Re: [PATCH net v3] vsock/virtio: cap TX credit to local buffer size

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Dec 11, 2025 at 01:51:04PM +0100, Melbin K Mathew wrote:
> The virtio vsock transport currently derives its TX credit directly from
> peer_buf_alloc, which is populated from the remote endpoint's
> SO_VM_SOCKETS_BUFFER_SIZE value.
> 
> On the host side, this means the amount of data we are willing to queue
> for a given connection is scaled purely by a peer-chosen value, rather
> than by the host's own vsock buffer configuration. A guest that
> advertises a very large buffer and reads slowly can cause the host to
> allocate a correspondingly large amount of sk_buff memory for that
> connection.
> 
> In practice, a malicious guest can:
> 
>   - set a large AF_VSOCK buffer size (e.g. 2 GiB) with
>     SO_VM_SOCKETS_BUFFER_MAX_SIZE / SO_VM_SOCKETS_BUFFER_SIZE, and
> 
>   - open multiple connections to a host vsock service that sends data
>     while the guest drains slowly.
> 
> On an unconstrained host this can drive Slab/SUnreclaim into the tens of
> GiB range, causing allocation failures and OOM kills in unrelated host
> processes while the offending VM remains running.
> 
> On non-virtio transports and compatibility:
> 
>   - VMCI uses the AF_VSOCK buffer knobs to size its queue pairs per
>     socket based on the local vsk->buffer_* values; the remote side
>     can’t enlarge those queues beyond what the local endpoint
>     configured.
> 
>   - Hyper-V’s vsock transport uses fixed-size VMBus ring buffers and
>     an MTU bound; there is no peer-controlled credit field comparable
>     to peer_buf_alloc, and the remote endpoint can’t drive in-flight
>     kernel memory above those ring sizes.
> 
>   - The loopback path reuses virtio_transport_common.c, so it
>     naturally follows the same semantics as the virtio transport.
> 
> Make virtio-vsock consistent with that model by intersecting the peer’s
> advertised receive window with the local vsock buffer size when
> computing TX credit. We introduce a small helper and use it in
> virtio_transport_get_credit(), virtio_transport_has_space() and
> virtio_transport_seqpacket_enqueue(), so that:
> 
>     effective_tx_window = min(peer_buf_alloc, buf_alloc)
> 
> This prevents a remote endpoint from forcing us to queue more data than
> our own configuration allows, while preserving the existing credit
> semantics and keeping virtio-vsock compatible with the other transports.
> 
> On an unpatched Ubuntu 22.04 host (~64 GiB RAM), running a PoC with
> 32 guest vsock connections advertising 2 GiB each and reading slowly
> drove Slab/SUnreclaim from ~0.5 GiB to ~57 GiB and the system only
> recovered after killing the QEMU process.
> 
> With this patch applied, rerunning the same PoC yields:
> 
>   Before:
>     MemFree:        ~61.6 GiB
>     MemAvailable:   ~62.3 GiB
>     Slab:           ~142 MiB
>     SUnreclaim:     ~117 MiB
> 
>   After 32 high-credit connections:
>     MemFree:        ~61.5 GiB
>     MemAvailable:   ~62.3 GiB
>     Slab:           ~178 MiB
>     SUnreclaim:     ~152 MiB
> 
> i.e. only ~35 MiB increase in Slab/SUnreclaim, no host OOM, and the
> guest remains responsive.
> 
> Fixes: 06a8fc78367d ("VSOCK: Introduce virtio_vsock_common.ko")
> Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
> Signed-off-by: Melbin K Mathew <mlbnkm1@gmail.com>
> ---
>  net/vmw_vsock/virtio_transport_common.c | 27 ++++++++++++++++++++++---
>  1 file changed, 24 insertions(+), 3 deletions(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index dcc8a1d58..02eeb96dd 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -491,6 +491,25 @@ void virtio_transport_consume_skb_sent(struct sk_buff *skb, bool consume)
>  }
>  EXPORT_SYMBOL_GPL(virtio_transport_consume_skb_sent);
>  
> +/* Return the effective peer buffer size for TX credit computation.
> + *
> + * The peer advertises its receive buffer via peer_buf_alloc, but we
> + * cap that to our local buf_alloc (derived from
> + * SO_VM_SOCKETS_BUFFER_SIZE and already clamped to buffer_max_size)
> + * so that a remote endpoint cannot force us to queue more data than
> + * our own configuration allows.
> + */
> +static u32 virtio_transport_tx_buf_alloc(struct virtio_vsock_sock *vvs)
> +{
> +	return min(vvs->peer_buf_alloc, vvs->buf_alloc);
> +}
> +
>  u32 virtio_transport_get_credit(struct virtio_vsock_sock *vvs, u32 credit)
>  {
>  	u32 ret;
> @@ -499,7 +518,8 @@ u32 virtio_transport_get_credit(struct virtio_vsock_sock *vvs, u32 credit)
>  		return 0;
>  
>  	spin_lock_bh(&vvs->tx_lock);
> -	ret = vvs->peer_buf_alloc - (vvs->tx_cnt - vvs->peer_fwd_cnt);
> +	ret = virtio_transport_tx_buf_alloc(vvs) -
> +		(vvs->tx_cnt - vvs->peer_fwd_cnt);
>  	if (ret > credit)
>  		ret = credit;
>  	vvs->tx_cnt += ret;
> @@ -831,7 +851,7 @@ virtio_transport_seqpacket_enqueue(struct vsock_sock *vsk,
>  
>  	spin_lock_bh(&vvs->tx_lock);
>  
> -	if (len > vvs->peer_buf_alloc) {
> +	if (len > virtio_transport_tx_buf_alloc(vvs)) {
>  		spin_unlock_bh(&vvs->tx_lock);
>  		return -EMSGSIZE;
>  	}
> @@ -882,7 +902,8 @@ static s64 virtio_transport_has_space(struct vsock_sock *vsk)
>  	struct virtio_vsock_sock *vvs = vsk->trans;
>  	s64 bytes;
>  
> -	bytes = (s64)vvs->peer_buf_alloc - (vvs->tx_cnt - vvs->peer_fwd_cnt);
> +	bytes = (s64)virtio_transport_tx_buf_alloc(vvs) -
> +		(vvs->tx_cnt - vvs->peer_fwd_cnt);
>  	if (bytes < 0)
>  		bytes = 0;
>  

Acked-by: Michael S. Tsirkin <mst@redhat.com>


Looking at this, why is one place casting to s64 the other is not?




> -- 
> 2.34.1