Subject: Re: [PATCH net 2/2] ip6_gre: Fix skb_under_panic in __gre6_xmit()

From: Jakub Kicinski <[email-masked]>
Date: Thu, 14 Apr 2022 13:14:24 +0200

On Mon, 11 Apr 2022 15:33:00 -0700 Peilin Ye wrote:
> The following sequence of events caused the BUG:
> 
> 1. During ip6gretap device initialization, tunnel->tun_hlen (e.g. 4) is
>    calculated based on old flags (see ip6gre_calc_hlen());
> 2. packet_snd() reserves header room for skb A, assuming
>    tunnel->tun_hlen is 4;
> 3. Later (in clsact Qdisc), the eBPF program sets a new tunnel key for
>    skb A using bpf_skb_set_tunnel_key() (see _ip6gretap_set_tunnel());
> 4. __gre6_xmit() detects the new tunnel key, and recalculates
>    "tun_hlen" (e.g. 12) based on new flags (e.g. TUNNEL_KEY and
>    TUNNEL_SEQ);
> 5. gre_build_header() calls skb_push() with insufficient reserved header
>    room, triggering the BUG.
> 
> As sugguested by Cong, fix it by moving the call to skb_cow_head() after
> the recalculation of tun_hlen.
> 
> Reported-by: Feng Zhou <[email-masked]>
> Co-developed-by: Cong Wang <[email-masked]>
> Signed-off-by: Cong Wang <[email-masked]>
> Signed-off-by: Peilin Ye <[email-masked]>
> ---
> Hi all,
> 
> I couldn't find a proper Fixes: tag for this fix; please comment if you
> have any sugguestions.  Thanks!

What's wrong with

Fixes: 6712abc168eb ("ip6_gre: add ip6 gre and gretap collect_md mode")

?

> diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
> index b43a46449130..976236736146 100644
> --- a/net/ipv6/ip6_gre.c
> +++ b/net/ipv6/ip6_gre.c
> @@ -733,9 +733,6 @@ static netdev_tx_t __gre6_xmit(struct sk_buff *skb,
>  	else
>  		fl6->daddr = tunnel->parms.raddr;
>  
> -	if (skb_cow_head(skb, dev->needed_headroom ?: tunnel->hlen))
> -		return -ENOMEM;
> -
>  	/* Push GRE header. */
>  	protocol = (dev->type == ARPHRD_ETHER) ? htons(ETH_P_TEB) : proto;
>  
> @@ -763,6 +760,9 @@ static netdev_tx_t __gre6_xmit(struct sk_buff *skb,
>  			(TUNNEL_CSUM | TUNNEL_KEY | TUNNEL_SEQ);

We should also reject using SEQ with collect_md, but that's a separate
issue.

>  		tun_hlen = gre_calc_hlen(flags);
>  
> +		if (skb_cow_head(skb, dev->needed_headroom ?: tun_hlen + tunnel->encap_hlen))
> +			return -ENOMEM;
> +
>  		gre_build_header(skb, tun_hlen,
>  				 flags, protocol,
>  				 tunnel_id_to_key32(tun_info->key.tun_id),