netdev - Re: [PATCH nf] netfilter: nf_tables: check for valid chain type pointer before dereference

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Date:   Tue, 21 Jan 2020 14:26:18 +0100
From:   Pablo Neira Ayuso <pablo@...filter.org>
To:     Florian Westphal <fw@...len.de>
Cc:     netfilter-devel@...r.kernel.org, netdev@...r.kernel.org,
        syzkaller-bugs@...glegroups.com,
        syzbot+156a04714799b1d480bc@...kaller.appspotmail.com
Subject: Re: [PATCH nf] netfilter: nf_tables: check for valid chain type
 pointer before dereference

On Sat, Jan 18, 2020 at 09:30:57PM +0100, Pablo Neira Ayuso wrote:
> On Thu, Jan 16, 2020 at 10:11:09PM +0100, Florian Westphal wrote:
> > Its possible to create tables in a family that isn't supported/known.
> > Then, when adding a base chain, the table pointer can be NULL.
> > 
> > This gets us a NULL ptr dereference in nf_tables_addchain().
> > 
> > Fixes: baae3e62f31618 ("netfilter: nf_tables: fix chain type module reference handling")
> > Reported-by: syzbot+156a04714799b1d480bc@...kaller.appspotmail.com
> > Signed-off-by: Florian Westphal <fw@...len.de>
> > ---
> >  net/netfilter/nf_tables_api.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
> > index 65f51a2e9c2a..e8976128cdb1 100644
> > --- a/net/netfilter/nf_tables_api.c
> > +++ b/net/netfilter/nf_tables_api.c
> > @@ -953,6 +953,9 @@ static int nf_tables_newtable(struct net *net, struct sock *nlsk,
> >  	struct nft_ctx ctx;
> >  	int err;
> >  
> > +	if (family >= NFPROTO_NUMPROTO)
> > +		return -EAFNOSUPPORT;
> > +
> >  	lockdep_assert_held(&net->nft.commit_mutex);
> >  	attr = nla[NFTA_TABLE_NAME];
> >  	table = nft_table_lookup(net, attr, family, genmask);
> > @@ -1765,6 +1768,9 @@ static int nft_chain_parse_hook(struct net *net,
> >  	    ha[NFTA_HOOK_PRIORITY] == NULL)
> >  		return -EINVAL;
> >  
> > +	if (family >= NFPROTO_NUMPROTO)
> > +		return -EAFNOSUPPORT;
> > +
> >  	hook->num = ntohl(nla_get_be32(ha[NFTA_HOOK_HOOKNUM]));
> >  	hook->priority = ntohl(nla_get_be32(ha[NFTA_HOOK_PRIORITY]));
> >  
> > @@ -1774,6 +1780,8 @@ static int nft_chain_parse_hook(struct net *net,
> >  						   family, autoload);
> >  		if (IS_ERR(type))
> >  			return PTR_ERR(type);
> > +	} else if (!type) {
> > +		return -EOPNOTSUPP;
> 
> I think this check should be enough.
> 
> I mean, NFPROTO_NUMPROTO still allows for creating tables for families
> that don't exist (<= NFPROTO_NUMPROTO) and why bother on creating such
> table. As long as such table does not crash the kernel, I think it's
> fine. No changes can be attached anymore anyway.
> 
> Otherwise, if a helper function to check for the families that are
> really supported could be another alternative. But not sure it is
> worth?

Not worth.

Probably this patch instead? Just make sure that access to the chain
type array is safe, no direct access to chain_type[][] anymore.

This includes the check for the default type too, since it cannot be
assume to always have a filter chain for unsupported families.

Thanks for explaining.

View attachment "x.patch" of type "text/x-diff" (1964 bytes)