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

[Stefano Garzarella <sgarzare@redhat.com>]

On Wed, May 06, 2026 at 12:50:04PM +0300, Arseniy Krasnov wrote:
>
>
>05.05.2026 19:37, Bobby Eshleman wrote:
>> On Tue, May 05, 2026 at 06:11:13PM +0200, Stefano Garzarella wrote:
>>> 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.
>
>Thanks!
>
>>>
>>>>
>>>> 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
>>>
>>>
>>
>> Indeed the old pre-fix skb code would have the same issue.
>>
>> I can't think of any way around this without extending the spec.
>
>Hi, thanks, agree with Bobby, that accounting metadata (e.g. skb size here) was not implemented "by
>design" in credit logic - another side of data exchange knows nothing about that. Also the same
>situation was before skb implementation was added by Bobby. So looks like need to update spec may be.
>

Even if we change the specifications, we still need to work with older 
devices, so we should find a solution for this as well.

My main concern is data loss, so I'm considering the following options:

1. Notify the other peer of a smaller buf_alloc from the start, leave 
some room for overhead, and when it's running out, notify them that 
buf_alloc = 0. This way, the peer realizes it can’t send anything else.

2. Or update buf_alloc each time by removing the overhead, similar to 
what’s currently done in virtio_transport_inc_rx_pkt(), but also do it 
in virtio_transport_inc_tx_pkt().

As I said, IMO this patch alone is incomplete; we need to communicate 
with the peer somehow regarding space. I don’t think including the 
overhead in fwd_cnt is spec compliant, since the other peer has no idea 
how much overhead is needed, but reducing buf_alloc should be okay, even 
though I’m concerned about packets in flight.

As a quick fix, I think option 2 might be the easiest; I’ll run some 
tests and send over a patch.

But in the long run, I think we absolutely need to improve memory 
management in vsock, perhaps by avoiding custom solutions.

Thanks,
Stefano