[PATCH v3] virtio_net: Fix mismatched buf address when unmapping for small packets

[Wenbo Li]

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 into 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: 295525e29a5b ("virtio_net: merge dma operations when filling mergeable 
buffers")
Signed-off-by: Wenbo Li <liwenbo.mar...@bytedance.com>
Signed-off-by: Jiahui Cen <cenjia...@bytedance.com>
Signed-off-by: Ying Fang <fangying.to...@bytedance.com>
---
 drivers/net/virtio_net.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 6f4781ec2b36..9446666c84aa 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1804,9 +1804,15 @@ static struct sk_buff *receive_small(struct net_device 
*dev,
                                     struct virtnet_rq_stats *stats)
 {
        unsigned int xdp_headroom = (unsigned long)ctx;
-       struct page *page = virt_to_head_page(buf);
+       struct page *page;
        struct sk_buff *skb;
 
+       // We passed the address of virtnet header to virtio-core,
+       // so truncate the padding.
+       buf -= VIRTNET_RX_PAD + xdp_headroom;
+
+       page = virt_to_head_page(buf);
+
        len -= vi->hdr_len;
        u64_stats_add(&stats->bytes, len);
 
@@ -2422,8 +2428,9 @@ 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) {
-- 
2.20.1