netdev - Re: [PATCH net] geneve: fix header validation in geneve[6]_xmit

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Message-ID: <Zg1l9L2BNoZWZDZG@hog>
Date: Wed, 3 Apr 2024 16:21:40 +0200
From: Sabrina Dubroca <sd@...asysnail.net>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, eric.dumazet@...il.com,
	syzbot+9ee20ec1de7b3168db09@...kaller.appspotmail.com,
	Phillip Potter <phil@...lpotter.co.uk>
Subject: Re: [PATCH net] geneve: fix header validation in geneve[6]_xmit_skb

2024-04-03, 11:38:53 +0000, Eric Dumazet wrote:
> syzbot is able to trigger an uninit-value in geneve_xmit() [1]
> 
> Problem : While most ip tunnel helpers (like ip_tunnel_get_dsfield())
> uses skb_protocol(skb, true), pskb_inet_may_pull() is only using
> skb->protocol.
> 
> If anything else than ETH_P_IPV6 or ETH_P_IP is found in skb->protocol,
> pskb_inet_may_pull() does nothing at all.
> 
> If a vlan tag was provided by the caller (af_packet in the syzbot case),
> the network header might not point to the correct location, and skb
> linear part could be smaller than expected.
> 
> Add skb_vlan_inet_prepare() to perform a complete validation and pull.
> If no IPv4/IPv6 header is found, it returns 0.

And then geneve_xmit_skb/geneve6_xmit_skb drops the packet, which
breaks ARP over a geneve tunnel, and other valid things like macsec.

> diff --git a/include/net/ip_tunnels.h b/include/net/ip_tunnels.h
> index 5cd64bb2104df389250fb3c518ba00a3826c53f7..41537d5dce52412e15d7871ec604546582b10098 100644
> --- a/include/net/ip_tunnels.h
> +++ b/include/net/ip_tunnels.h
> @@ -361,6 +361,37 @@ static inline bool pskb_inet_may_pull(struct sk_buff *skb)
>  	return pskb_network_may_pull(skb, nhlen);
>  }
>  
> +/* Strict version of pskb_inet_may_pull().
> + * Once vlan headers are skipped, only accept
> + * ETH_P_IPV6 and ETH_P_IP.
> + */
> +static inline __be16 skb_vlan_inet_prepare(struct sk_buff *skb)
> +{
> +	int nhlen, maclen;
> +	__be16 type;

Should that be:

    type = skb->protocol

?

Otherwise it's used uninitialized here:

> +
> +	type = __vlan_get_protocol(skb, type, &maclen);

-- 
Sabrina