Message-ID: <uo63g2tsmgcqqg3tpzm7tdtfn2pr73kymfyl4woulpwcobevuw@vr3d4i4konge>
Date: Thu, 11 Dec 2025 14:15:49 +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 v3] vsock/virtio: cap TX credit to local buffer size

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@...hat.com>
>Signed-off-by: Melbin K Mathew <mlbnkm1@...il.com>
>---
> net/vmw_vsock/virtio_transport_common.c | 27 ++++++++++++++++++++++---
> 1 file changed, 24 insertions(+), 3 deletions(-)

Changes LGTM, but the patch seems corrupted.

$ git am ./v3_20251211_mlbnkm1_vsock_virtio_cap_tx_credit_to_local_buffer_size.mbx
Applying: vsock/virtio: cap TX credit to local buffer size
error: corrupt patch at line 29
Patch failed at 0001 vsock/virtio: cap TX credit to local buffer size

See also 
https://patchwork.kernel.org/project/netdevbpf/patch/20251211125104.375020-1-mlbnkm1@gmail.com/

Stefano

>
>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;
>
>-- 
>2.34.1
>

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