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

[Melbin K Mathew <mlbnkm1@gmail.com>]

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@redhat.com>
Signed-off-by: Melbin K Mathew <mlbnkm1@gmail.com>
---
 net/vmw_vsock/virtio_transport_common.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

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