netdev - Re: [PATCH v5 nf] netfilter: seqadj: Drop the packet directly when fail to add seqadj extension to avoid dereference NULL pointer later

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Date:   Tue, 6 Sep 2016 16:51:17 +0200
From:   Florian Westphal <fw@...len.de>
To:     fgao@...ai8.com
Cc:     pablo@...filter.org, netfilter-devel@...r.kernel.org, fw@...len.de,
        coreteam@...filter.org, netdev@...r.kernel.org,
        gfree.wind@...il.com
Subject: Re: [PATCH v5 nf] netfilter: seqadj: Drop the packet directly when
 fail to add seqadj extension to avoid dereference NULL pointer later

fgao@...ai8.com <fgao@...ai8.com> wrote:
> From: Gao Feng <fgao@...ai8.com>
> 
> When memory is exhausted, nfct_seqadj_ext_add may fail to add the seqadj
> extension. But the function nf_ct_seqadj_init doesn't check if get valid
> seqadj pointer by the nfct_seqadj.
> 
> Now drop the packet directly when fail to add seqadj extension to avoid
> dereference NULL pointer in nf_ct_seqadj_init.
> 
> Signed-off-by: Gao Feng <fgao@...ai8.com>
> ---
>  v5: Return NF_ACCEPT instead of NF_DROP when nfct_seqadj_ext_add failed in nf_nat_setup_info
>  v4: Drop the packet directly when fail to add seqadj extension;
>  v3: Remove the warning log when seqadj is null;
>  v2: Remove the unnessary seqadj check in nf_ct_seq_adjust
>  v1: Initial patch
> 
>  net/netfilter/nf_conntrack_core.c | 6 +++++-
>  net/netfilter/nf_nat_core.c       | 3 ++-
>  2 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
> index dd2c43a..dfa76ce 100644
> --- a/net/netfilter/nf_conntrack_core.c
> +++ b/net/netfilter/nf_conntrack_core.c
> @@ -1036,7 +1036,11 @@ init_conntrack(struct net *net, struct nf_conn *tmpl,
>  		return (struct nf_conntrack_tuple_hash *)ct;
>  
>  	if (tmpl && nfct_synproxy(tmpl)) {
> -		nfct_seqadj_ext_add(ct);
> +		if (!nfct_seqadj_ext_add(ct)) {
> +			nf_conntrack_free(ct);
> +			pr_debug("Can't add seqadj extension\n");
> +			return NULL;
> +		}

if (!nfct_add_synrpxy(ct, tmpl)) {
	nf_conntrack_free(ct);
	return NULL;
}

static bool nf_ct_add_synproxy(struct nf_conn *ct, const struct nf_conn *tmpl)
{
	if (tmpl && nfct_synproxy(tmpl)) {
		if (!nfct_seqadj_ext_add(ct))
			return false;

		if (!nfct_synproxy_ext_add(ct))
			return false;
	}

	return true;
}

> diff --git a/net/netfilter/nf_nat_core.c b/net/netfilter/nf_nat_core.c
> index de31818..f8b916a 100644
> --- a/net/netfilter/nf_nat_core.c
> +++ b/net/netfilter/nf_nat_core.c
> @@ -441,7 +441,8 @@ nf_nat_setup_info(struct nf_conn *ct,
>  			ct->status |= IPS_DST_NAT;
>  
>  		if (nfct_help(ct))
> -			nfct_seqadj_ext_add(ct);
> +			if (!nfct_seqadj_ext_add(ct))
> +				return NF_ACCEPT;
>  	}

Hmm, why accept?

We are asked to add extension to rewrite sequence numbers, but
we cannot.  How can the connection work if we cannot munge/track
seqno rewrites?