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From: Thomas Gleixner <tglx@linutronix.de>
To: Justin Stitt <justinstitt@google.com>, John Stultz <jstultz@google.com>,
 Stephen Boyd <sboyd@kernel.org>, Nathan Chancellor <nathan@kernel.org>,
 Bill Wendling <morbo@google.com>, Nick Desaulniers
 <ndesaulniers@google.com>
Cc: linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
 linux-hardening@vger.kernel.org, Justin Stitt <justinstitt@google.com>,
 Miroslav Lichvar <mlichvar@redhat.com>
Subject: Re: [PATCH v2] ntp: remove accidental integer wrap-around
In-Reply-To: <87ed9re7i4.ffs@tglx>
References: <20240517-b4-sio-ntp-usec-v2-1-d539180f2b79@google.com>
 <87ed9re7i4.ffs@tglx>
Date: Fri, 24 May 2024 14:44:19 +0200
Message-ID: <87bk4ve5wc.ffs@tglx>
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On Fri, May 24 2024 at 14:09, Thomas Gleixner wrote:
> On Fri, May 17 2024 at 20:22, Justin Stitt wrote:
> 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:
>
>     "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);
>
> or something like that.

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.

Thanks,

        tglx