Message-ID: <CAGXu5jLJd+JufiKd3azXgg1C-7or50BP_ShNq6VzR67J2PQe7w@mail.gmail.com>
Date:   Tue, 3 Jan 2017 12:44:41 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Paul Moore <paul@...l-moore.com>
Cc:     Tyler Hicks <tyhicks@...onical.com>,
        Eric Paris <eparis@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, linux-audit@...hat.com,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] Begin auditing SECCOMP_RET_ERRNO return actions

On Tue, Jan 3, 2017 at 11:42 AM, Paul Moore <paul@...l-moore.com> wrote:
> On Tue, Jan 3, 2017 at 8:31 AM, Tyler Hicks <tyhicks@...onical.com> wrote:
>> On 01/02/2017 04:47 PM, Paul Moore wrote:
>>> On Mon, Jan 2, 2017 at 11:53 AM, Tyler Hicks <tyhicks@...onical.com> wrote:
>>>> This patch set creates the basis for auditing information specific to a given
>>>> seccomp return action and then starts auditing SECCOMP_RET_ERRNO return
>>>> actions. The audit messages for SECCOMP_RET_ERRNO return actions include the
>>>> errno value that will be returned to userspace.
>>>
>>> I'm replying to this patchset posting because it his my inbox first,
>>> but my comments here apply to both this patchset and the other
>>> seccomp/audit patchset you posted.
>>>
>>> In my experience, we have two or three problems (the count varies
>>> depending on perspective) when it comes to seccomp filter reporting:
>>>
>>> 1. Inability to log all filter actions.
>>> 2. Inability to selectively enable filtering; e.g. devs want noisy
>>> logging, users want relative quiet.
>>> 3. Consistent behavior with audit enabled and disabled.
>>
>> Agreed. Those three logging issues are what have been nagging me the most.
>
> /me nods

I think this sounds fine too.

>>> My current thinking - forgive me, this has been kicking around in my
>>> head for the better part of six months (longer?) and I haven't
>>> attempted to code it up - is to create a sysctl knob for a system wide
>>> seccomp logging threshold that would be applied to the high 16-bits of
>>> *every* triggered action: if the action was at/below the threshold a
>>> record would be emitted, otherwise silence.  This should resolve
>>> problems #1 and #2, and the code should be relatively straightforward
>>> and small.
>>
>> I like that idea quite a bit. To be completely honest, for #1, I
>> personally only care about logging SECCOMP_RET_ERRNO actions but this
>> idea solves it in a nice and general way.
>
> Yeah, I'd much rather solve this problem generally; everybody has
> their favorite action and I'd like to avoid solving the same problem
> multiple times.
>
> Sooo ... you want to take a whack at coding this up? ;)
>
>>> As part of the code above, I expect that all seccomp logging would get
>>> routed through a single logging function (sort of like a better
>>> implementation of the existing audit_seccomp()) that would check the
>>> threshold and trigger the logging if needed.  This function could be
>>> augmented to check for CONFIG_AUDIT and in the case where audit was
>>> not built into the kernel, a simple printk could be used to log the
>>> seccomp event; solving problem #3.
>>
>> That doesn't fully solve #3 for me. In Ubuntu (and I think Debian), we
>> build with CONFIG_AUDIT enabled but don't ship auditd by default so
>> audit_enabled is false. In that default configuration, we still want
>> seccomp audit messages to be printk'ed. I'll need to figure out how to
>> cleanly allow opting into seccomp audit messages when CONFIG_AUDIT is
>> enabled and audit_enabled is false.
>
> Heh, so you've got audit built into the kernel but you're not using
> it; that sounds "fun".
>
> Anyway, I think the logging consolidation could still help you, if for
> no other reason than everything is going through the same function at
> that point.  We could do some other stuff there to handle the case
> where audit is compiled, but auditd is not running ... we already have
> some code in place to handle that for other reasons, check
> kernel/audit.c for more information.  I'd still work on the other
> stuff first and then we can add this in at the end of the patchset.

Yeah, I think the "should I report it?" threshold sysctl could just
check if audit is enabled...

I still wonder, though, isn't there a way to use auditctl to get all
the seccomp messages you need?

-Kees

>
>>> We could also add a SECCOMP_RET_AUDIT, or similar, if we still feel
>>> that is important (I personally waffle on this), but I think that is
>>> independent of the ideas above.
>>
>> I agree that it is independent but SECCOMP_RET_AUDIT would still be
>> important to Ubuntu.
>>
>> Tyler
>
> --
> paul moore
> www.paul-moore.com



-- 
Kees Cook
Nexus Security

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