[PATCH v2 1/3] virtio_net: Preserve virtio header before XDP program execution

[Liang Chen <liangchen.linux@gmail.com>]

The xdp program may overwrite the inline virtio header. To ensure the
integrity of the virtio header, it is saved in a data structure that
wraps both the xdp_buff and the header before running the xdp program.

Signed-off-by: Liang Chen <liangchen.linux@gmail.com>
---
 drivers/net/virtio_net.c | 43 +++++++++++++++++++++++++++++-----------
 1 file changed, 31 insertions(+), 12 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index d7ce4a1011ea..b56828804e5f 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -349,6 +349,11 @@ struct virtio_net_common_hdr {
 	};
 };
 
+struct virtnet_xdp_buff {
+	struct xdp_buff xdp;
+	struct virtio_net_common_hdr hdr;
+};
+
 static void virtnet_sq_free_unused_buf(struct virtqueue *vq, void *buf);
 
 static bool is_xdp_frame(void *ptr)
@@ -1199,9 +1204,10 @@ static struct sk_buff *receive_small_xdp(struct net_device *dev,
 	unsigned int headroom = vi->hdr_len + header_offset;
 	struct virtio_net_hdr_mrg_rxbuf *hdr = buf + header_offset;
 	struct page *page = virt_to_head_page(buf);
+	struct virtnet_xdp_buff virtnet_xdp;
 	struct page *xdp_page;
+	struct xdp_buff *xdp;
 	unsigned int buflen;
-	struct xdp_buff xdp;
 	struct sk_buff *skb;
 	unsigned int metasize = 0;
 	u32 act;
@@ -1233,17 +1239,23 @@ static struct sk_buff *receive_small_xdp(struct net_device *dev,
 		page = xdp_page;
 	}
 
-	xdp_init_buff(&xdp, buflen, &rq->xdp_rxq);
-	xdp_prepare_buff(&xdp, buf + VIRTNET_RX_PAD + vi->hdr_len,
+	xdp = &virtnet_xdp.xdp;
+	xdp_init_buff(xdp, buflen, &rq->xdp_rxq);
+	xdp_prepare_buff(xdp, buf + VIRTNET_RX_PAD + vi->hdr_len,
 			 xdp_headroom, len, true);
 
-	act = virtnet_xdp_handler(xdp_prog, &xdp, dev, xdp_xmit, stats);
+	/* Copy out the virtio header, as it may be overwritten by the
+	 * xdp program.
+	 */
+	memcpy(&virtnet_xdp.hdr, hdr, vi->hdr_len);
+
+	act = virtnet_xdp_handler(xdp_prog, xdp, dev, xdp_xmit, stats);
 
 	switch (act) {
 	case XDP_PASS:
 		/* Recalculate length in case bpf program changed it */
-		len = xdp.data_end - xdp.data;
-		metasize = xdp.data - xdp.data_meta;
+		len = xdp->data_end - xdp->data;
+		metasize = xdp->data - xdp->data_meta;
 		break;
 
 	case XDP_TX:
@@ -1254,7 +1266,7 @@ static struct sk_buff *receive_small_xdp(struct net_device *dev,
 		goto err_xdp;
 	}
 
-	skb = virtnet_build_skb(buf, buflen, xdp.data - buf, len);
+	skb = virtnet_build_skb(buf, buflen, xdp->data - buf, len);
 	if (unlikely(!skb))
 		goto err;
 
@@ -1591,10 +1603,11 @@ static struct sk_buff *receive_mergeable_xdp(struct net_device *dev,
 	int num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers);
 	struct page *page = virt_to_head_page(buf);
 	int offset = buf - page_address(page);
+	struct virtnet_xdp_buff virtnet_xdp;
 	unsigned int xdp_frags_truesz = 0;
 	struct sk_buff *head_skb;
 	unsigned int frame_sz;
-	struct xdp_buff xdp;
+	struct xdp_buff *xdp;
 	void *data;
 	u32 act;
 	int err;
@@ -1604,16 +1617,22 @@ static struct sk_buff *receive_mergeable_xdp(struct net_device *dev,
 	if (unlikely(!data))
 		goto err_xdp;
 
-	err = virtnet_build_xdp_buff_mrg(dev, vi, rq, &xdp, data, len, frame_sz,
+	xdp = &virtnet_xdp.xdp;
+	err = virtnet_build_xdp_buff_mrg(dev, vi, rq, xdp, data, len, frame_sz,
 					 &num_buf, &xdp_frags_truesz, stats);
 	if (unlikely(err))
 		goto err_xdp;
 
-	act = virtnet_xdp_handler(xdp_prog, &xdp, dev, xdp_xmit, stats);
+	/* Copy out the virtio header, as it may be overwritten by the
+	 * xdp program.
+	 */
+	memcpy(&virtnet_xdp.hdr, hdr, vi->hdr_len);
+
+	act = virtnet_xdp_handler(xdp_prog, xdp, dev, xdp_xmit, stats);
 
 	switch (act) {
 	case XDP_PASS:
-		head_skb = build_skb_from_xdp_buff(dev, vi, &xdp, xdp_frags_truesz);
+		head_skb = build_skb_from_xdp_buff(dev, vi, xdp, xdp_frags_truesz);
 		if (unlikely(!head_skb))
 			break;
 		return head_skb;
@@ -1626,7 +1645,7 @@ static struct sk_buff *receive_mergeable_xdp(struct net_device *dev,
 		break;
 	}
 
-	put_xdp_frags(&xdp);
+	put_xdp_frags(xdp);
 
 err_xdp:
 	put_page(page);
-- 
2.40.1