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 <[email protected]>
> Signed-off-by: Melbin K Mathew <[email protected]>
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
