[PATCH 12/15] block: switch polling to be bio based

[Ming Lei]

On Wed, May 12, 2021 at 03:15:42PM +0200, Christoph Hellwig wrote:
> Replace the blk_poll interface that requires the caller to keep a queue
> and cookie from the submissions with polling based on the bio.
> 
> Polling for the bio itself leads to a few advantages:
> 
>  - the cookie construction can made entirely private in blk-mq.c
>  - the caller does not need to remember the request_queue and cookie
>    separately and thus sidesteps their lifetime issues
>  - keeping the device and the cookie inside the bio allows to trivially
>    support polling BIOs remapping by stacking drivers
>  - a lot of code to propagate the cookie back up the submission path can
>    be removed entirely.
> 
> Signed-off-by: Christoph Hellwig <hch at lst.de>

...

> +
> +/*
> + * Helper to implement file_operations.iopoll.  Requires the bio to be stored
> + * in iocb->private, and cleared before freeing the bio.
> + */
> +int iocb_bio_iopoll(struct kiocb *kiocb, unsigned int flags)
> +{
> +	struct bio *bio;
> +	int ret = 0;
> +
> +	/*
> +	 * Note: the bio cache only uses SLAB_TYPESAFE_BY_RCU, so bio can
> +	 * point to a freshly allocated bio at this point.  If that happens
> +	 * we have a few cases to consider:
> +	 *
> +	 *  1) the bio is beeing initialized and bi_bdev is NULL.  We can just
> +	 *     simply nothing in this case
> +	 *  2) the bio points to a not poll enabled device.  bio_poll will catch
> +	 *     this and return 0
> +	 *  3) the bio points to a poll capable device, including but not
> +	 *     limited to the one that the original bio pointed to.  In this
> +	 *     case we will call into the actual poll method and poll for I/O,
> +	 *     even if we don't need to, but it won't cause harm either.
> +	 *
> +	 * For cases 2) and 3) above the RCU grace period ensures that the
> +	 * bi_bdev is still allocated, and because partitions hold a reference
> +	 * to the whole device bdev and thus disk it is still valid.
> +	 */
> +	rcu_read_lock();
> +	bio = READ_ONCE(kiocb->private);
> +	if (bio && bio->bi_bdev)
> +		ret = bio_poll(bio, flags);

The bio can be re-allocated from another IO path & bdev after checking on
bio->bi_bdev in the above code, then this bio is freed, its ->bi_bdev is freed,
same with its associated disk/hw queues/request queue.

Both bdev and request queue are freed via rcu, but disk/hw queues are freed
immediately, so there is still UAF risk in bio_poll().

Thanks, 
Ming