Re: [PATCH net-next v3] virtio_net: Introduce skb_vnet_common_hdr to avoid typecasting

[Jason Wang <jasowang@redhat.com>]

On Mon, Aug 21, 2023 at 10:27 PM Feng Liu <feliu@nvidia.com> wrote:
>
> The virtio_net driver currently deals with different versions and types
> of virtio net headers, such as virtio_net_hdr_mrg_rxbuf,
> virtio_net_hdr_v1_hash, etc. Due to these variations, the code relies
> on multiple type casts to convert memory between different structures,
> potentially leading to bugs when there are changes in these structures.
>
> Introduces the "struct skb_vnet_common_hdr" as a unifying header
> structure using a union. With this approach, various virtio net header
> structures can be converted by accessing different members of this
> structure, thus eliminating the need for type casting and reducing the
> risk of potential bugs.
>
> For example following code:
> static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>                 struct receive_queue *rq,
>                 struct page *page, unsigned int offset,
>                 unsigned int len, unsigned int truesize,
>                 unsigned int headroom)
> {
> [...]
>         struct virtio_net_hdr_mrg_rxbuf *hdr;
> [...]
>         hdr_len = vi->hdr_len;
> [...]
> ok:
>         hdr = skb_vnet_hdr(skb);
>         memcpy(hdr, hdr_p, hdr_len);
> [...]
> }
>
> When VIRTIO_NET_F_HASH_REPORT feature is enabled, hdr_len = 20
> But the sizeof(*hdr) is 12,
> memcpy(hdr, hdr_p, hdr_len); will copy 20 bytes to the hdr,
> which make a potential risk of bug. And this risk can be avoided by
> introducing struct skb_vnet_common_hdr.
>
> Change log
> v1->v2
> feedback from Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> feedback from Simon Horman <horms@kernel.org>
> 1. change to use net-next tree.
> 2. move skb_vnet_common_hdr inside kernel file instead of the UAPI header.
>
> v2->v3
> feedback from Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> 1. fix typo in commit message.
> 2. add original struct virtio_net_hdr into union
> 3. remove virtio_net_hdr_mrg_rxbuf variable in receive_buf;
>
> Signed-off-by: Feng Liu <feliu@nvidia.com>
> Reviewed-by: Jiri Pirko <jiri@nvidia.com>

Acked-by: Jason Wang <jasowang@redhat.com>

Thanks

> ---
>  drivers/net/virtio_net.c | 27 ++++++++++++++++++---------
>  1 file changed, 18 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 8e9f4cfe941f..8c74bc8cfe68 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -303,6 +303,14 @@ struct padded_vnet_hdr {
>         char padding[12];
>  };
>
> +struct virtio_net_common_hdr {
> +       union {
> +               struct virtio_net_hdr hdr;
> +               struct virtio_net_hdr_mrg_rxbuf mrg_hdr;
> +               struct virtio_net_hdr_v1_hash hash_v1_hdr;
> +       };
> +};
> +
>  static void virtnet_rq_free_unused_buf(struct virtqueue *vq, void *buf);
>  static void virtnet_sq_free_unused_buf(struct virtqueue *vq, void *buf);
>
> @@ -344,9 +352,10 @@ static int rxq2vq(int rxq)
>         return rxq * 2;
>  }
>
> -static inline struct virtio_net_hdr_mrg_rxbuf *skb_vnet_hdr(struct sk_buff *skb)
> +static inline struct virtio_net_common_hdr *
> +skb_vnet_common_hdr(struct sk_buff *skb)
>  {
> -       return (struct virtio_net_hdr_mrg_rxbuf *)skb->cb;
> +       return (struct virtio_net_common_hdr *)skb->cb;
>  }
>
>  /*
> @@ -469,7 +478,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>                                    unsigned int headroom)
>  {
>         struct sk_buff *skb;
> -       struct virtio_net_hdr_mrg_rxbuf *hdr;
> +       struct virtio_net_common_hdr *hdr;
>         unsigned int copy, hdr_len, hdr_padded_len;
>         struct page *page_to_free = NULL;
>         int tailroom, shinfo_size;
> @@ -554,7 +563,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>                 give_pages(rq, page);
>
>  ok:
> -       hdr = skb_vnet_hdr(skb);
> +       hdr = skb_vnet_common_hdr(skb);
>         memcpy(hdr, hdr_p, hdr_len);
>         if (page_to_free)
>                 put_page(page_to_free);
> @@ -966,7 +975,7 @@ static struct sk_buff *receive_small_build_skb(struct virtnet_info *vi,
>                 return NULL;
>
>         buf += header_offset;
> -       memcpy(skb_vnet_hdr(skb), buf, vi->hdr_len);
> +       memcpy(skb_vnet_common_hdr(skb), buf, vi->hdr_len);
>
>         return skb;
>  }
> @@ -1577,7 +1586,7 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
>  {
>         struct net_device *dev = vi->dev;
>         struct sk_buff *skb;
> -       struct virtio_net_hdr_mrg_rxbuf *hdr;
> +       struct virtio_net_common_hdr *hdr;
>
>         if (unlikely(len < vi->hdr_len + ETH_HLEN)) {
>                 pr_debug("%s: short packet %i\n", dev->name, len);
> @@ -1597,9 +1606,9 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
>         if (unlikely(!skb))
>                 return;
>
> -       hdr = skb_vnet_hdr(skb);
> +       hdr = skb_vnet_common_hdr(skb);
>         if (dev->features & NETIF_F_RXHASH && vi->has_rss_hash_report)
> -               virtio_skb_set_hash((const struct virtio_net_hdr_v1_hash *)hdr, skb);
> +               virtio_skb_set_hash(&hdr->hash_v1_hdr, skb);
>
>         if (hdr->hdr.flags & VIRTIO_NET_HDR_F_DATA_VALID)
>                 skb->ip_summed = CHECKSUM_UNNECESSARY;
> @@ -2105,7 +2114,7 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
>         if (can_push)
>                 hdr = (struct virtio_net_hdr_mrg_rxbuf *)(skb->data - hdr_len);
>         else
> -               hdr = skb_vnet_hdr(skb);
> +               hdr = &skb_vnet_common_hdr(skb)->mrg_hdr;
>
>         if (virtio_net_hdr_from_skb(skb, &hdr->hdr,
>                                     virtio_is_little_endian(vi->vdev), false,
> --
> 2.37.1 (Apple Git-137.1)
>