Re: [PATCH 1/2] vsock/virtio: Coalesce only linear skb

[Stefano Garzarella <sgarzare@redhat.com>]

On Thu, Jan 08, 2026 at 10:54:54AM +0100, Michal Luczaj wrote:
>Vsock/virtio common tries to coalesce buffers in rx queue: if a linear skb
>(with a spare tail room) is followed by a small skb (length limited by
>GOOD_COPY_LEN = 128), an attempt is made to join them.
>
>Since the introduction of MSG_ZEROCOPY support, assumption that a small skb
>will always be linear is incorrect (see loopback transport). In the
>zerocopy case, data is lost and the linear skb is appended with
>uninitialized kernel memory.
>
>Ensure only linear skbs are coalesced. Note that skb_tailroom(last_skb) > 0
>guarantees last_skb is linear.
>
>Fixes: 581512a6dc93 ("vsock/virtio: MSG_ZEROCOPY flag support")
>Signed-off-by: Michal Luczaj <mhal@rbox.co>
>---
> net/vmw_vsock/virtio_transport_common.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
>diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>index dcc8a1d5851e..cf35eb7190cc 100644
>--- a/net/vmw_vsock/virtio_transport_common.c
>+++ b/net/vmw_vsock/virtio_transport_common.c
>@@ -1375,7 +1375,8 @@ virtio_transport_recv_enqueue(struct vsock_sock *vsk,
> 		 * of a new message.
> 		 */
> 		if (skb->len < skb_tailroom(last_skb) &&
>-		    !(le32_to_cpu(last_hdr->flags) & VIRTIO_VSOCK_SEQ_EOM)) {
>+		    !(le32_to_cpu(last_hdr->flags) & VIRTIO_VSOCK_SEQ_EOM) &&
>+		    !skb_is_nonlinear(skb)) {

Why here? I mean we can do the check even early, something like this:

--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -1361,7 +1361,8 @@ virtio_transport_recv_enqueue(struct vsock_sock *vsk,
          * to avoid wasting memory queueing the entire buffer with a small
          * payload.
          */
-       if (len <= GOOD_COPY_LEN && !skb_queue_empty(&vvs->rx_queue)) {
+       if (len <= GOOD_COPY_LEN && !skb_queue_empty(&vvs->rx_queue) &&
+           !skb_is_nonlinear(skb)) {
                 struct virtio_vsock_hdr *last_hdr;
                 struct sk_buff *last_skb;


I would also add the reason in the comment before that to make it clear.

Thanks for the fix!
Stefano

> 			memcpy(skb_put(last_skb, skb->len), skb->data, skb->len);
> 			free_pkt = true;
> 			last_hdr->flags |= hdr->flags;
>
>-- 
>2.52.0
>