linux-kernel - Re: [RFC PATCH ghak10 v4 0/2] audit: Log modifying adjtimex(2) calls

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Message-ID: <20180821072114.GC23069@localhost>
Date:   Tue, 21 Aug 2018 09:21:14 +0200
From:   Miroslav Lichvar <mlichvar@...hat.com>
To:     Ondrej Mosnacek <omosnace@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-audit@...hat.com,
        Paul Moore <paul@...l-moore.com>,
        Richard Guy Briggs <rgb@...hat.com>,
        Steve Grubb <sgrubb@...hat.com>,
        John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH ghak10 v4 0/2] audit: Log modifying adjtimex(2) calls

> 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).

> >     - time_freq (maybe not important?)

This can speed up or slow down by up to about 0.05%.

> >     - 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.

> >     - 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?

> >     - time_constant (???)

This controls the speed of the clock adjustments that are made when
time_offset is set. Probably not important for the audit.

> >     - 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%.

> >     - tick_usec (???)

This is a more extreme version of time_freq. It can speed up or slow
down the clock by up to 10%.

-- 
Miroslav Lichvar