Re: [PATCH net] vsock/virtio: fix potential unbounded skb queue

[Stefano Garzarella <sgarzare@redhat.com>]

On Thu, Apr 30, 2026 at 12:26:52PM +0000, Eric Dumazet wrote:
>virtio_transport_inc_rx_pkt() checks vvs->rx_bytes + len > vvs->buf_alloc.
>
>virtio_transport_recv_enqueue() skips coalescing for packets
>with VIRTIO_VSOCK_SEQ_EOM.
>
>If fed with packets with len == 0 and VIRTIO_VSOCK_SEQ_EOM,
>a very large number of packets can be queued
>because vvs->rx_bytes stays at 0.
>
>Fix this by estimating the skb metadata size:
>
>	(Number of skbs in the queue) * SKB_TRUESIZE(0)
>
>Fixes: 077706165717 ("virtio/vsock: don't use skbuff state to account credit")
>Signed-off-by: Eric Dumazet <edumazet@google.com>
>Cc: Arseniy Krasnov <AVKrasnov@sberdevices.ru>
>Cc: Stefan Hajnoczi <stefanha@redhat.com>
>Cc: Stefano Garzarella <sgarzare@redhat.com>
>Cc: "Michael S. Tsirkin" <mst@redhat.com>
>Cc: Jason Wang <jasowang@redhat.com>
>Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
>Cc: "Eugenio Pérez" <eperezma@redhat.com>
>Cc: kvm@vger.kernel.org
>Cc: virtualization@lists.linux.dev
>---
> net/vmw_vsock/virtio_transport_common.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
>diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>index 416d533f493d7b07e9c77c43f741d28cfcd0953e..9b8014516f4fb1130ae184635fbba4dfee58bd64 100644
>--- a/net/vmw_vsock/virtio_transport_common.c
>+++ b/net/vmw_vsock/virtio_transport_common.c
>@@ -447,7 +447,9 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
> static bool virtio_transport_inc_rx_pkt(struct virtio_vsock_sock *vvs,
> 					u32 len)
> {
>-	if (vvs->buf_used + len > vvs->buf_alloc)
>+	u64 skb_overhead = (skb_queue_len(&vvs->rx_queue) + 1) * SKB_TRUESIZE(0);
>+
>+	if (skb_overhead + vvs->buf_used + len > vvs->buf_alloc)
> 		return false;

I'm not sure about this fix, I mean that maybe this is incomplete.
In virtio-vsock, there is a credit mechanism between the two peers:
https://docs.oasis-open.org/virtio/virtio/v1.3/csd01/virtio-v1.3-csd01.html#x1-4850003

This takes only the payload into account, so it’s true that this problem 
exists; however, perhaps we should also inform the other peer of a lower 
credit balance, otherwise the other peer will believe it has much more 
credit than it actually does, send a large payload, and then the packet 
will be discarded and the data lost (there are no retransmissions, 
etc.).

Thanks,
Stefano