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

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Date: Mon, 6 May 2024 22:54:30 -0700
From: John Stultz <jstultz@...gle.com>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Stephen Boyd <sboyd@...nel.org>, 
	Nathan Chancellor <nathan@...nel.org>, Bill Wendling <morbo@...gle.com>, linux-kernel@...r.kernel.org, 
	llvm@...ts.linux.dev, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] ntp: remove accidental integer wrap-around

On Mon, May 6, 2024 at 9:34 PM Justin Stitt <justinstitt@...gle.com> wrote:
> Let's introduce a new macro and use that against NTP_PHASE_LIMIT to
> properly limit the max size of time_maxerror without overflowing during
> the check itself.
>
> Link: https://github.com/llvm/llvm-project/pull/82432 [1]
> Closes: https://github.com/KSPP/linux/issues/354
> Cc: linux-hardening@...r.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> ---
>  include/linux/timex.h | 1 +
>  kernel/time/ntp.c     | 8 ++++----
>  2 files changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/timex.h b/include/linux/timex.h
> index 3871b06bd302..976490a06915 100644
> --- a/include/linux/timex.h
> +++ b/include/linux/timex.h
> @@ -138,6 +138,7 @@ unsigned long random_get_entropy_fallback(void);
>  #define MINSEC 256             /* min interval between updates (s) */
>  #define MAXSEC 2048            /* max interval between updates (s) */
>  #define NTP_PHASE_LIMIT ((MAXPHASE / NSEC_PER_USEC) << 5) /* beyond max. dispersion */
> +#define NTP_MAXFREQ_USEC (MAXFREQ / NSEC_PER_USEC) /* scaled to microseconds */
>
>  /*
>   * kernel variables
> diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
> index 406dccb79c2b..19027b6d0827 100644
> --- a/kernel/time/ntp.c
> +++ b/kernel/time/ntp.c
> @@ -454,12 +454,12 @@ int second_overflow(time64_t secs)
>         }
>
>
> -       /* Bump the maxerror field */
> -       time_maxerror += MAXFREQ / NSEC_PER_USEC;
> -       if (time_maxerror > NTP_PHASE_LIMIT) {
> +       /* Bump the maxerror field, making sure not to exceed NTP_PHASE_LIMIT */
> +       if (NTP_PHASE_LIMIT - NTP_MAXFREQ_USEC < time_maxerror) {
>                 time_maxerror = NTP_PHASE_LIMIT;
>                 time_status |= STA_UNSYNC;
> -       }
> +       } else
> +               time_maxerror += NTP_MAXFREQ_USEC;
>
>         /* Compute the phase adjustment for the next second */
>         tick_length      = tick_length_base;
>

Looks reasonable to me.
Acked-by: John Stultz <jstultz@...gle.com>

thanks
-john