On Tue, May 5, 2026 at 6:52 AM Stefano Garzarella <[email protected]> wrote:
>
> 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 <[email protected]>
> >Cc: Arseniy Krasnov <[email protected]>
> >Cc: Stefan Hajnoczi <[email protected]>
> >Cc: Stefano Garzarella <[email protected]>
> >Cc: "Michael S. Tsirkin" <[email protected]>
> >Cc: Jason Wang <[email protected]>
> >Cc: Xuan Zhuo <[email protected]>
> >Cc: "Eugenio Pérez" <[email protected]>
> >Cc: [email protected]
> >Cc: [email protected]
> >---
> > 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.).
I dunno, perhaps revert 077706165717 ("virtio/vsock: don't use skbuff
state to account credit")
and find a better fix then?
There is always a discrepancy between skb->len and skb->truesize.
You will not be able to announce a 1MB window, and accept one milliion
skb of 1-byte each.
This kind of contract is broken.
