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From:   Paul Moore <paul@paul-moore.com>
Date:   Wed, 22 Aug 2018 17:27:17 -0400
Message-ID: <CAHC9VhQCB5kB9Vy-TWwu-Z62p2XcPpKwsFLB647=usVM2EE=LA@mail.gmail.com>
Subject: Re: [RFC PATCH ghak10 v4 0/2] audit: Log modifying adjtimex(2) calls
To:     mlichvar@redhat.com, omosnace@redhat.com
Cc:     tglx@linutronix.de, linux-audit@redhat.com, rgb@redhat.com,
        sgrubb@redhat.com, john.stultz@linaro.org, sboyd@kernel.org,
        linux-kernel@vger.kernel.org
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On Tue, Aug 21, 2018 at 3:21 AM Miroslav Lichvar <mlichvar@redhat.com> wrote:
> > On Mon, 20 Aug 2018, Ondrej Mosnacek wrote:
> > > @John or other timekeeping/NTP folks: We had a discussion on the audit
> > > ML on which of the internal timekeeping/NTP variables we should actually
> > > log changes for. We are only interested in variables that can (directly
> > > or indirectly) cause noticeable changes to the system clock, but since we
> > > have only limited understanding of the NTP code, we would like to ask
> > > you for advice on which variables are security relevant.
>
> I guess that mostly depends on whether you consider setting the clock
> to run faster or slower than real time to be an important event for
> the audit.
>
> > >   - NTP value adjustments:
> > >     - time_offset (probably important)
>
> This can adjust the clock by up to 0.5 seconds per call and also speed
> it up or slow down by up to about 0.05% (43 seconds per day).

This seems worthwhile.

> > >     - time_freq (maybe not important?)
>
> This can speed up or slow down by up to about 0.05%.

This too.

> > >     - time_status (likely important, can cause leap second injection)
>
> Yes, it can insert/delete leap seconds and it also enables/disables
> synchronization of the hardware real-time clock.

This one as well.

> > >     - time_maxerror (maybe not important?)
> > >     - time_esterror (maybe not important?)
>
> These two change the error estimates that are reported to applications
> using ntp_gettime()/adjtimex(). If an application was periodically
> checking that the clock is synchronized with some specified accuracy
> and setting the maxerror to a larger value would cause the application
> to abort, would it be an important event in the audit?

Since these don't really affect the time, just the expected error, I'm
not sure this is important.

> > >     - time_constant (???)
>
> This controls the speed of the clock adjustments that are made when
> time_offset is set. Probably not important for the audit.

Agreed.  I think we can skip this.

> > >     - time_adjust (sounds important)
>
> This is similar to time_freq. It can temporarily speed up or slow down
> the clock by up to 0.05%.

Like time_freq, we should probably log this too.

> > >     - tick_usec (???)
>
> This is a more extreme version of time_freq. It can speed up or slow
> down the clock by up to 10%.

Let's audit this one too.

-- 
paul moore
www.paul-moore.com