Subject: Re: [PATCH net] ip: fix IP_CHECKSUM handling

From: Stephen Hemminger <[email-masked]>
Date: Mon, 20 Feb 2017 09:23:25 -0800

On Mon, 20 Feb 2017 17:55:39 +0100
Paolo Abeni <[email-masked]> wrote:

> The skbs processed by ip_cmsg_recv() are not guaranteed to
> be linear e.g. when sending UDP packets over loopback with
> MSGMORE.
> Using csum_partial() on [potentially] the whole skb len
> is dangerous; instead be on the safe side and use skb_checksum().
> 
> Thanks to syzkaller team to detect the issue and provide the
> reproducer.
> 
> Fixes: ad6f939ab193 ("ip: Add offset parameter to ip_cmsg_recv")
> Reported-by: Andrey Konovalov <[email-masked]>
> Signed-off-by: Paolo Abeni <[email-masked]>
> ---
>  net/ipv4/ip_sockglue.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
> index 9000117..d88bbe2 100644
> --- a/net/ipv4/ip_sockglue.c
> +++ b/net/ipv4/ip_sockglue.c
> @@ -112,14 +112,15 @@ static void ip_cmsg_recv_checksum(struct msghdr *msg, struct sk_buff *skb,
>  				  int tlen, int offset)
>  {
>  	__wsum csum = skb->csum;
> +	int tend_off;
>  
>  	if (skb->ip_summed != CHECKSUM_COMPLETE)
>  		return;
>  
> -	if (offset != 0)
> -		csum = csum_sub(csum,
> -				csum_partial(skb_transport_header(skb) + tlen,
> -					     offset, 0));
> +	if (offset != 0) {
> +		tend_off = skb_transport_offset(skb) + tlen;
> +		csum = csum_sub(csum, skb_checksum(skb, tend_off, offset, 0));
> +	}

If you need to add a local variable (tend_off) then please make its scope
as tight as possible. i.e.
	
	if (offset != 0) {
		int tend_off = skb_transport_offset(skb) + tlen;