linux-kernel - Re: [PATCH v2] ntp: remove accidental integer wrap-around

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Message-ID: <CAFhGd8o0eFh0sqXtOcw=E+WBaKCTatpv18fA1Pzr_M1aC9OQ=A@mail.gmail.com>
Date: Fri, 24 May 2024 15:43:35 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: John Stultz <jstultz@...gle.com>, Stephen Boyd <sboyd@...nel.org>, 
	Nathan Chancellor <nathan@...nel.org>, Bill Wendling <morbo@...gle.com>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, linux-kernel@...r.kernel.org, 
	llvm@...ts.linux.dev, linux-hardening@...r.kernel.org, 
	Miroslav Lichvar <mlichvar@...hat.com>
Subject: Re: [PATCH v2] ntp: remove accidental integer wrap-around

Thomas,

I appreciate you reviewing my patches.

On Fri, May 24, 2024 at 5:09 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Fri, May 17 2024 at 20:22, Justin Stitt wrote:
> > time_maxerror is unconditionally incremented and the result is checked
> > against NTP_PHASE_LIMIT, but the increment itself can overflow,
> > resulting in wrap-around to negative space.
> >
> > The user can supply some crazy values which is causing the overflow. Add
> > an extra validation step checking that maxerror is reasonable.
>
> The user can supply any value which can cause an overflow as the input
> is unchecked. Add ...
>
> Hmm?
>
> > diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> > index b58dffc58a8f..321f251c02aa 100644
> > --- a/kernel/time/timekeeping.c
> > +++ b/kernel/time/timekeeping.c
> > @@ -2388,6 +2388,11 @@ static int timekeeping_validate_timex(const struct __kernel_timex *txc)
> >               }
> >       }
> >
> > +     if (txc->modes & ADJ_MAXERROR) {
> > +             if (txc->maxerror < 0 || txc->maxerror > NTP_PHASE_LIMIT)
> > +                     return -EINVAL;
> > +     }
>
> I dug into history to find a Fixes tag. That unearthed something
> interesting.  Exactly this check used to be there until commit
> eea83d896e31 ("ntp: NTP4 user space bits update") which landed in
> 2.6.30. The change log says:

Thanks for doing the archaeology.

>
>     "If some values for adjtimex() are outside the acceptable range, they
>      are now simply normalized instead of letting the syscall fail."
>
> The problem with that commit is that it did not do any normalization at
> all and just relied on the actual time_maxerror handling in
> second_overflow(), which is both insufficient and also prone to that
> overflow issue.
>
> So instead of turning the clock back, we might be better off to actually
> put the normalization in place at the assignment:
>
>     time_maxerror = min(max(0, txc->maxerror), NTP_PHASE_LIMIT);

A saturating resolution strategy is one that I've taken with some of
my other overflow patches.

.. but how about: clamp(txc->maxerror, 0, NTP_PHASE_LIMIT)

>
> or something like that.
>
> Miroslav: Any opinion on that?
>
> Thanks,
>
>         tglx

Anyways, I'm waiting to see how the whole overflow/wraparound
discussion in general evolves and, of course, how the local discussion
about this patch shapes up.

Thanks
Justin