netfilter: nftables: disallow updates on table ownership

Disallow updating the ownership bit on an existing table: Do not allow
to grab ownership on an existing table. Do not allow to drop ownership
on an existing table.

Fixes: 6001a930 ("netfilter: nftables: introduce table ownership")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -916,6 +916,12 @@ static int nf_tables_updtable(struct nft_ctx *ctx)
 	if (flags == ctx->table->flags)
 		return 0;
 
+	if ((nft_table_has_owner(ctx->table) &&
+	     !(flags & NFT_TABLE_F_OWNER)) ||
+	    (!nft_table_has_owner(ctx->table) &&
+	     flags & NFT_TABLE_F_OWNER))
+		return -EOPNOTSUPP;
+
 	trans = nft_trans_alloc(ctx, NFT_MSG_NEWTABLE,
 				sizeof(struct nft_trans_table));
 	if (trans == NULL)