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

[Stefano Garzarella <sgarzare@redhat.com>]

On Wed, May 06, 2026 at 11:37:45AM -0400, Michael S. Tsirkin 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.
>>
>> >
>> > 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
>
>Why do we need the metadata? Just don't keep it around if you begin
>running low on memory.

I don't think removing the skuffs will be easy; we added them for ebpf, 
zero-copy, and seqpacket as well. For now, we're already doing 
something: merging the skuffs if they don't have EOM set.

As a quick fix, I'm thinking of reducing the `buf_alloc` value to 
account for the overhead and notifying the other peer, at least until we 
find a better solution.

Stefano