netfilter: nftables: introduce table ownership

A userspace daemon like firewalld might need to monitor for netlink
updates to detect its ruleset removal by the (global) flush ruleset
command to ensure ruleset persistency. This adds extra complexity from
userspace and, for some little time, the firewall policy is not in
place.

This patch adds the NFT_TABLE_F_OWNER flag which allows a userspace
program to own the table that creates in exclusivity.

Tables that are owned...

- can only be updated and removed by the owner, non-owners hit EPERM if
  they try to update it or remove it.
- are destroyed when the owner closes the netlink socket or the process
  is gone (implicit netlink socket closure).
- are skipped by the global flush ruleset command.
- are listed in the global ruleset.

The userspace process that sets on the NFT_TABLE_F_OWNER flag need to
leave open the netlink socket.

A new NFTA_TABLE_OWNER netlink attribute specifies the netlink port ID
to identify the owner from userspace.

This patch also updates error reporting when an unknown table flag is
specified to change it from EINVAL to EOPNOTSUPP given that EINVAL is
usually reserved to report for malformed netlink messages to userspace.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_tables.h

@@ -1106,11 +1106,17 @@ struct nft_table {
 	u16				family:6,
 					flags:8,
 					genmask:2;
+	u32				nlpid;
 	char				*name;
 	u16				udlen;
 	u8				*udata;
 };
 
+static inline bool nft_table_has_owner(const struct nft_table *table)
+{
+	return table->flags & NFT_TABLE_F_OWNER;
+}
+
 static inline bool nft_base_chain_netdev(int family, u32 hooknum)
 {
 	return family == NFPROTO_NETDEV ||

include/uapi/linux/netfilter/nf_tables.h

@@ -164,7 +164,10 @@ enum nft_hook_attributes {
  */
 enum nft_table_flags {
 	NFT_TABLE_F_DORMANT	= 0x1,
+	NFT_TABLE_F_OWNER	= 0x2,
 };
+#define NFT_TABLE_F_MASK	(NFT_TABLE_F_DORMANT | \
+				 NFT_TABLE_F_OWNER)
 
 /**
  * enum nft_table_attributes - nf_tables table netlink attributes
@@ -173,6 +176,7 @@ enum nft_table_flags {
  * @NFTA_TABLE_FLAGS: bitmask of enum nft_table_flags (NLA_U32)
  * @NFTA_TABLE_USE: number of chains in this table (NLA_U32)
  * @NFTA_TABLE_USERDATA: user data (NLA_BINARY)
+ * @NFTA_TABLE_OWNER: owner of this table through netlink portID (NLA_U32)
  */
 enum nft_table_attributes {
 	NFTA_TABLE_UNSPEC,
@@ -182,6 +186,7 @@ enum nft_table_attributes {
 	NFTA_TABLE_HANDLE,
 	NFTA_TABLE_PAD,
 	NFTA_TABLE_USERDATA,
+	NFTA_TABLE_OWNER,
 	__NFTA_TABLE_MAX
 };
 #define NFTA_TABLE_MAX		(__NFTA_TABLE_MAX - 1)