to free requests now.
>  	 */
>  	mutex_lock(&q->sysfs_lock);
> -	if (q->elevator)
> +	if (q->elevator) {
> +		blk_mq_wait_for_tag_iter(q->tag_set);
>  		blk_mq_sched_free_requests(q);
> +	}
>  	mutex_unlock(&q->sysfs_lock);
>  
>  	percpu_ref_exit(&q->q_usage_counter);
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index d8eaa38a1bd1..39d5c9190a6b 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -209,14 +209,24 @@ static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
>  
>  	if (!reserved)
>  		bitnr += tags->nr_reserved_tags;
> -	rq = tags->rqs[bitnr];
> -
> +	rcu_read_lock();
> +	/*
> +	 * The request 'rq' points at is protected by an RCU read lock until
> +	 * its queue pointer has been verified and by q_usage_count while the
> +	 * callback function is being invoked. See also the
> +	 * percpu_ref_tryget() and blk_queue_exit() calls in
> +	 * blk_mq_queue_tag_busy_iter().
> +	 */
> +	rq = rcu_dereference(tags->rqs[bitnr]);
>  	/*
>  	 * We can hit rq == NULL here, because the tagging functions
>  	 * test and set the bit before assigning ->rqs[].
>  	 */
> -	if (rq && rq->q == hctx->queue && rq->mq_hctx == hctx)
> +	if (rq && rq->q == hctx->queue && rq->mq_hctx == hctx) {
> +		rcu_read_unlock();
>  		return iter_data->fn(hctx, rq, iter_data->data, reserved);
> +	}
> +	rcu_read_unlock();
>  	return true;
>  }
>  
> @@ -254,11 +264,17 @@ struct bt_tags_iter_data {
>  	unsigned int flags;
>  };
>  
> +/* Include reserved tags. */
>  #define BT_TAG_ITER_RESERVED		(1 << 0)
> +/* Only include started requests. */
>  #define BT_TAG_ITER_STARTED		(1 << 1)
> +/* Iterate over tags->static_rqs[] instead of tags->rqs[]. */
>  #define BT_TAG_ITER_STATIC_RQS		(1 << 2)
> +/* The callback function may sleep. */
> +#define BT_TAG_ITER_MAY_SLEEP		(1 << 3)
>  
> -static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
> +static bool __bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr,
> +			   void *data)
>  {
>  	struct bt_tags_iter_data *iter_data = data;
>  	struct blk_mq_tags *tags = iter_data->tags;
> @@ -275,7 +291,8 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
>  	if (iter_data->flags & BT_TAG_ITER_STATIC_RQS)
>  		rq = tags->static_rqs[bitnr];
>  	else
> -		rq = tags->rqs[bitnr];
> +		rq = rcu_dereference_check(tags->rqs[bitnr],
> +					   lockdep_is_held(&tags->iter_rwsem));
>  	if (!rq)
>  		return true;
>  	if ((iter_data->flags & BT_TAG_ITER_STARTED) &&
> @@ -284,6 +301,25 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
>  	return iter_data->fn(rq, iter_data->data, reserved);
>  }
>  
> +static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
> +{
> +	struct bt_tags_iter_data *iter_data = data;
> +	struct blk_mq_tags *tags = iter_data->tags;
> +	bool res;
> +
> +	if (iter_data->flags & BT_TAG_ITER_MAY_SLEEP) {
> +		down_read(&tags->iter_rwsem);
> +		res = __bt_tags_iter(bitmap, bitnr, data);
> +		up_read(&tags->iter_rwsem);
> +	} else {
> +		rcu_read_lock();
> +		res = __bt_tags_iter(bitmap, bitnr, data);
> +		rcu_read_unlock();
> +	}
> +
> +	return res;
> +}

Holding one rwsem or rcu read lock won't avoid the issue completely
because request may be completed remotely in iter_data->fn(), such as
nbd_clear_req(), nvme_cancel_request(), complete_all_cmds_iter(),
mtip_no_dev_cleanup(), because blk_mq_complete_request() may complete
request in softirq, remote IPI, even wq, and the request is still
referenced in these contexts after bt_tags_iter() returns.


Thanks,
Ming