Re: [PATCH RFC v2 08/10] vdpa: add vdpa simulator for block device

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Jan 28, 2021 at 03:41:25PM +0100, Stefano Garzarella wrote:
> +static void vdpasim_blk_work(struct work_struct *work)
> +{
> +	struct vdpasim *vdpasim = container_of(work, struct vdpasim, work);
> +	u8 status = VIRTIO_BLK_S_OK;
> +	int i;
> +
> +	spin_lock(&vdpasim->lock);
> +
> +	if (!(vdpasim->status & VIRTIO_CONFIG_S_DRIVER_OK))
> +		goto out;
> +
> +	for (i = 0; i < VDPASIM_BLK_VQ_NUM; i++) {
> +		struct vdpasim_virtqueue *vq = &vdpasim->vqs[i];
> +
> +		if (!vq->ready)
> +			continue;
> +
> +		while (vringh_getdesc_iotlb(&vq->vring, &vq->out_iov,
> +					    &vq->in_iov, &vq->head,
> +					    GFP_ATOMIC) > 0) {
> +			int write;
> +
> +			vq->in_iov.i = vq->in_iov.used - 1;
> +			write = vringh_iov_push_iotlb(&vq->vring, &vq->in_iov,
> +						      &status, 1);
> +			if (write <= 0)
> +				break;

This code looks fragile:

1. Relying on unsigned underflow and the while loop in
   vringh_iov_push_iotlb() to handle the case where in_iov.used == 0 is
   risky and could break.

2. Does this assume that the last in_iov element has size 1? For
   example, the guest driver may send a single "in" iovec with size 513
   when reading 512 bytes (with an extra byte for the request status).

Please validate inputs fully, even in test/development code, because
it's likely to be copied by others when writing production code (or
deployed in production by unsuspecting users) :).

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