netdev - Re: iptables userspace API broken due to added value in nf_inet

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Message-ID: <20201014130156.GB21886@salvia>
Date:   Wed, 14 Oct 2020 15:01:56 +0200
From:   Pablo Neira Ayuso <pablo@...filter.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     netfilter-devel@...r.kernel.org, Netdev <netdev@...r.kernel.org>
Subject: Re: iptables userspace API broken due to added value in nf_inet_hooks

On Wed, Oct 14, 2020 at 03:01:15PM +0200, Pablo Neira Ayuso wrote:
> On Wed, Oct 14, 2020 at 02:59:47PM +0200, Jason A. Donenfeld wrote:
> > Hey Pablo,
> > 
> > In 60a3815da702fd9e4759945f26cce5c47d3967ad, you added another enum
> > value to nf_inet_hooks:
> > 
> > --- a/include/uapi/linux/netfilter.h
> > +++ b/include/uapi/linux/netfilter.h
> > @@ -45,6 +45,7 @@ enum nf_inet_hooks {
> >        NF_INET_FORWARD,
> >        NF_INET_LOCAL_OUT,
> >        NF_INET_POST_ROUTING,
> > +       NF_INET_INGRESS,
> >        NF_INET_NUMHOOKS
> > };
> > 
> > That seems fine, but actually it changes the value of
> > NF_INET_NUMHOOKS, which is used in struct ipt_getinfo:
> > 
> > /* The argument to IPT_SO_GET_INFO */
> > struct ipt_getinfo {
> >        /* Which table: caller fills this in. */
> >        char name[XT_TABLE_MAXNAMELEN];
> > 
> >        /* Kernel fills these in. */
> >        /* Which hook entry points are valid: bitmask */
> >        unsigned int valid_hooks;
> > 
> >        /* Hook entry points: one per netfilter hook. */
> >        unsigned int hook_entry[NF_INET_NUMHOOKS];
> > 
> >        /* Underflow points. */
> >        unsigned int underflow[NF_INET_NUMHOOKS];
> > 
> >        /* Number of entries */
> >        unsigned int num_entries;
> > 
> >        /* Size of entries. */
> >        unsigned int size;
> > };
> > 
> > This in turn makes that struct bigger, which means this check in
> > net/ipv4/netfilter/ip_tables.c fails:
> > 
> > static int get_info(struct net *net, void __user *user, const int *len)
> > {
> >        char name[XT_TABLE_MAXNAMELEN];
> >        struct xt_table *t;
> >        int ret;
> > 
> >        if (*len != sizeof(struct ipt_getinfo))
> >                return -EINVAL;
> > 
> > This is affecting my CI, which attempts to use an older iptables with
> > net-next and fails with:
> > 
> > iptables v1.8.4 (legacy): can't initialize iptables table `filter':
> > Module is wrong version
> > Perhaps iptables or your kernel needs to be upgraded.
> > 
> > Is this kind of breakage okay? If there's an exception carved out for
> > breaking the iptables API, just let me know, and I'll look into making
> > adjustments to work around it in my CI. On the other hand, if this
> > breakage was unintentional, now you know.
> 
> Oh right, I'll need a new IPT_INET_NUMHOOKS for this.
> 
> I'll submit a patch, that's for the heads up.

s/that's/thanks