[PATCH net 2/2] vsock/virtio: fix empty payload in tap skb for non-linear buffers

[Stefano Garzarella <sgarzare@redhat.com>]

From: Stefano Garzarella <sgarzare@redhat.com>

For non-linear skbs, virtio_transport_build_skb() goes through
virtio_transport_copy_nonlinear_skb() to copy the original payload
in the new skb to be delivered to the vsockmon tap device.
This manually initializes an iov_iter but does not set iov_iter.count.
Since the iov_iter is zero-initialized, the copy length is zero and no
payload is actually copied to the monitor interface, leaving data
un-initialized.

Fix this by removing the linear vs non-linear split and using
skb_copy_datagram_iter() with iov_iter_kvec() for all cases, as
vhost-vsock already does. This handles both linear and non-linear skbs,
properly initializes the iov_iter, and removes the now unused
virtio_transport_copy_nonlinear_skb().

While touching this code, let's also check the return value of
skb_copy_datagram_iter(), even though it's unlikely to fail.

Fixes: 4b0bf10eb077 ("vsock/virtio: non-linear skb handling for tap")
Reported-by: Yiqi Sun <sunyiqixm@gmail.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
 net/vmw_vsock/virtio_transport_common.c | 40 ++++++++-----------------
 1 file changed, 12 insertions(+), 28 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index a678d5d75704..989cc252d3d3 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -136,27 +136,6 @@ static void virtio_transport_init_hdr(struct sk_buff *skb,
 	hdr->fwd_cnt	= cpu_to_le32(0);
 }
 
-static void virtio_transport_copy_nonlinear_skb(const struct sk_buff *skb,
-						void *dst,
-						size_t len)
-{
-	struct iov_iter iov_iter = { 0 };
-	struct kvec kvec;
-	size_t to_copy;
-
-	kvec.iov_base = dst;
-	kvec.iov_len = len;
-
-	iov_iter.iter_type = ITER_KVEC;
-	iov_iter.kvec = &kvec;
-	iov_iter.nr_segs = 1;
-
-	to_copy = min_t(size_t, len, skb->len);
-
-	skb_copy_datagram_iter(skb, VIRTIO_VSOCK_SKB_CB(skb)->offset,
-			       &iov_iter, to_copy);
-}
-
 /* Packet capture */
 static struct sk_buff *virtio_transport_build_skb(void *opaque)
 {
@@ -214,13 +193,18 @@ static struct sk_buff *virtio_transport_build_skb(void *opaque)
 	skb_put_data(skb, pkt_hdr, sizeof(*pkt_hdr));
 
 	if (payload_len) {
-		if (skb_is_nonlinear(pkt)) {
-			void *data = skb_put(skb, payload_len);
-
-			virtio_transport_copy_nonlinear_skb(pkt, data, payload_len);
-		} else {
-			skb_put_data(skb, pkt->data + VIRTIO_VSOCK_SKB_CB(pkt)->offset,
-				     payload_len);
+		struct iov_iter iov_iter;
+		struct kvec kvec;
+		void *data = skb_put(skb, payload_len);
+
+		kvec.iov_base = data;
+		kvec.iov_len = payload_len;
+		iov_iter_kvec(&iov_iter, ITER_DEST, &kvec, 1, payload_len);
+
+		if (skb_copy_datagram_iter(pkt, VIRTIO_VSOCK_SKB_CB(pkt)->offset,
+					   &iov_iter, payload_len)) {
+			kfree_skb(skb);
+			return NULL;
 		}
 	}
 
-- 
2.54.0