linux-kernel - Re: [PATCH net v4 2/4] vsock/virtio: cap TX credit to local buffer size

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Message-ID: <vekrofb2syrh35whi7tznmypvekvqmcbjjlv5bhyknrghhnxr7@b6v3yxubtu5y>
Date: Thu, 18 Dec 2025 10:24:38 +0100
From: Stefano Garzarella <sgarzare@...hat.com>
To: Melbin K Mathew <mlbnkm1@...il.com>
Cc: stefanha@...hat.com, kvm@...r.kernel.org, netdev@...r.kernel.org, 
	virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org, mst@...hat.com, 
	jasowang@...hat.com, xuanzhuo@...ux.alibaba.com, eperezma@...hat.com, 
	davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, 
	horms@...nel.org
Subject: Re: [PATCH net v4 2/4] vsock/virtio: cap TX credit to local buffer
 size

On Wed, Dec 17, 2025 at 07:12:04PM +0100, Melbin K Mathew wrote:
>The virtio vsock transport 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_tx_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; the system only
>recovered after killing the QEMU process.
>
>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 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@...hat.com>
>Signed-off-by: Melbin K Mathew <mlbnkm1@...il.com>
>---
> net/vmw_vsock/virtio_transport_common.c | 18 +++++++++++++++---
> 1 file changed, 15 insertions(+), 3 deletions(-)

This LGTM, but I'd like to see the final version.

Stefano

>
>diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>index d692b227912d..92575e9d02cd 100644
>--- a/net/vmw_vsock/virtio_transport_common.c
>+++ b/net/vmw_vsock/virtio_transport_common.c
>@@ -491,6 +491,18 @@ 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.
>+ *
>+ * 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.
>+ */
>+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;
>@@ -508,7 +520,7 @@ u32 virtio_transport_get_credit(struct virtio_vsock_sock *vvs, u32 credit)
> 	 * its advertised buffer while data is in flight).
> 	 */
> 	inflight = vvs->tx_cnt - vvs->peer_fwd_cnt;
>-	bytes = (s64)vvs->peer_buf_alloc - inflight;
>+	bytes = (s64)virtio_transport_tx_buf_alloc(vvs) - inflight;
> 	if (bytes < 0)
> 		bytes = 0;
>
>@@ -842,7 +854,7 @@ 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;
> 	}
>@@ -893,7 +905,7 @@ 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;
>
>-- 
>2.34.1
>