Re: [PATCH 4.4 002/193] net: replace dst_cache ip6_tunnel implementation with the generic one

[Michal Kubecek]

On Fri, Feb 23, 2018 at 07:23:55PM +0100, Greg Kroah-Hartman wrote:
> 4.4-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Paolo Abeni <pabeni@xxxxxxxxxx>
> 
> commit 607f725f6f7d5ec3759fbc16224afb60e2152a5b upstream.
> 
> This also fix a potential race into the existing tunnel code, which
> could lead to the wrong dst to be permanenty cached:
> 
> CPU1:					CPU2:
>   <xmit on ip6_tunnel>
>   <cache lookup fails>
>   dst = ip6_route_output(...)
> 					<tunnel params are changed via nl>
> 					dst_cache_reset() // no effect,
> 							// the cache is empty
>   dst_cache_set() // the wrong dst
> 	// is permanenty stored
> 	// into the cache
> 
> With the new dst implementation the above race is not possible
> since the first cache lookup after dst_cache_reset will fail due
> to the timestamp check
> 
> Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
> Suggested-and-acked-by: Hannes Frederic Sowa <hannes@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
> Signed-off-by: Manoj Boopathi Raj <manojboopathi@xxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> 
> ---
...
> --- a/net/ipv6/ip6_gre.c
> +++ b/net/ipv6/ip6_gre.c
...
> @@ -1053,7 +962,6 @@ static int ip6_tnl_xmit2(struct sk_buff
>  	struct ipv6_tel_txoption opt;
>  	struct dst_entry *dst = NULL, *ndst = NULL;
>  	struct net_device *tdev;
> -	bool use_cache = false;
>  	int mtu;
>  	unsigned int max_headroom = sizeof(struct ipv6hdr);
>  	u8 proto;
> @@ -1061,39 +969,28 @@ static int ip6_tnl_xmit2(struct sk_buff
>  
>  	/* NBMA tunnel */
>  	if (ipv6_addr_any(&t->parms.raddr)) {
> -		if (skb->protocol == htons(ETH_P_IPV6)) {
> -			struct in6_addr *addr6;
> -			struct neighbour *neigh;
> -			int addr_type;
> -
> -			if (!skb_dst(skb))
> -				goto tx_err_link_failure;
> -
> -			neigh = dst_neigh_lookup(skb_dst(skb),
> -						 &ipv6_hdr(skb)->daddr);
> -			if (!neigh)
> -				goto tx_err_link_failure;
> +		struct in6_addr *addr6;
> +		struct neighbour *neigh;
> +		int addr_type;
> +
> +		if (!skb_dst(skb))
> +			goto tx_err_link_failure;
>  
> -			addr6 = (struct in6_addr *)&neigh->primary_key;
> -			addr_type = ipv6_addr_type(addr6);
> +		neigh = dst_neigh_lookup(skb_dst(skb),
> +					 &ipv6_hdr(skb)->daddr);
> +		if (!neigh)
> +			goto tx_err_link_failure;
>  
> -			if (addr_type == IPV6_ADDR_ANY)
> -				addr6 = &ipv6_hdr(skb)->daddr;
> +		addr6 = (struct in6_addr *)&neigh->primary_key;
> +		addr_type = ipv6_addr_type(addr6);
>  
> -			memcpy(&fl6->daddr, addr6, sizeof(fl6->daddr));
> -			neigh_release(neigh);
> -		}
> -	} else if (t->parms.proto != 0 && !(t->parms.flags &
> -					    (IP6_TNL_F_USE_ORIG_TCLASS |
> -					     IP6_TNL_F_USE_ORIG_FWMARK))) {
> -		/* enable the cache only if neither the outer protocol nor the
> -		 * routing decision depends on the current inner header value
> -		 */
> -		use_cache = true;
> -	}
> +		if (addr_type == IPV6_ADDR_ANY)
> +			addr6 = &ipv6_hdr(skb)->daddr;
>  
> -	if (use_cache)
> -		dst = ip6_tnl_dst_get(t);
> +		memcpy(&fl6->daddr, addr6, sizeof(fl6->daddr));
> +		neigh_release(neigh);
> +	} else if (!fl6->flowi6_mark)
> +		dst = dst_cache_get(&t->dst_cache);
>  
>  	if (!ip6_tnl_xmit_ctl(t, &fl6->saddr, &fl6->daddr))
>  		goto tx_err_link_failure;
> @@ -1156,8 +1053,8 @@ static int ip6_tnl_xmit2(struct sk_buff
>  		skb = new_skb;
>  	}
>  
> -	if (use_cache && ndst)
> -		ip6_tnl_dst_set(t, ndst);
> +	if (!fl6->flowi6_mark && ndst)
> +		dst_cache_set_ip6(&t->dst_cache, ndst, &fl6->saddr);
>  	skb_dst_set(skb, dst);
>  
>  	skb->transport_header = skb->network_header;

This part looks essentially like a revert of earlier commit befb92542439
("ipv6: check skb->protocol before lookup for nexthop", mainline
199ab00f3cdb). Is it possible that it comes from an incorrect resolution
of the conflict caused by these two being backported in opposite order
(befb92542439 < b8c7f80cbdcd) than the original mainline commits
(607f725f6f7d < 199ab00f3cdb)?

Michal Kubecek