Subject: Re: [PATCH net v2] vsock/virtio: fix skb overhead accounting to preserve full buf_alloc

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 12 May 2026 04:54:34 -0400

On Tue, May 12, 2026 at 10:07:37AM +0200, Stefano Garzarella wrote:
> From: Stefano Garzarella <[email-masked]>
> 
> After commit 059b7dbd20a6 ("vsock/virtio: fix potential unbounded skb
> queue"), virtio_transport_inc_rx_pkt() subtracts per-skb overhead from
> buf_alloc when checking whether a new packet fits. This reduces the
> effective receive buffer below what the user configured via
> SO_VM_SOCKETS_BUFFER_SIZE, causing legitimate data packets to be
> silently dropped and applications that rely on the full buffer size
> to deadlock.
> 
> Also, the reduced space is not communicated to the remote peer, so
> its credit calculation accounts more credit than the receiver will
> actually accept, causing data loss (there is no retransmission).
> 
> With this approach we currently have failures in
> tools/testing/vsock/vsock_test.c. Test 18 sometimes fails, while
> test 22 always fails in this way:
>     18 - SOCK_STREAM MSG_ZEROCOPY...hash mismatch
> 
>     22 - SOCK_STREAM virtio credit update + SO_RCVLOWAT...send failed:
>     Resource temporarily unavailable
> 
> Fix this by using `buf_alloc * 2` as the total budget for payload plus
> skb overhead in virtio_transport_inc_rx_pkt(), similar to how SO_RCVBUF
> is doubled to reserve space for sk_buff metadata. This preserves the
> full buf_alloc for payload under normal operation, while still bounding
> the skb queue growth.
> 
> When the total budget (buf_alloc * 2) is exceeded (e.g. under small-packet
> flooding where overhead dominates), the connection is reset and local
> socket error set to ENOBUFS, so both peers are explicitly notified of
> the failure rather than silently losing data.
> 
> With this patch, all tests in tools/testing/vsock/vsock_test.c are
> now passing again.
> 
> A solution to handle small-packet overhead efficiently also for
> SEQPACKET (we already do that for STREAM) is planned as follow-up work.
> This patch is needed in any case to prevent silent data loss, because
> even if we reduce the overhead, we can't eliminate it entirely.
> 
> Fixes: 059b7dbd20a6 ("vsock/virtio: fix potential unbounded skb queue")
> Signed-off-by: Stefano Garzarella <[email-masked]>

Thanks for the patch!  I'd like to split this: 
1. buf alloc boost
2. reset when out of credits

this way we can revert 2 easier later.

> ---
> v2:
> - Close the connection when we can no longer queue new packets instead
>   of losing data.
> - No longer announce the reduced buf_alloc to avoid violating the
>   spec. [MST]
> 
> v1: https://lore.kernel.org/netdev/[email-masked]/
> ---
>  net/vmw_vsock/virtio_transport_common.c | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index 9b8014516f4f..f23bf8a11319 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -449,7 +449,10 @@ static bool virtio_transport_inc_rx_pkt(struct virtio_vsock_sock *vvs,
>  {
>  	u64 skb_overhead = (skb_queue_len(&vvs->rx_queue) + 1) * SKB_TRUESIZE(0);
>  
> -	if (skb_overhead + vvs->buf_used + len > vvs->buf_alloc)
> +	/* Use buf_alloc * 2 as total budget (payload + overhead), similar to
> +	 * how SO_RCVBUF is doubled to reserve space for sk_buff metadata.
> +	 */
> +	if (skb_overhead + vvs->buf_used + len > (u64)vvs->buf_alloc * 2)
>  		return false;
>  
>  	vvs->rx_bytes += len;
> @@ -1365,7 +1368,7 @@ virtio_transport_recv_connecting(struct sock *sk,
>  	return err;
>  }
>  
> -static void
> +static bool
>  virtio_transport_recv_enqueue(struct vsock_sock *vsk,
>  			      struct sk_buff *skb)
>  {
> @@ -1380,10 +1383,8 @@ virtio_transport_recv_enqueue(struct vsock_sock *vsk,
>  	spin_lock_bh(&vvs->rx_lock);
>  
>  	can_enqueue = virtio_transport_inc_rx_pkt(vvs, len);
> -	if (!can_enqueue) {
> -		free_pkt = true;
> +	if (!can_enqueue)
>  		goto out;
> -	}
>  
>  	if (le32_to_cpu(hdr->flags) & VIRTIO_VSOCK_SEQ_EOM)
>  		vvs->msg_count++;
> @@ -1423,6 +1424,8 @@ virtio_transport_recv_enqueue(struct vsock_sock *vsk,
>  	spin_unlock_bh(&vvs->rx_lock);
>  	if (free_pkt)
>  		kfree_skb(skb);
> +
> +	return can_enqueue;
>  }
>  
>  static int
> @@ -1435,7 +1438,16 @@ virtio_transport_recv_connected(struct sock *sk,
>  
>  	switch (le16_to_cpu(hdr->op)) {
>  	case VIRTIO_VSOCK_OP_RW:
> -		virtio_transport_recv_enqueue(vsk, skb);
> +		if (!virtio_transport_recv_enqueue(vsk, skb)) {
> +			/* There is no more space to queue the packet, so let's
> +			 * close the connection; otherwise, we'll lose data.
> +			 */
> +			(void)virtio_transport_reset(vsk, skb);
> +			sk->sk_state = TCP_CLOSE;
> +			sk->sk_err = ENOBUFS;
> +			sk_error_report(sk);
> +			break;
> +		}
>  		vsock_data_ready(sk);
>  		return err;
>  	case VIRTIO_VSOCK_OP_CREDIT_REQUEST:
> -- 
> 2.54.0