netdev - Re: [patch v3, kernel version 3.2.1] net/ipv4/ip

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Date:	Wed, 18 Jan 2012 00:17:42 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Stefan Gula <steweg@...t.sk>
Cc:	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	"David S. Miller" <davem@...emloft.net>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch v3, kernel version 3.2.1] net/ipv4/ip_gre: Ethernet
 multipoint GRE over IP

Le mardi 17 janvier 2012 à 23:20 +0100, Stefan Gula a écrit :
> From: Stefan Gula <steweg@...il.com>
> 
> This patch is an extension for current Ethernet over GRE
> implementation, which allows user to create virtual bridge (multipoint
> VPN) and forward traffic based on Ethernet MAC address information in
> it. It simulates the Bridge behavior learning mechanism, but instead
> of learning port ID from which given MAC address comes, it learns IP
> address of peer which encapsulated given packet. Multicast, Broadcast
> and unknown-multicast traffic is send over network as multicast
> encapsulated GRE packet, so one Ethernet multipoint GRE tunnel can be
> represented as one single virtual switch on logical level and be also
> represented as one multicast IPv4 address on network level.
> 
> Signed-off-by: Stefan Gula <steweg@...il.com>
> 
> ---
> 
> code was merged with Eric Dumazet proposal (all except the reordering
> of orig_source as that needed to be previous value), tested and fixed
> with additional lines in ipgre_tap_netdev_ops struct
> 

Sorry, this is buggy (again...)

Its even clearly commented in the code :

/* Warning: All skb pointers will be invalidated! */

>  
>          if (!pskb_may_pull(skb, 16))
>                  goto drop_nolock;
> @@ -659,10 +836,38 @@ static int ipgre_rcv(struct sk_buff *skb
>                                  tunnel->dev->stats.rx_errors++;
>                                  goto drop;
>                          }

At this point, iph can point to freed memory and its dereference can
crash, since pskb_may_pull() can reallocate skb head.

> -
> +#ifdef CONFIG_NET_IPGRE_BRIDGE
> +                        orig_source = iph->saddr;
> +#endif

Without any doubt, you know here there is a bug.

>                          iph = ip_hdr(skb);
>                          skb->protocol = eth_type_trans(skb, tunnel->dev);
>                          skb_postpull_rcsum(skb, eth_hdr(skb), ETH_HLEN);


So if you need orig_source as the previous iph->saddr value, you must
fetch it _before_ the pskb_may_pull()


/* Warning: All skb pointers will be invalidated! */
if (tunnel->dev->type == ARPHRD_ETHER) {
#ifdef CONFIG_NET_IPGRE_BRIDGE
 	orig_source = iph->saddr; /* must be done before pskb_may_pull() */
#endif
	if (!pskb_may_pull(skb, ETH_HLEN)) {
		tunnel->dev->stats.rx_length_errors++;
		tunnel->dev->stats.rx_errors++;
		goto drop;
	}

	iph = ip_hdr(skb);
	...



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