netfilter: nf_tables: validate NFTA_SET_TABLE parameter

If the NFTA_SET_TABLE parameter is missing and the NLM_F_DUMP flag is
not set, then a NULL pointer dereference is triggered in
nf_tables_set_lookup because ctx.table is NULL.
Signed-off-by: Phil Turnbull <phil.turnbull@oracle.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -2641,6 +2641,8 @@ static int nf_tables_getset(struct net *net, struct sock *nlsk,
 	/* Only accept unspec with dump */
 	if (nfmsg->nfgen_family == NFPROTO_UNSPEC)
 		return -EAFNOSUPPORT;
+	if (!nla[NFTA_SET_TABLE])
+		return -EINVAL;
 
 	set = nf_tables_set_lookup(ctx.table, nla[NFTA_SET_NAME]);
 	if (IS_ERR(set))