[PATCH ghak25 v4 3/3] audit: add subj creds to NETFILTER_CFG record to cover async unregister

[Casey Schaufler]

On 5/17/2020 7:15 AM, Richard Guy Briggs wrote:
> On 2020-04-28 18:25, Paul Moore wrote:
>> On Wed, Apr 22, 2020 at 5:40 PM Richard Guy Briggs <rgb at redhat.com> wrote:
>>> Some table unregister actions seem to be initiated by the kernel to
>>> garbage collect unused tables that are not initiated by any userspace
>>> actions.  It was found to be necessary to add the subject credentials to
>>> cover this case to reveal the source of these actions.  A sample record:
>>>
>>>   type=NETFILTER_CFG msg=audit(2020-03-11 21:25:21.491:269) : table=nat family=bridge entries=0 op=unregister pid=153 uid=root auid=unset tty=(none) ses=unset subj=system_u:system_r:kernel_t:s0 comm=kworker/u4:2 exe=(null)
>> [I'm going to comment up here instead of in the code because it is a
>> bit easier for everyone to see what the actual impact might be on the
>> records.]
>>
>> Steve wants subject info in this case, okay, but let's try to trim out
>> some of the fields which simply don't make sense in this record; I'm
>> thinking of fields that are unset/empty in the kernel case and are
>> duplicates of other records in the userspace/syscall case.  I think
>> that means we can drop "tty", "ses", "comm", and "exe" ... yes?
>>
>> While "auid" is a potential target for removal based on the
>> dup-or-unset criteria, I think it falls under Steve's request for
>> subject info here, even if it is garbage in this case.
> Can you explain why auid falls under this criteria but ses does not if
> both are unset?  If auid is unset then we know ses is unset?  If subj
> contains *:kernel_t:* then uid can also be dropped even though it is
> set, no?

That's going to be up to the security module. SELinux may know that a
task with a subj= *:kernel_t:* doesn't need an uid, but that's not
going to be true with Smack, or if in the (distant?) future you
have both SELinux and Smack. Creating a way for the security module
to inform the audit system that it believes fields are unnecessary
sounds tricky. Not to mention that it's likely to create cases where
the audit user-space has to know which, if any, security modules are
active.

>  I figure if we are going to start dropping fields, might as
> well drop enough to make it worth the effort, even though this is a rare
> event.
>
> As for searchability, I have solved that easily in the parser.
>
>>> Signed-off-by: Richard Guy Briggs <rgb at redhat.com>
>>> ---
>>>  kernel/auditsc.c | 18 ++++++++++++++++++
>>>  1 file changed, 18 insertions(+)
>>>
>>> diff --git a/kernel/auditsc.c b/kernel/auditsc.c
>>> index d281c18d1771..d7a45b181be0 100644
>>> --- a/kernel/auditsc.c
>>> +++ b/kernel/auditsc.c
>>> @@ -2557,12 +2557,30 @@ void __audit_log_nfcfg(const char *name, u8 af, unsigned int nentries,
>>>                        enum audit_nfcfgop op)
>>>  {
>>>         struct audit_buffer *ab;
>>> +       const struct cred *cred;
>>> +       struct tty_struct *tty;
>>> +       char comm[sizeof(current->comm)];
>>>
>>>         ab = audit_log_start(audit_context(), GFP_KERNEL, AUDIT_NETFILTER_CFG);
>>>         if (!ab)
>>>                 return;
>>>         audit_log_format(ab, "table=%s family=%u entries=%u op=%s",
>>>                          name, af, nentries, audit_nfcfgs[op].s);
>>> +
>>> +       cred = current_cred();
>>> +       tty = audit_get_tty();
>>> +       audit_log_format(ab, " pid=%u uid=%u auid=%u tty=%s ses=%u",
>>> +                        task_pid_nr(current),
>>> +                        from_kuid(&init_user_ns, cred->uid),
>>> +                        from_kuid(&init_user_ns, audit_get_loginuid(current)),
>>> +                        tty ? tty_name(tty) : "(none)",
>>> +                        audit_get_sessionid(current));
>>> +       audit_put_tty(tty);
>>> +       audit_log_task_context(ab); /* subj= */
>>> +       audit_log_format(ab, " comm=");
>>> +       audit_log_untrustedstring(ab, get_task_comm(comm, current));
>>> +       audit_log_d_path_exe(ab, current->mm); /* exe= */
>>> +
>>>         audit_log_end(ab);
>>>  }
>>>  EXPORT_SYMBOL_GPL(__audit_log_nfcfg);
>> paul moore
> - RGB
>
> --
> Richard Guy Briggs <rgb at redhat.com>
> Sr. S/W Engineer, Kernel Security, Base Operating Systems
> Remote, Ottawa, Red Hat Canada
> IRC: rgb, SunRaycer
> Voice: +1.647.777.2635, Internal: (81) 32635
>
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