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

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

thanks for the patch! yet something to improve:

On Wed, Dec 10, 2025 at 02:32:59PM +0100, Melbin K Mathew wrote:
> The virtio vsock transport currently derives its TX credit directly
> from peer_buf_alloc, which is set from the remote endpoint's
> SO_VM_SOCKETS_BUFFER_SIZE value.
> 
> On the host side this means that the amount of data we are willing to
> queue for a connection is scaled by a guest-chosen buffer size,
> rather than the host's own vsock configuration. A malicious guest can
> advertise a large buffer and read slowly, causing the host to allocate
> a correspondingly large amount of sk_buff memory.
> 
> Introduce a small helper, virtio_transport_peer_buf_alloc(), that
> returns min(peer_buf_alloc, buf_alloc), and use it wherever we consume
> peer_buf_alloc:
> 
>   - virtio_transport_get_credit()
>   - virtio_transport_has_space()
>   - virtio_transport_seqpacket_enqueue()
> 
> This ensures the effective TX window is bounded by both the peer's
> advertised buffer and our own buf_alloc (already clamped to
> buffer_max_size via SO_VM_SOCKETS_BUFFER_MAX_SIZE), so a remote guest
> cannot force the host to queue more data than allowed by the host's
> own vsock settings.
> 
> 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.
> 

what is missing here, is how do non-virtio transports behave?
because I think we want transports to be compatible.

> Fixes: d021c344051a ("VSOCK: Introduce VM Sockets")

that commit does not even include the patched file.
how can it be the right commit to fix?

> Reported-by: Melbin K Mathew <mlbnkm1@gmail.com>
> Signed-off-by: Melbin K Mathew <mlbnkm1@gmail.com>


this is the fix suggested by Stefano, right?
maybe mention this.

> ---
>  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..f5afedf01 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_peer_buf_alloc(struct virtio_vsock_sock *vvs)
> +{
> +	u32 peer  = vvs->peer_buf_alloc;
> +	u32 local = vvs->buf_alloc;
> +
> +	if (peer > local)
> +		return local;

> +	return peer;

is this just
 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_peer_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_peer_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_peer_buf_alloc(vvs) -
> +               (vvs->tx_cnt - vvs->peer_fwd_cnt);
>  	if (bytes < 0)
>  		bytes = 0;
>  
> -- 
> 2.34.1