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

[Pablo Neira Ayuso <pablo@netfilter.org>]

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@syzkaller.appspotmail.com
> Signed-off-by: Florian Westphal <fw@strlen.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?

Let me know, thanks.