RE: [PATCH v2 1/1] vhost: fix a double fetch when dequeue offloading

["Wangyunjian(wangyunjian,TongTu)" <wangyunjian@huawei.com>]

> -----Original Message-----
> From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> Sent: Saturday, December 21, 2024 1:11 AM
> To: Wangyunjian(wangyunjian,TongTu) <wangyunjian@huawei.com>
> Cc: dev@dpdk.org; maxime.coquelin@redhat.com; chenbox@nvidia.com;
> Lilijun (Jerry) <jerry.lilijun@huawei.com>; xiawei (H) <xiawei40@huawei.com>;
> wangzengyuan <wangzengyuan@huawei.com>; stable@dpdk.org
> Subject: Re: [PATCH v2 1/1] vhost: fix a double fetch when dequeue offloading
> 
> On Fri, 20 Dec 2024 11:49:55 +0800
> Yunjian Wang <wangyunjian@huawei.com> wrote:
> 
> > The hdr->csum_start does two successive reads from user space to read a
> > variable length data structure. The result overflow if the data structure
> > changes between the two reads.
> >
> > To fix this, we can prevent double fetch issue by copying virtio_hdr to
> > the temporary variable.
> >
> > Fixes: 4dc4e33ffa10 ("net/virtio: fix Rx checksum calculation")
> > Cc: stable@dpdk.org
> >
> > Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
> 
> 
> How about something like the following *untested*

I agree. Can you fix it?

Thanks,
Yunjian

> 
> diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c
> index 69901ab3b5..c65cb639b2 100644
> --- a/lib/vhost/virtio_net.c
> +++ b/lib/vhost/virtio_net.c
> @@ -2861,25 +2861,28 @@ vhost_dequeue_offload(struct virtio_net *dev,
> struct virtio_net_hdr *hdr,
>  	}
>  }
> 
> -static __rte_noinline void
> +static inline int
>  copy_vnet_hdr_from_desc(struct virtio_net_hdr *hdr,
> -		struct buf_vector *buf_vec)
> +			const struct buf_vector *buf_vec,
> +			uint16_t nr_vec)
>  {
> -	uint64_t len;
> -	uint64_t remain = sizeof(struct virtio_net_hdr);
> -	uint64_t src;
> -	uint64_t dst = (uint64_t)(uintptr_t)hdr;
> +	size_t remain = sizeof(struct virtio_net_hdr);
> +	uint8_t *dst = (uint8_t *)hdr;
> 
> -	while (remain) {
> -		len = RTE_MIN(remain, buf_vec->buf_len);
> -		src = buf_vec->buf_addr;
> -		rte_memcpy((void *)(uintptr_t)dst,
> -				(void *)(uintptr_t)src, len);
> +	while (remain > 0) {
> +		size_t len = RTE_MIN(remain, buf_vec->buf_len);
> +		const void *src = (const void *)(uintptr_t)buf_vec->buf_addr;
> 
> +		if (unlikely(nr_vec == 0))
> +			return -1;
> +
> +		memcpy(dst, src, len);
>  		remain -= len;
>  		dst += len;
>  		buf_vec++;
> +		--nr_vec;
>  	}
> +	return 0;
>  }
> 
>  static __rte_always_inline int
> @@ -2908,16 +2911,12 @@ desc_to_mbuf(struct virtio_net *dev, struct
> vhost_virtqueue *vq,
>  	 */
> 
>  	if (virtio_net_with_host_offload(dev)) {
> -		if (unlikely(buf_vec[0].buf_len < sizeof(struct virtio_net_hdr))) {
> -			/*
> -			 * No luck, the virtio-net header doesn't fit
> -			 * in a contiguous virtual area.
> -			 */
> -			copy_vnet_hdr_from_desc(&tmp_hdr, buf_vec);
> -			hdr = &tmp_hdr;
> -		} else {
> -			hdr = (struct virtio_net_hdr *)((uintptr_t)buf_vec[0].buf_addr);
> -		}
> +		if (unlikely(copy_vnet_hdr_from_desc(&tmp_hdr, buf_vec, nr_vec) !=
> 0))
> +		    return -1;
> +
> +		/* ensure that compiler does not delay copy */
> +		rte_compiler_barrier();
> +		hdr = &tmp_hdr;
>  	}
> 
>  	for (vec_idx = 0; vec_idx < nr_vec; vec_idx++) {
> @@ -3363,7 +3362,6 @@ virtio_dev_tx_batch_packed(struct virtio_net *dev,
>  {
>  	uint16_t avail_idx = vq->last_avail_idx;
>  	uint32_t buf_offset = sizeof(struct virtio_net_hdr_mrg_rxbuf);
> -	struct virtio_net_hdr *hdr;
>  	uintptr_t desc_addrs[PACKED_BATCH_SIZE];
>  	uint16_t ids[PACKED_BATCH_SIZE];
>  	uint16_t i;
> @@ -3382,8 +3380,12 @@ virtio_dev_tx_batch_packed(struct virtio_net *dev,
> 
>  	if (virtio_net_with_host_offload(dev)) {
>  		vhost_for_each_try_unroll(i, 0, PACKED_BATCH_SIZE) {
> -			hdr = (struct virtio_net_hdr *)(desc_addrs[i]);
> -			vhost_dequeue_offload(dev, hdr, pkts[i], legacy_ol_flags);
> +			struct virtio_net_hdr hdr;
> +
> +			memcpy(&hdr, (void *)desc_addrs[i], sizeof(struct
> virtio_net_hdr));
> +			rte_compiler_barrier();
> +
> +			vhost_dequeue_offload(dev, &hdr, pkts[i], legacy_ol_flags);
>  		}
>  	}
>