Address NETFILTER_CFG issues

There were questions about the presence and cause of unsolicited syscall events in the logs containing NETFILTER_CFG records and sometimes unaccompanied NETFILTER_CFG records.

During testing at least the following list of events trigger NETFILTER_CFG records and the syscalls related (There may be more events that will trigger this message type.):
init_module, finit_module: modprobe
delete_module: rmmod
setsockopt: iptables-restore, ip6tables-restore, ebtables-restore
unshare: (h?)ostnamed, updatedb
clone: libvirtd
kernel threads garbage collecting empty ebtables

The syscall events unsolicited by any audit rule were found to be caused by a missing !audit_dummy_context() check before issuing a NETFILTER_CFG record. In fact, since this is a configuration change it is required, and we always want the accompanying syscall record even with no rules present, so this has been addressed by ghak120.

The vast majority of unaccompanied records are caused by the fedora default
rule: "-a never,task" and the occasional early startup one is I believe caused by the iptables filter table module hard linked into the kernel rather than a loadable module.

A couple of other factors should help eliminate unaccompanied records which include commit cb74ed278f80 ("audit: always enable syscall auditing when supported and audit is enabled") which makes sure that when audit is enabled, so automatically is syscall auditing, and ghak66 which addressed initializing audit before PID 1.

Ebtables module initialization to register tables doesn't generate records because it was never hooked in to audit. Recommend adding audit hooks to log this covered by ghak43 covered by patch 1.

Table unregistration was never logged, which is now covered by ghak44 in patch 2. Unaccompanied records were caused by kernel threads automatically unregistering empty ebtables, which necessitates adding subject credentials covered in patch 3.

Seemingly duplicate records are not actually exact duplicates that are caused by netfilter table initialization in different network namespaces from the same syscall. Recommend adding the network namespace ID (proc inode and dev) to the record to make this obvious (address later with ghak79 after nsid patches).

See: https://github.com/linux-audit/audit-kernel/issues/25
See: https://github.com/linux-audit/audit-kernel/issues/35
See: https://github.com/linux-audit/audit-kernel/issues/43
See: https://github.com/linux-audit/audit-kernel/issues/44

Changelog:
v4
- rebase on audit/next v5.7-rc1
- fix checkpatch.pl errors/warnings in 1/3 and 2/3

v3
- rebase on v5.6-rc1 audit/next
- change audit_nf_cfg to audit_log_nfcfg
- squash 2,3,4,5 to 1 and update patch descriptions
- add subject credentials to cover garbage collecting kernel threads

v2
- Rebase (audit/next 5.5-rc1) to get audit_context access and ebt_register_table ret code
- Split x_tables and ebtables updates
- Check audit_dummy_context
- Store struct audit_nfcfg params in audit_context, abstract to audit_nf_cfg() call
- Restore back to "table, family, entries" from "family, table, entries"
- Log unregistration of tables
- Add "op=" at the end of the AUDIT_NETFILTER_CFG record
- Defer nsid patch (ghak79) to once nsid patchset upstreamed (ghak32)
- Add ghak refs
- Ditch NETFILTER_CFGSOLO record

Richard Guy Briggs (3):
audit: tidy and extend netfilter_cfg x_tables and ebtables logging
netfilter: add audit table unregister actions
audit: add subj creds to NETFILTER_CFG record to cover async
unregister

 include/linux/audit.h           | 22 +++++++++++++++++++++
 kernel/auditsc.c                | 43 +++++++++++++++++++++++++++++++++++++++++
 net/bridge/netfilter/ebtables.c | 14 ++++++--------
 net/netfilter/x_tables.c        | 14 +++++---------
 4 files changed, 76 insertions(+), 17 deletions(-)

1.8.3.1