Subject: Re: [PATCH net V2] net: always reevalulate autoflowlabel setting for reset packet

From: Eric Dumazet <[email-masked]>
Date: Tue, 19 Dec 2017 14:33:30 -0800

On Tue, 2017-12-19 at 12:58 -0800, Shaohua Li wrote:
> From: Shaohua Li <[email-masked]>
> 
> ipv6_pinfo.autoflowlabel is set in sock creation. Later if we change
> sysctl.ip6.auto_flowlabels, the ipv6_pinfo.autoflowlabel isn't changed,
> so the sock will keep the old behavior in terms of auto flowlabel. Reset
> packet is suffering from this problem, because reset packset is sent
> from a special control socket, which is created at boot time. Since
> sysctl.ipv6.auto_flowlabels is 2 by default, the control socket will
> always have its ipv6_pinfo.autoflowlabel set, even after user set
> sysctl.ipv6.auto_flowlabels to 1, so reset packset will always have
> flowlabel.
> 
> To fix this, we always reevaluate autoflowlabel setting for reset
> packet. Normal sock has the same issue too, but since the
> sysctl.ipv6.auto_flowlabels is usually set at host startup, this isn't a
> big issue for normal sock.
> 
> Cc: Martin KaFai Lau <[email-masked]>
> Signed-off-by: Shaohua Li <[email-masked]>
> ---
>  net/ipv6/tcp_ipv6.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index 7178476..5fba548 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/net/ipv6/tcp_ipv6.c
> @@ -787,9 +787,11 @@ static void tcp_v6_send_response(const struct sock *sk, struct sk_buff *skb, u32
>  	struct net *net = sk ? sock_net(sk) : dev_net(skb_dst(skb)->dev);
>  	struct sock *ctl_sk = net->ipv6.tcp_sk;
>  	unsigned int tot_len = sizeof(struct tcphdr);
> +	struct ipv6_pinfo *np = inet6_sk(ctl_sk);
>  	struct dst_entry *dst;
>  	__be32 *topt;
>  
> +	np->autoflowlabel = ip6_default_np_autolabel(net);

This looks unsafe to set a bitfield on a shared socket (one ctl_sk per
netns)

Compiler could play strange things here.