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

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Date:   Tue, 2 Feb 2021 16:49:50 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Stefan Hajnoczi <stefanha@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org,
        Xie Yongji <xieyongji@...edance.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Laurent Vivier <lvivier@...hat.com>,
        linux-kernel@...r.kernel.org, Max Gurtovoy <mgurtovoy@...dia.com>,
        Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH RFC v2 08/10] vdpa: add vdpa simulator for block device

On Tue, Feb 02, 2021 at 09:34:12AM +0000, Stefan Hajnoczi wrote:
>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) :).

Perfectly agree on that, so I addressed these things, also following 
your review on the previous version, on the next patch of this series:
"vdpa_sim_blk: implement ramdisk behaviour".

Do you think should I move these checks in this patch?

I did this to leave Max credit for this patch and add more code to 
emulate a ramdisk in later patches.

Thanks,
Stefano