Subject: Re: [BUG] skb corruption and kernel panic at forwarding with fragmentation

From: Florian Westphal <[email-masked]>
Date: Wed, 6 Jan 2016 23:03:34 +0100

Thadeu Lima de Souza Cascardo <[email-masked]> wrote:
> On Wed, Jan 06, 2016 at 11:11:41PM +0300, Konstantin Khlebnikov wrote:
> > On Wed, Jan 6, 2016 at 10:59 PM, Cong Wang <[email-masked]> wrote:
> > > On Wed, Jan 6, 2016 at 11:15 AM, Konstantin Khlebnikov <[email-masked]> wrote:
> > >> Looks like this happens because ip_options_fragment() relies on
> > >> correct ip options length in ip control block in skb. But in
> > >> ip_finish_output_gso() control block in segments is reused by
> > >> skb_gso_segment(). following ip_fragment() sees some garbage.
> > >>
> > >> In my case there was no ip options but length becomes non-zero and
> > >> ip_options_fragment() picked some bytes from payload and decides to
> > >> fill huge range with IPOPT_NOOP (1). One of that ones flipped nr_frags
> > >> in skb_shared_info at the end of data =)
> > >>
> > >
> > > Hmm, it looks like SKB_GSO_CB should be cleared after skb_gso_segment()
> > > since all the gso information should be saved in shared_info after it finishes.
> > >
> > > Does a memset(0) on SKB_GSO_CB after skb_gso_segment() work as well?
> > 
> > This will break present logic around ip_options_fragment() - it clears
> > options from
> > second and following fragments. With zeroed cb it will do nothing.
> > 
> > ip_options_fragment() can get required information directly from ip header but
> > it also resets fields in IPCB -- probably it should stay valid here
> > and somebody else will use it later.
[..]
> I have hit this as well, this fixes it for me on an older kernel. Can you try it
> on latest kernel?

> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index d8a1745..f44bc91 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -216,6 +216,7 @@ static int ip_finish_output_gso(struct sk_buff *skb)
>  	netdev_features_t features;
>  	struct sk_buff *segs;
>  	int ret = 0;
> +	struct inet_skb_parm ipcb;
>  
>  	if (skb_gso_network_seglen(skb) <= ip_skb_dst_mtu(skb))
>  		return ip_finish_output2(skb);
> @@ -227,6 +228,10 @@ static int ip_finish_output_gso(struct sk_buff *skb)
>  	 * 2) skb arrived via virtio-net, we thus get TSO/GSO skbs directly
>  	 * from host network stack.
>  	 */
> +	/* We need to save IPCB here because skb_gso_segment will use
> +	 * SKB_GSO_CB.
> +	 */
> +	ipcb = *IPCB(skb);
>  	features = netif_skb_features(skb);
>  	segs = skb_gso_segment(skb, features & ~NETIF_F_GSO_MASK);
>  	if (IS_ERR_OR_NULL(segs)) {
> @@ -241,6 +246,7 @@ static int ip_finish_output_gso(struct sk_buff *skb)
>  		int err;
>  
>  		segs->next = NULL;
> +		*IPCB(segs) = ipcb;
>  		err = ip_fragment(segs, ip_finish_output2);
>  
>  		if (err && ret == 0)

I'm worried that this doesn't solve all cases. f.e. xfrm may also
call skb_gso_segment(), and it will call into ipv4/ipv6 netfilter
postrouting + ipv4 output functions...

nfqnl_enqueue_packet() is also affected.