seccomp and audit_enabled

[Tony Jones]

On 10/13/2015 09:11 AM, Paul Moore wrote:
> On Mon, Oct 12, 2015 at 4:45 PM, Kees Cook <keescook at chromium.org> wrote:
>> On Mon, Oct 12, 2015 at 10:53 AM, Tony Jones <tonyj at suse.de> wrote:
>>> From d6971ec9508244f7a1ab42f9ac4c59b7e1ca6145 Mon Sep 17 00:00:00 2001
>>> From: Tony Jones <tonyj at suse.de>
>>> Date: Sat, 10 Oct 2015 19:30:49 -0700
>>> Subject: [PATCH] Don't log seccomp messages when audit is disabled
>>>
>>> Don't log seccomp messages when audit is disabled.
>>
>> This is intentional since violation of a seccomp policy ought to
>> indicate a misbehaving program, and we want these to always be
>> presented to the system log, regardless of audit being enabled. (I'd
>> like to even produce system log entries when there is no CONFIG_AUDIT
>> too, but that's for the future.)
> 
> I agree.  As I mentioned earlier these AUDIT_SECCOMP records are very handy.
> 
>>> diff --git a/include/linux/audit.h b/include/linux/audit.h
>>> index b2abc99..8f70f3f 100644
>>> --- a/include/linux/audit.h
>>> +++ b/include/linux/audit.h
>>> @@ -113,6 +113,12 @@ struct filename;
>>>
>>>  extern void audit_log_session_info(struct audit_buffer *ab);
>>>
>>> +#ifdef CONFIG_AUDIT
>>> +extern u32 audit_enabled;
>>> +#else
>>> +#define audit_enabled 0
>>> +#endif
>>> +
>>>  #ifdef CONFIG_AUDIT_COMPAT_GENERIC
>>>  #define audit_is_compat(arch)  (!((arch) & __AUDIT_ARCH_64BIT))
>>>  #else
>>> @@ -213,7 +219,7 @@ void audit_core_dumps(long signr);
>>>  static inline void audit_seccomp(unsigned long syscall, long signr, int code)
>>>  {
>>>         /* Force a record to be reported if a signal was delivered. */
>>> -       if (signr || unlikely(!audit_dummy_context()))
>>
>> What is dummy_context part of this actually do? I don't think reports
>> should be made when signr == 0.
> 
> The idea behind audit_dummy_context() is to skip auditing when there
> are no audit rules configured, it's a performance tweak.  My guess is
> that Tony's system loads some audit configuration at boot which
> enables audit (the kernel starts with audit_enabled=0 ...) and loads a
> few syscall filter rules which are enough to make
> audit_dummy_context() return false.  Can you confirm that Tony?

No, it's the default audit.rules (-D, -b320).   No actual rules loaded. 
Let me add some instrumentation and figure out what's going on.  auditd
is masked (via systemd) but systemd-journal seems to set audit_enabled=1 
during startup (at least on our systems).

> As for logging seccomp actions when signr == 0, I personally think
> that still might be useful as the normal behavior has been altered; I
> tend to think any action != ALLOW is worth logging.  However, I'm open
> to discussion on this if others feel strongly.
> 
>>> +       if (audit_enabled && (signr || unlikely(!audit_dummy_context())))
>>>                 __audit_seccomp(syscall, signr, code);
>>>  }

I'm of the opinion that nothing should get output (through the audit system) if 
audit_enabled == 0.  What you advocate calls for more than 2 possible states for 
audit_enabled or logging the information through another mechanism than audit.

Tony