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

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Date:   Fri, 16 Aug 2019 10:23:55 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Hangbin Liu <liuhangbin@...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



On 8/16/19 5:24 AM, Hangbin Liu wrote:
> 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?
>

dst are inherently shared.

This is why we have a refcount on them.

Only when the dst has been allocated by the current thread we can make changes on them.