Re: [PATCH net v2] virtio/vsock: fix leaks due to missing skb owner

[Bobby Eshleman <bobbyeshleman@gmail.com>]

On Wed, Mar 29, 2023 at 09:16:19AM +0200, Stefano Garzarella wrote:
> On Tue, Mar 28, 2023 at 04:29:09PM +0000, Bobby Eshleman wrote:
> > This patch sets the skb owner in the recv and send path for virtio.
> > 
> > For the send path, this solves the leak caused when
> > virtio_transport_purge_skbs() finds skb->sk is always NULL and therefore
> > never matches it with the current socket. Setting the owner upon
> > allocation fixes this.
> > 
> > For the recv path, this ensures correctness of accounting and also
> > correct transfer of ownership in vsock_loopback (when skbs are sent from
> > one socket and received by another).
> > 
> > Fixes: 71dc9ec9ac7d ("virtio/vsock: replace virtio_vsock_pkt with sk_buff")
> > Signed-off-by: Bobby Eshleman <bobby.eshleman@bytedance.com>
> > Reported-by: Cong Wang <xiyou.wangcong@gmail.com>
> > Link: https://lore.kernel.org/all/ZCCbATwov4U+GBUv@pop-os.localdomain/
> > ---
> > Changes in v2:
> > - virtio/vsock: add skb_set_owner_r to recv_pkt()
> > - Link to v1: https://lore.kernel.org/r/20230327-vsock-fix-leak-v1-1-3fede367105f@bytedance.com
> > ---
> > net/vmw_vsock/virtio_transport_common.c | 5 +++++
> > 1 file changed, 5 insertions(+)
> > 
> > diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> > index 957cdc01c8e8..900e5dca05f5 100644
> > --- a/net/vmw_vsock/virtio_transport_common.c
> > +++ b/net/vmw_vsock/virtio_transport_common.c
> > @@ -94,6 +94,9 @@ virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *info,
> > 					 info->op,
> > 					 info->flags);
> > 
> > +	if (info->vsk)
> > +		skb_set_owner_w(skb, sk_vsock(info->vsk));
> > +
> > 	return skb;
> > 
> > out:
> > @@ -1294,6 +1297,8 @@ void virtio_transport_recv_pkt(struct virtio_transport *t,
> > 		goto free_pkt;
> > 	}
> > 
> > +	skb_set_owner_r(skb, sk);
> > +
> > 	vsk = vsock_sk(sk);
> > 
> > 	lock_sock(sk);
> 
> Can you explain why we are using skb_set_owner_w/skb_set_owner_r?
> 
> I'm a little concerned about 2 things:
> - skb_set_owner_r() documentation says: "Stream and sequenced
>   protocols can't normally use this as they need to fit buffers in
>   and play with them."
> - they increment sk_wmem_alloc and sk_rmem_alloc that we never used
>   (IIRC)
> 
> For the long run, I think we should manage memory better, and using
> socket accounting makes sense to me, but since we now have a different
> system (which we have been carrying around since the introduction of
> vsock), I think this change is a bit risky, especially as a fix.
> 
> So my suggestion is to use skb_set_owner_sk_safe() for now, unless I
> missed something about why to use skb_set_owner_w/skb_set_owner_r.
> 

I think that makes sense. I was honestly unaware of
skb_set_owner_sk_safe(), but given the reasons you stated and after
reading its code, I agree it is a better fit in light of vsock's
different accounting scheme.

I'll switch it over in v3.

Best,
Bobby