linux-kernel - Re: [PATCH] ntp: verify offset doesn't overflow in ntp_update

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Message-ID: <CALAqxLWEvJJCKA0dS7QFTXy0oA2=NsKRwJ3WbKwfdCXUufxcPA@mail.gmail.com>
Date:	Mon, 7 Dec 2015 16:02:44 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ntp: verify offset doesn't overflow in ntp_update_offset

On Thu, Dec 3, 2015 at 12:46 PM, Sasha Levin <sasha.levin@...cle.com> wrote:
> We need to make sure that the offset is valid before manipulating it,
> otherwise it might overflow on the multiplication.
>
> Signed-off-by: Sasha Levin <sasha.levin@...cle.com>
> ---
>  kernel/time/ntp.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
> index 149cc80..36616c3 100644
> --- a/kernel/time/ntp.c
> +++ b/kernel/time/ntp.c
> @@ -297,6 +297,9 @@ static void ntp_update_offset(long offset)
>         if (!(time_status & STA_PLL))
>                 return;
>
> +       /* Make sure the multiplication below won't overflow */
> +       offset = clamp(offset, -MAXPHASE, MAXPHASE);
> +
>         if (!(time_status & STA_NANO))
>                 offset *= NSEC_PER_USEC;

So looking at this a bit closer, this bit looks sort of crazy since we
clam the offset, do the multiply and then do the exact same clamp.

I'd much rather do a more logical  clamp(offset, -USEC_PER_SEC,
USEC_PER_SEC), but only in the case where we do the multiply.

Any objection to that?

thanks
-john
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