linux-kernel - Re: [PATCH] timekeeping: contribute wall clock to rng on time change

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Message-ID: <CAHmME9qy0N3BvDo-0jkS+om0N3Yk--ZAyKvSKshzDBzvuoP+UA@mail.gmail.com>
Date:   Thu, 23 Jun 2022 20:04:39 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] timekeeping: contribute wall clock to rng on time change

Hi Eric,

On Thu, Jun 23, 2022 at 7:39 PM Eric Biggers <ebiggers@...nel.org> wrote:
> This doesn't compile:

Doh. I had the missing include, but missed it in `git add -p`. Will be
fixed for v2.

> > @@ -1346,6 +1346,9 @@ int do_settimeofday64(const struct timespec64 *ts)
> >       if (!ret)
> >               audit_tk_injoffset(ts_delta);
> >
> > +     ktime_get_real_ts64(&xt);
> > +     add_device_randomness(&xt, sizeof(xt));
> > +
> >       return ret;
>
> Isn't the new time already available in 'ts'?  Is the call to
> ktime_get_real_ts64() necessary?

Good point; you're right. Will simplify as such.

> >       ntp_notify_cmos_timer();
> >
> > +     ktime_get_real_ts64(&ts);
> > +     add_device_randomness(&ts, sizeof(ts));
> > +
> >       return ret;
> >  }
>
> adjtimex() actually triggers a gradual adjustment of the clock, rather than
> setting it immediately.  Is there a way to mix in the target time rather than
> the current time as this does?

Hmm... I have no idea. But maybe instead I can just call
`add_device_randomness()` on that big struct of offsets and things. If
the kernel is able to use it to set the time, then probably it's a
sufficient encoding of that end state.

Jason