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

[Stefano Garzarella <sgarzare@redhat.com>]

On Tue, May 05, 2026 at 07:14:36AM -0700, Eric Dumazet wrote:
>On Tue, May 5, 2026 at 6:52 AM Stefano Garzarella <sgarzare@redhat.com> 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 <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.).
>
>I dunno, perhaps revert 077706165717 ("virtio/vsock: don't use skbuff
>state to account credit")
>and find a better fix then?

IIRC the same issue was there before the commit fixed by that one 
(commit 71dc9ec9ac7d ("virtio/vsock: replace virtio_vsock_pkt with 
sk_buff")), so not sure about reverting it TBH.

CCing Arseniy and Bobby.

>
>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.
>

Yep, I agree, but before we start discarding data (and losing it), IMHO 
we should at least inform the other peer that we're out of space.

@Stefan, @Michael, do you think we can do something in the spec to avoid 
this issue and in some way take into account also the metadata in the 
credit. I mean to avoid the 1-byte packets flooding.

Thanks,
Stefano