netdev - Re: [PATCH net-next] virtio-net: invoke zerocopy callback on xmit path if no tx napi

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Date:   Tue, 22 Aug 2017 11:28:28 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     mst@...hat.com, jasowang@...hat.com,
        willemdebruijn.kernel@...il.com, den@...ipeden.com,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] virtio-net: invoke zerocopy callback on xmit
 path if no tx napi

On Tue, 2017-08-22 at 11:01 -0700, David Miller wrote:
> From: "Michael S. Tsirkin" <mst@...hat.com>
> Date: Tue, 22 Aug 2017 20:55:56 +0300
> 
> > Which reminds me that skb_linearize in net core seems to be
> > fundamentally racy - I suspect that if skb is cloned, and someone is
> > trying to use the shared frags while another thread calls skb_linearize,
> > we get some use after free bugs which likely mostly go undetected
> > because the corrupted packets mostly go on wire and get dropped
> > by checksum code.
> 
> Indeed, it does assume that the skb from which the clone was made
> never has it's geometry changed.
> 
> I don't think even the TCP retransmit queue has this guarantee.

TCP retransmit makes sure to avoid that.

if (skb_unclone(skb, GFP_ATOMIC))
     return -ENOMEM;

( Before cloning again skb )