Re: [Qemu-devel] [PATCH V2 1/3] virtio: guard against NULL pfn

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Mon, 13 Mar 2017 14:29:41 +0800
Jason Wang <jasowang@redhat.com> wrote:

> To avoid access stale memory region cache after reset, this patch
> check the existence of virtqueue pfn for all exported virtqueue access
> helpers before trying to use them.
> 
> Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  hw/virtio/virtio.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index efce4b3..76cc81b 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -322,6 +322,10 @@ static int virtio_queue_empty_rcu(VirtQueue *vq)
>          return 0;
>      }
> 
> +    if (unlikely(!vq->vring.avail)) {
> +        return 0;

Shouldn't that rather return !0 (denoting a non-existing queue as
empty)?

> +    }
> +
>      return vring_avail_idx(vq) == vq->last_avail_idx;
>  }
> 
> @@ -333,6 +337,10 @@ int virtio_queue_empty(VirtQueue *vq)
>          return 0;
>      }
> 
> +    if (unlikely(!vq->vring.avail)) {
> +        return 0;

Likewise.

> +    }
> +
>      rcu_read_lock();
>      empty = vring_avail_idx(vq) == vq->last_avail_idx;
>      rcu_read_unlock();
> @@ -431,6 +439,10 @@ void virtqueue_fill(VirtQueue *vq, const VirtQueueElement *elem,
>          return;
>      }
> 
> +    if (unlikely(!vq->vring.used)) {
> +        return;
> +    }
> +
>      idx = (idx + vq->used_idx) % vq->vring.num;
> 
>      uelem.id = elem->index;
> @@ -448,6 +460,10 @@ void virtqueue_flush(VirtQueue *vq, unsigned int count)
>          return;
>      }
> 
> +    if (unlikely(!vq->vring.used)) {
> +        return;
> +    }
> +
>      /* Make sure buffer is written before we update index. */
>      smp_wmb();
>      trace_virtqueue_flush(vq, count);
> @@ -546,6 +562,11 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
>      int64_t len = 0;
>      int rc;
> 
> +    if (unlikely(!vq->vring.desc)) {
> +        *in_bytes = *out_bytes = 0;

I think you need to check for in_bytes and out_bytes being !NULL first.

> +        return;
> +    }
> +
>      rcu_read_lock();
>      idx = vq->last_avail_idx;
>      total_bufs = in_total = out_total = 0;