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

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Date:   Wed, 29 Mar 2023 09:16:19 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Bobby Eshleman <bobby.eshleman@...edance.com>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Cong Wang <xiyou.wangcong@...il.com>
Subject: Re: [PATCH net v2] virtio/vsock: fix leaks due to missing skb owner

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@...edance.com>
>Reported-by: Cong Wang <xiyou.wangcong@...il.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.

Thanks,
Stefano