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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 8 May 2026 05:53:07 -0400

On Fri, May 08, 2026 at 11:23:30AM +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).
> 
> This also causes 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 introducing virtio_transport_rx_buf_size() to calculate the
> size of the RX buffer based on the overhead. Using it in the acceptance
> check, the advertised buf_alloc, and the credit update decision.
> Use buf_alloc * 2 as total budget (payload + overhead), similar to how
> SO_RCVBUF is doubled to reserve space for sk_buff metadata.
> The function returns buf_alloc as long as overhead fits within the
> reservation, then gradually reduces toward 0 as overhead exceeds
> buf_alloc (e.g. under small-packet flooding), informing the peer to
> slow down.
> 
> Fixes: 059b7dbd20a6 ("vsock/virtio: fix potential unbounded skb queue")
> Signed-off-by: Stefano Garzarella <[email-masked]>

unfortunately, this is a bit of a spec violation and there is no guarantee
it helps.

a spec violation because the spec says:
Only payload bytes are counted and header bytes are not
included

and the implication is that a side can not reduce its own buf_alloc.

no guarantee because the other side is not required to process your
packets, so it might not see your buf alloc reduction.

as designed in the current spec, you can only increase your buf alloc,
not decrease it.

what can be done:
- more efficient storage for small packets (poc i posted)
- reduce buf alloc ahead of the time

> ---
>  net/vmw_vsock/virtio_transport_common.c | 31 +++++++++++++++++++++----
>  1 file changed, 27 insertions(+), 4 deletions(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index 9b8014516f4f..94a4beb8fd61 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -444,12 +444,32 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>  	return ret;
>  }
>  
> +/* vvs->rx_lock held by the caller */
> +static u32 virtio_transport_rx_buf_size(struct virtio_vsock_sock *vvs)
> +{
> +	u64 skb_overhead = (skb_queue_len(&vvs->rx_queue) + 1) * SKB_TRUESIZE(0);
> +	/* Use buf_alloc * 2 as total budget (payload + overhead), similar to
> +	 * how SO_RCVBUF is doubled to reserve space for sk_buff metadata.
> +	 */
> +	u64 total_budget = (u64)vvs->buf_alloc * 2;
> +
> +	/* Overhead within buf_alloc: full buf_alloc available for payload */
> +	if (skb_overhead < vvs->buf_alloc)
> +		return vvs->buf_alloc;
> +
> +	/* Overhead exceeded buf_alloc: gradually reduce to bound skb queue */
> +	if (skb_overhead < total_budget)
> +		return total_budget - skb_overhead;
> +
> +	return 0;
> +}
> +
>  static bool virtio_transport_inc_rx_pkt(struct virtio_vsock_sock *vvs,
>  					u32 len)
>  {
> -	u64 skb_overhead = (skb_queue_len(&vvs->rx_queue) + 1) * SKB_TRUESIZE(0);
> +	u32 rx_buf_size = virtio_transport_rx_buf_size(vvs);
>  
> -	if (skb_overhead + vvs->buf_used + len > vvs->buf_alloc)
> +	if (!rx_buf_size || vvs->buf_used + len > rx_buf_size)
>  		return false;
>  
>  	vvs->rx_bytes += len;
> @@ -472,7 +492,7 @@ void virtio_transport_inc_tx_pkt(struct virtio_vsock_sock *vvs, struct sk_buff *
>  	spin_lock_bh(&vvs->rx_lock);
>  	vvs->last_fwd_cnt = vvs->fwd_cnt;
>  	hdr->fwd_cnt = cpu_to_le32(vvs->fwd_cnt);
> -	hdr->buf_alloc = cpu_to_le32(vvs->buf_alloc);
> +	hdr->buf_alloc = cpu_to_le32(virtio_transport_rx_buf_size(vvs));
>  	spin_unlock_bh(&vvs->rx_lock);
>  }
>  EXPORT_SYMBOL_GPL(virtio_transport_inc_tx_pkt);
> @@ -594,6 +614,7 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>  	bool low_rx_bytes;
>  	int err = -EFAULT;
>  	size_t total = 0;
> +	u32 rx_buf_size;
>  	u32 free_space;
>  
>  	spin_lock_bh(&vvs->rx_lock);
> @@ -639,7 +660,9 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>  	}
>  
>  	fwd_cnt_delta = vvs->fwd_cnt - vvs->last_fwd_cnt;
> -	free_space = vvs->buf_alloc - fwd_cnt_delta;
> +	rx_buf_size = virtio_transport_rx_buf_size(vvs);
> +	free_space = rx_buf_size > fwd_cnt_delta ?
> +		     rx_buf_size - fwd_cnt_delta : 0;
>  	low_rx_bytes = (vvs->rx_bytes <
>  			sock_rcvlowat(sk_vsock(vsk), 0, INT_MAX));
>  
> -- 
> 2.54.0