[PATCH net v1] virtio_net: Introduce skb_vnet_common_hdr to avoid typecasting

[Feng Liu <feliu@nvidia.com>]

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 virtio_net_hdr_mrg_rxbuf.

Signed-off-by: Feng Liu <feliu@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
---
 drivers/net/virtio_net.c        | 29 ++++++++++++++++-------------
 include/uapi/linux/virtio_net.h |  7 +++++++
 2 files changed, 23 insertions(+), 13 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 1270c8d23463..6ce0fbcabda9 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -344,9 +344,10 @@ 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 +470,7 @@ 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 +555,7 @@ 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 +967,7 @@ 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 +1578,8 @@ 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 *common_hdr;
+	struct virtio_net_hdr_mrg_rxbuf	*mrg_hdr;
 
 	if (unlikely(len < vi->hdr_len + ETH_HLEN)) {
 		pr_debug("%s: short packet %i\n", dev->name, len);
@@ -1597,18 +1599,19 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
 	if (unlikely(!skb))
 		return;
 
-	hdr = skb_vnet_hdr(skb);
+	common_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(&common_hdr->hash_v1_hdr, skb);
 
-	if (hdr->hdr.flags & VIRTIO_NET_HDR_F_DATA_VALID)
+	mrg_hdr = &common_hdr->mrg_hdr;
+	if (mrg_hdr->hdr.flags & VIRTIO_NET_HDR_F_DATA_VALID)
 		skb->ip_summed = CHECKSUM_UNNECESSARY;
 
-	if (virtio_net_hdr_to_skb(skb, &hdr->hdr,
+	if (virtio_net_hdr_to_skb(skb, &mrg_hdr->hdr,
 				  virtio_is_little_endian(vi->vdev))) {
 		net_warn_ratelimited("%s: bad gso: type: %u, size: %u\n",
-				     dev->name, hdr->hdr.gso_type,
-				     hdr->hdr.gso_size);
+				     dev->name, mrg_hdr->hdr.gso_type,
+				     mrg_hdr->hdr.gso_size);
 		goto frame_err;
 	}
 
@@ -2105,7 +2108,7 @@ 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,
diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
index 12c1c9699935..db40f93ae8b3 100644
--- a/include/uapi/linux/virtio_net.h
+++ b/include/uapi/linux/virtio_net.h
@@ -201,6 +201,13 @@ struct virtio_net_hdr_mrg_rxbuf {
 	struct virtio_net_hdr hdr;
 	__virtio16 num_buffers;	/* Number of merged rx buffers */
 };
+
+struct virtio_net_common_hdr {
+	union {
+		struct virtio_net_hdr_mrg_rxbuf	mrg_hdr;
+		struct virtio_net_hdr_v1_hash hash_v1_hdr;
+	};
+};
 #endif /* ...VIRTIO_NET_NO_LEGACY */
 
 /*
-- 
2.37.1 (Apple Git-137.1)