Subject: Re: [PATCH 18/29] netfilter: notify about NF_QUEUE vs emergency skbs

From: Patrick McHardy <[email-masked]>
Date: Sat, 24 Feb 2007 16:27:16 +0100

Peter Zijlstra wrote:
> Emergency skbs should never touch user-space, however NF_QUEUE is fully user
> configurable. Notify the user of his mistake and try to continue.
>
> --- linux-2.6-git.orig/net/netfilter/core.c	2007-02-14 12:09:07.000000000 +0100
> +++ linux-2.6-git/net/netfilter/core.c	2007-02-14 12:09:18.000000000 +0100
> @@ -187,6 +187,11 @@ next_hook:
>  		kfree_skb(*pskb);
>  		ret = -EPERM;
>  	} else if ((verdict & NF_VERDICT_MASK)  == NF_QUEUE) {
> +		if (unlikely((*pskb)->emergency)) {
> +			printk(KERN_ERR "nf_hook: NF_QUEUE encountered for "
> +					"emergency skb - skipping rule.\n");
> +			goto next_hook;
> +		}

If I'm not mistaken any skb on the receive side might get
allocated from the reserve. I don't see how the user could
avoid this except by not using queueing at all.

I also didn't see a patch dropping packets allocated from
the reserve that are forwarded or processed directly without
getting queued to a socket, so this would allow them to
bypass userspace queueing and still go through.

I think the user should just exclude packets necessary for
swapping from queueing manually, based on IP addresses,
port numbers or something like that.