virtio_net: Fix mismatched buf address when unmapping for small packets

Currently, the virtio-net driver will perform a pre-dma-mapping for
small or mergeable RX buffer. But for small packets, a mismatched address
without VIRTNET_RX_PAD and xdp_headroom is used for unmapping.

That will result in unsynchronized buffers when SWIOTLB is enabled, for
example, when running as a TDX guest.

This patch unifies the address passed to the virtio core as the address of
the virtnet header and fixes the mismatched buffer address.

Changes from v2: unify the buf that passed to the virtio core in small
and merge mode.
Changes from v1: Use ctx to get xdp_headroom.

Fixes: 295525e ("virtio_net: merge dma operations when filling mergeable buffers")
Signed-off-by: Wenbo Li <liwenbo.martin@bytedance.com>
Signed-off-by: Jiahui Cen <cenjiahui@bytedance.com>
Signed-off-by: Ying Fang <fangying.tommy@bytedance.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Link: https://patch.msgid.link/20240919081351.51772-1-liwenbo.martin@bytedance.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/virtio_net.c

@@ -1807,6 +1807,11 @@ static struct sk_buff *receive_small(struct net_device *dev,
 	struct page *page = virt_to_head_page(buf);
 	struct sk_buff *skb;
 
+	/* We passed the address of virtnet header to virtio-core,
+	 * so truncate the padding.
+	 */
+	buf -= VIRTNET_RX_PAD + xdp_headroom;
+
 	len -= vi->hdr_len;
 	u64_stats_add(&stats->bytes, len);
 
@@ -2422,8 +2427,9 @@ static int add_recvbuf_small(struct virtnet_info *vi, struct receive_queue *rq,
 	if (unlikely(!buf))
 		return -ENOMEM;
 
-	virtnet_rq_init_one_sg(rq, buf + VIRTNET_RX_PAD + xdp_headroom,
-			       vi->hdr_len + GOOD_PACKET_LEN);
+	buf += VIRTNET_RX_PAD + xdp_headroom;
+
+	virtnet_rq_init_one_sg(rq, buf, vi->hdr_len + GOOD_PACKET_LEN);
 
 	err = virtqueue_add_inbuf_ctx(rq->vq, rq->sg, 1, buf, ctx, gfp);
 	if (err < 0) {