netdev - Re: [RFC net-next] net: guard drivers against shared skbs

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Date:   Mon, 15 Nov 2021 08:56:10 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>, davem@...emloft.net
Cc:     netdev@...r.kernel.org, eric.dumazet@...il.com, hawk@...nel.org,
        syzbot+4c63f36709a642f801c5@...kaller.appspotmail.com
Subject: Re: [RFC net-next] net: guard drivers against shared skbs



On 11/15/21 8:32 AM, Jakub Kicinski wrote:
> Commit d8873315065f ("net: add IFF_SKB_TX_SHARED flag to priv_flags")
> introduced IFF_SKB_TX_SHARED to protect drivers which are not ready
> for getting shared skbs from pktgen sending such frames.
> 
> Some drivers dutifully clear the flag but most don't, even though
> they modify the skb or call skb helpers which expect private skbs.
> 
> syzbot has also discovered more sources of shared skbs than just
> pktgen (e.g. llc).
> 
> I think defaulting to opt-in is doing more harm than good, those
> who care about fast pktgen should inspect their drivers and opt-in.
> It's far too risky to enable this flag in ether_setup().
> 
> Reported-by: syzbot+4c63f36709a642f801c5@...kaller.appspotmail.com
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> ---
>  drivers/net/dummy.c | 1 +
>  net/core/dev.c      | 4 ++++
>  net/ethernet/eth.c  | 1 -
>  3 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/dummy.c b/drivers/net/dummy.c
> index f82ad7419508..530eaaee2d25 100644
> --- a/drivers/net/dummy.c
> +++ b/drivers/net/dummy.c
> @@ -123,6 +123,7 @@ static void dummy_setup(struct net_device *dev)
>  	dev->flags |= IFF_NOARP;
>  	dev->flags &= ~IFF_MULTICAST;
>  	dev->priv_flags |= IFF_LIVE_ADDR_CHANGE | IFF_NO_QUEUE;
> +	dev->priv_flags |= IFF_TX_SKB_SHARING;
>  	dev->features	|= NETIF_F_SG | NETIF_F_FRAGLIST;
>  	dev->features	|= NETIF_F_GSO_SOFTWARE;
>  	dev->features	|= NETIF_F_HW_CSUM | NETIF_F_HIGHDMA | NETIF_F_LLTX;
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 15ac064b5562..476a826bb4f0 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3661,6 +3661,10 @@ static struct sk_buff *validate_xmit_skb(struct sk_buff *skb, struct net_device
>  	if (unlikely(!skb))
>  		goto out_null;
>  
> +	if (unlikely(skb_shared(skb)) &&
> +	    !(dev->priv_flags & IFF_TX_SKB_SHARING))
> +		goto out_kfree_skb;


So this will break llc, right ?

I am sad we are adding so much tests in fast path.