netfilter: nfnetlink: nfnetlink_unicast() reports EAGAIN instead of ENOBUFS

Frontend callback reports EAGAIN to nfnetlink to retry a command, this
is used to signal that module autoloading is required. Unfortunately,
nlmsg_unicast() reports EAGAIN in case the receiver socket buffer gets
full, so it enters a busy-loop.

This patch updates nfnetlink_unicast() to turn EAGAIN into ENOBUFS and
to use nlmsg_unicast(). Remove the flags field in nfnetlink_unicast()
since this is always MSG_DONTWAIT in the existing code which is exactly
what nlmsg_unicast() passes to netlink_unicast() as parameter.

Fixes: 96518518cc41 ("netfilter: add nftables")
Reported-by: Phil Sutter <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/linux/netfilter/nfnetlink.h

···
 int nfnetlink_send(struct sk_buff *skb, struct net *net, u32 portid,
 		   unsigned int group, int echo, gfp_t flags);
 int nfnetlink_set_err(struct net *net, u32 portid, u32 group, int error);
-int nfnetlink_unicast(struct sk_buff *skb, struct net *net, u32 portid,
-		      int flags);
+int nfnetlink_unicast(struct sk_buff *skb, struct net *net, u32 portid);
 
 static inline u16 nfnl_msg_type(u8 subsys, u8 msg_type)
 {

net/netfilter/nf_tables_api.c

···
 					nlh->nlmsg_seq, NFT_MSG_NEWTABLE, 0,
 					family, table);
 	if (err < 0)
-		goto err;
+		goto err_fill_table_info;
 
-	return nlmsg_unicast(nlsk, skb2, NETLINK_CB(skb).portid);
+	return nfnetlink_unicast(skb2, net, NETLINK_CB(skb).portid);
 
-err:
+err_fill_table_info:
 	kfree_skb(skb2);
 	return err;
 }
···
 					nlh->nlmsg_seq, NFT_MSG_NEWCHAIN, 0,
 					family, table, chain);
 	if (err < 0)
-		goto err;
+		goto err_fill_chain_info;
 
-	return nlmsg_unicast(nlsk, skb2, NETLINK_CB(skb).portid);
+	return nfnetlink_unicast(skb2, net, NETLINK_CB(skb).portid);
 
-err:
+err_fill_chain_info:
 	kfree_skb(skb2);
 	return err;
 }
···
 				       nlh->nlmsg_seq, NFT_MSG_NEWRULE, 0,
 				       family, table, chain, rule, NULL);
 	if (err < 0)
-		goto err;
+		goto err_fill_rule_info;
 
-	return nlmsg_unicast(nlsk, skb2, NETLINK_CB(skb).portid);
+	return nfnetlink_unicast(skb2, net, NETLINK_CB(skb).portid);
 
-err:
+err_fill_rule_info:
 	kfree_skb(skb2);
 	return err;
 }
···
 
 	err = nf_tables_fill_set(skb2, &ctx, set, NFT_MSG_NEWSET, 0);
 	if (err < 0)
-		goto err;
+		goto err_fill_set_info;
 
-	return nlmsg_unicast(nlsk, skb2, NETLINK_CB(skb).portid);
+	return nfnetlink_unicast(skb2, net, NETLINK_CB(skb).portid);
 
-err:
+err_fill_set_info:
 	kfree_skb(skb2);
 	return err;
 }
···
 	err = -ENOMEM;
 	skb = nlmsg_new(NLMSG_GOODSIZE, GFP_ATOMIC);
 	if (skb == NULL)
-		goto err1;
+		return err;
 
 	err = nf_tables_fill_setelem_info(skb, ctx, ctx->seq, ctx->portid,
 					  NFT_MSG_NEWSETELEM, 0, set, &elem);
 	if (err < 0)
-		goto err2;
+		goto err_fill_setelem;
 
-	err = nfnetlink_unicast(skb, ctx->net, ctx->portid, MSG_DONTWAIT);
-	/* This avoids a loop in nfnetlink. */
-	if (err < 0)
-		goto err1;
+	return nfnetlink_unicast(skb, ctx->net, ctx->portid);
 
-	return 0;
-err2:
+err_fill_setelem:
 	kfree_skb(skb);
-err1:
-	/* this avoids a loop in nfnetlink. */
-	return err == -EAGAIN ? -ENOBUFS : err;
+	return err;
 }
 
 /* called with rcu_read_lock held */
···
 				      nlh->nlmsg_seq, NFT_MSG_NEWOBJ, 0,
 				      family, table, obj, reset);
 	if (err < 0)
-		goto err;
+		goto err_fill_obj_info;
 
-	return nlmsg_unicast(nlsk, skb2, NETLINK_CB(skb).portid);
-err:
+	return nfnetlink_unicast(skb2, net, NETLINK_CB(skb).portid);
+
+err_fill_obj_info:
 	kfree_skb(skb2);
 	return err;
 }
···
 					    NFT_MSG_NEWFLOWTABLE, 0, family,
 					    flowtable, &flowtable->hook_list);
 	if (err < 0)
-		goto err;
+		goto err_fill_flowtable_info;
 
-	return nlmsg_unicast(nlsk, skb2, NETLINK_CB(skb).portid);
-err:
+	return nfnetlink_unicast(skb2, net, NETLINK_CB(skb).portid);
+
+err_fill_flowtable_info:
 	kfree_skb(skb2);
 	return err;
 }
···
 	err = nf_tables_fill_gen_info(skb2, net, NETLINK_CB(skb).portid,
 				      nlh->nlmsg_seq);
 	if (err < 0)
-		goto err;
+		goto err_fill_gen_info;
 
-	return nlmsg_unicast(nlsk, skb2, NETLINK_CB(skb).portid);
-err:
+	return nfnetlink_unicast(skb2, net, NETLINK_CB(skb).portid);
+
+err_fill_gen_info:
 	kfree_skb(skb2);
 	return err;
 }

net/netfilter/nfnetlink.c

···
 }
 EXPORT_SYMBOL_GPL(nfnetlink_set_err);
 
-int nfnetlink_unicast(struct sk_buff *skb, struct net *net, u32 portid,
-		      int flags)
+int nfnetlink_unicast(struct sk_buff *skb, struct net *net, u32 portid)
 {
-	return netlink_unicast(net->nfnl, skb, portid, flags);
+	int err;
+
+	err = nlmsg_unicast(net->nfnl, skb, portid);
+	if (err == -EAGAIN)
+		err = -ENOBUFS;
+
+	return err;
 }
 EXPORT_SYMBOL_GPL(nfnetlink_unicast);
 

net/netfilter/nfnetlink_log.c

···
 			goto out;
 		}
 	}
-	nfnetlink_unicast(inst->skb, inst->net, inst->peer_portid,
-			  MSG_DONTWAIT);
+	nfnetlink_unicast(inst->skb, inst->net, inst->peer_portid);
 out:
 	inst->qlen = 0;
 	inst->skb = NULL;

net/netfilter/nfnetlink_queue.c

···
 	*packet_id_ptr = htonl(entry->id);
 
 	/* nfnetlink_unicast will either free the nskb or add it to a socket */
-	err = nfnetlink_unicast(nskb, net, queue->peer_portid, MSG_DONTWAIT);
+	err = nfnetlink_unicast(nskb, net, queue->peer_portid);
 	if (err < 0) {
 		if (queue->flags & NFQA_CFG_F_FAIL_OPEN) {
 			failopen = 1;