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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Thu, 23 Jun 2022 21:11:32 +0200
Message-ID: <CAHmME9qGQrgCEGgQpomq6W2EMUy_D5AxqgYHykmmgND+PPVjjw@mail.gmail.com>
Subject: Re: [PATCH v3] timekeeping: contribute wall clock to rng on time change
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        stable <stable@vger.kernel.org>
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On Thu, Jun 23, 2022 at 9:10 PM Eric Biggers <ebiggers@kernel.org> wrote:
>
> On Thu, Jun 23, 2022 at 09:00:14PM +0200, Jason A. Donenfeld wrote:
> > The rng's random_init() function contributes the real time to the rng at
> > boot time, so that events can at least start in relation to something
> > particular in the real world. But this clock might not yet be set that
> > point in boot, so nothing is contributed. In addition, the relation
> > between minor clock changes from, say, NTP, and the cycle counter is
> > potentially useful entropic data.
> >
> > This commit addresses this by mixing in a time stamp on calls to
> > settimeofday and adjtimex. No entropy is credited in doing so, so it
> > doesn't make initialization faster, but it is still useful input to
> > have.
> >
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> > ---
> >  kernel/time/timekeeping.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> > index 8e4b3c32fcf9..49ee8ef16544 100644
> > --- a/kernel/time/timekeeping.c
> > +++ b/kernel/time/timekeeping.c
> > @@ -23,6 +23,7 @@
> >  #include <linux/pvclock_gtod.h>
> >  #include <linux/compiler.h>
> >  #include <linux/audit.h>
> > +#include <linux/random.h>
> >
> >  #include "tick-internal.h"
> >  #include "ntp_internal.h"
> > @@ -1343,8 +1344,10 @@ int do_settimeofday64(const struct timespec64 *ts)
> >       /* Signal hrtimers about time change */
> >       clock_was_set(CLOCK_SET_WALL);
> >
> > -     if (!ret)
> > +     if (!ret) {
> >               audit_tk_injoffset(ts_delta);
> > +             add_device_randomness(&ts, sizeof(ts));
> > +     }
>
> It should be:
>
> add_device_randomness(ts, sizeof(*ts));

This simple patch... I swear I know C, I promise...

Thanks again, and sorry.

Jason