Re: [PATCH net v6 3/4] vsock/virtio: cap TX credit to local buffer size

[Luigi Leonardi <leonardi@redhat.com>]

On Wed, Jan 21, 2026 at 10:36:27AM +0100, Stefano Garzarella wrote:
>From: Melbin K Mathew <mlbnkm1@gmail.com>
>
>The virtio transports 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.
>The same thing would happen in the guest with a malicious host, since
>virtio transports share the same code base.
>
>Introduce a small helper, virtio_transport_tx_buf_size(), that
>returns min(peer_buf_alloc, buf_alloc), and use it wherever we consume
>peer_buf_alloc.
>
>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 peer
>cannot force the other to queue more data than allowed by its 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; the system only
>recovered after killing the QEMU process. That said, if QEMU memory is
>limited with cgroups, the maximum memory used will be limited.
>
>With this patch applied:
>
>  Before:
>    MemFree:        ~61.6 GiB
>    Slab:           ~142 MiB
>    SUnreclaim:     ~117 MiB
>
>  After 32 high-credit connections:
>    MemFree:        ~61.5 GiB
>    Slab:           ~178 MiB
>    SUnreclaim:     ~152 MiB
>
>Only ~35 MiB increase in Slab/SUnreclaim, no host OOM, and the guest
>remains responsive.
>
>Compatibility with non-virtio transports:
>
>  - VMCI uses the AF_VSOCK buffer knobs to size its queue pairs per
>    socket based on the local vsk->buffer_* values; the remote side
>    cannot 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 cannot 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.
>
>This change is limited to virtio_transport_common.c and thus affects
>virtio-vsock, vhost-vsock, and loopback, bringing them in line with the
>"remote window intersected with local policy" behaviour that VMCI and
>Hyper-V already effectively have.
>
>Fixes: 06a8fc78367d ("VSOCK: Introduce virtio_vsock_common.ko")
>Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
>Signed-off-by: Melbin K Mathew <mlbnkm1@gmail.com>
>[Stefano: small adjustments after changing the previous patch]
>[Stefano: tweak the commit message]
>Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>---
> net/vmw_vsock/virtio_transport_common.c | 14 ++++++++++++--
> 1 file changed, 12 insertions(+), 2 deletions(-)
>
>diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>index 6175124d63d3..d3e26025ef58 100644
>--- a/net/vmw_vsock/virtio_transport_common.c
>+++ b/net/vmw_vsock/virtio_transport_common.c
>@@ -821,6 +821,15 @@ virtio_transport_seqpacket_dequeue(struct vsock_sock *vsk,
> }
> EXPORT_SYMBOL_GPL(virtio_transport_seqpacket_dequeue);
>
>+static u32 virtio_transport_tx_buf_size(struct virtio_vsock_sock *vvs)
>+{
>+	/* The peer advertises its receive buffer via peer_buf_alloc, but we
>+	 * cap it to our local buf_alloc so a remote peer cannot force us to
>+	 * queue more data than our own buffer configuration allows.
>+	 */
>+	return min(vvs->peer_buf_alloc, vvs->buf_alloc);
>+}
>+
> int
> virtio_transport_seqpacket_enqueue(struct vsock_sock *vsk,
> 				   struct msghdr *msg,
>@@ -830,7 +839,7 @@ 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_size(vvs)) {
> 		spin_unlock_bh(&vvs->tx_lock);
> 		return -EMSGSIZE;
> 	}
>@@ -884,7 +893,8 @@ static s64 virtio_transport_has_space(struct virtio_vsock_sock *vvs)
> 	 * we have bytes in flight (tx_cnt - peer_fwd_cnt), the subtraction
> 	 * does not underflow.
> 	 */
>-	bytes = (s64)vvs->peer_buf_alloc - (vvs->tx_cnt - vvs->peer_fwd_cnt);
>+	bytes = (s64)virtio_transport_tx_buf_size(vvs) -
>+		(vvs->tx_cnt - vvs->peer_fwd_cnt);
> 	if (bytes < 0)
> 		bytes = 0;
>
>-- 
>2.52.0
>

LGTM!

Reviewed-by: Luigi Leonardi <leonardi@redhat.com>