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

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Message-ID: <ZlRDoyJk2Erzje_z@localhost>
Date: Mon, 27 May 2024 10:26:11 +0200
From: Miroslav Lichvar <mlichvar@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Justin Stitt <justinstitt@...gle.com>, 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
Subject: Re: [PATCH v2] ntp: remove accidental integer wrap-around

On Fri, May 24, 2024 at 02:44:19PM +0200, Thomas Gleixner wrote:
> On Fri, May 24 2024 at 14:09, Thomas Gleixner wrote:
> > 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);
> >
> > or something like that.

Yes, I think that's a better approach. Failing the system call could
break existing applications, e.g. ntpd can be configured to accept a
large root distance and it doesn't clamp the maxerror value, while
updating the PLL offset in the same adjtimex() call.

> So that commit also removed the sanity check for time_esterror, but
> that's not doing anything in the kernel other than being reset in
> clear_ntp() and being handed back to user space. No idea what this is
> actually used for.

It's a lower-bound estimate of the clock error, which applications can
check if it's acceptable for them. I think it should be clamped too.
It doesn't make much sense for it to be larger than the maximum error.

Another possible improvement of adjtimex() would be to set the UNSYNC
flag immediately in the call if maxerror >= 16s to avoid the delay of
up to 1 second for applications which check only that flag instead of
the maxerror value.

-- 
Miroslav Lichvar