netdev - Re: [PATCH net] tunnel: fix dev null pointer dereference when send pkg larger than mtu in collect

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Message-ID: <20190816032418.GX18865@dhcp-12-139.nay.redhat.com>
Date:   Fri, 16 Aug 2019 11:24:18 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org, Stefano Brivio <sbrivio@...hat.com>,
        wenxu <wenxu@...oud.cn>, Alexei Starovoitov <ast@...com>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH net] tunnel: fix dev null pointer dereference when send
 pkg larger than mtu in collect_md mode

Hi Eric,

Thanks for the review.
On Thu, Aug 15, 2019 at 11:16:58AM +0200, Eric Dumazet wrote:
> > diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
> > index 38c02bb62e2c..c6713c7287df 100644
> > --- a/net/ipv4/ip_tunnel.c
> > +++ b/net/ipv4/ip_tunnel.c
> > @@ -597,6 +597,9 @@ void ip_md_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
> >  		goto tx_error;
> >  	}
> >  
> > +	if (skb_dst(skb) && !skb_dst(skb)->dev)
> > +		skb_dst(skb)->dev = rt->dst.dev;
> > +
> 
> 
> IMO this looks wrong.
> This dst seems shared. 

If the dst is shared, it may cause some problem. Could you point me where the
dst may be shared possibly?

> Once set, we will reuse the same dev ?

If yes, how about just set the skb dst to rt->dst, as the
iptunnel_xmit would do later.

skb_dst_drop(skb);
skb_dst_set(skb, &rt->dst);

or do you have any other idea?
> 
> If intended, why not doing this in __metadata_dst_init() instead of in the fast path ?

I'm afraid we couldn't do this, I didn't find a way to init dev in
__metadata_dst_init(). Do you?

Thanks
Hangbin