From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH net-next 2/2] net: sched: add em_ipt ematch for calling
 xtables matches
Date: Wed, 24 Jan 2018 16:37:16 -0500 (EST)
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From: Eyal Birger <eyal.birger@gmail.com>
Date: Tue, 23 Jan 2018 11:17:32 +0200

> +	network_offset = skb_network_offset(skb);
> +	skb_pull(skb, network_offset);
> +
> +	rcu_read_lock();
> +
> +	if (skb->skb_iif)
> +		indev = dev_get_by_index_rcu(em->net, skb->skb_iif);
> +
> +	nf_hook_state_init(&state, im->hook, im->nfproto, indev ?: skb->dev,
> +			   skb->dev, NULL, em->net, NULL);
> +
> +	acpar.match = im->match;
> +	acpar.matchinfo = im->match_data;
> +	acpar.state = &state;
> +
> +	ret = im->match->match(skb, &acpar);
> +
> +	rcu_read_unlock();
> +
> +	skb_push(skb, network_offset);

If the SKB is shared in any way, this pull/push around the NF hook
invocation is illegal.