Re: [PATCH net-next 6/6] tools: virtio: introduce vhost_net_test

[Yunsheng Lin]

On 2023/12/12 12:35, Jason Wang wrote:>>>> +done:
>>>> +       backend.fd = tun_alloc();
>>>> +       assert(backend.fd >= 0);
>>>> +       vdev_info_init(&dev, features);
>>>> +       vq_info_add(&dev, 256);
>>>> +       run_test(&dev, &dev.vqs[0], delayed, batch, reset, nbufs);
>>>
>>> I'd expect we are testing some basic traffic here. E.g can we use a
>>> packet socket then we can test both tx and rx?
>>
>> Yes, only rx for tun is tested.
>> Do you have an idea how to test the tx too? As I am not familar enough
>> with vhost_net and tun yet.
> 
> Maybe you can have a packet socket to bind to the tun/tap. Then you can test:
> 
> 1) TAP RX: by write a packet via virtqueue through vhost_net and read
> it from packet socket
> 2) TAP TX:  by write via packet socket and read it from the virtqueue
> through vhost_net

When implementing the TAP TX by adding VHOST_NET_F_VIRTIO_NET_HDR,
I found one possible use of uninitialized data in vhost_net_build_xdp().

And vhost_hlen is set to sizeof(struct virtio_net_hdr_mrg_rxbuf) and
sock_hlen is set to zero in vhost_net_set_features() for both tx and rx
queue.

For vhost_net_build_xdp() called by handle_tx_copy():

The (gso->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) checking below may cause a
read of uninitialized data if sock_hlen is zero.

And it seems vhost_hdr is skipped in get_tx_bufs():
https://elixir.bootlin.com/linux/latest/source/drivers/vhost/net.c#L616

static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
			       struct iov_iter *from)
{
...
	buflen += SKB_DATA_ALIGN(len + pad);
	alloc_frag->offset = ALIGN((u64)alloc_frag->offset, SMP_CACHE_BYTES);
	if (unlikely(!vhost_net_page_frag_refill(net, buflen,
						 alloc_frag, GFP_KERNEL)))
		return -ENOMEM;

	buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset;
	copied = copy_page_from_iter(alloc_frag->page,
				     alloc_frag->offset +
				     offsetof(struct tun_xdp_hdr, gso),
				     sock_hlen, from);
	if (copied != sock_hlen)
		return -EFAULT;

	hdr = buf;
	gso = &hdr->gso;

	if ((gso->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) &&
	    vhost16_to_cpu(vq, gso->csum_start) +
	    vhost16_to_cpu(vq, gso->csum_offset) + 2 >
	    vhost16_to_cpu(vq, gso->hdr_len)) {
...
}

I seems the handle_tx_copy() does not handle the VHOST_NET_F_VIRTIO_NET_HDR
case correctly, Or do I miss something obvious here?

> 
> Thanks
> 
>>
>>>
>>> Thanks
>>
> 
> .
>