linux-kernel - Re: [PATCH] timekeeping/vsyscall: Prevent math overflow in BOOTTIME update

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Date:   Thu, 22 Aug 2019 13:52:40 +0100
From:   Chris Clayton <chris2553@...glemail.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, vincenzo.frascino@....com,
        catalin.marinas@....com, Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] timekeeping/vsyscall: Prevent math overflow in BOOTTIME
 update

Thanks Thomas.

On 22/08/2019 12:00, Thomas Gleixner wrote:
> The VDSO update for CLOCK_BOOTTIME has a overflow issue as it shifts the
> nanoseconds based boot time offset left by the clocksource shift. That
> overflows once the boot time offset becomes large enough. As a consequence
> CLOCK_BOOTTIME in the VDSO becomes a random number causing applications to
> misbehave.
> 
> Fix it by storing a timespec64 representation of the offset when boot time
> is adjusted and add that to the MONOTONIC base time value in the vdso data
> page. Using the timespec64 representation avoids a 64bit division in the
> update code.
> 

I've tested resume from both suspend and hibernate and this patch fixes the problem I reported.

Tested-by: Chris Clayton <chris2553@...glemail.com>

> Fixes: 44f57d788e7d ("timekeeping: Provide a generic update_vsyscall() implementation")
> Reported-by: Chris Clayton <chris2553@...glemail.com>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  include/linux/timekeeper_internal.h |    5 +++++
>  kernel/time/timekeeping.c           |    5 +++++
>  kernel/time/vsyscall.c              |   22 +++++++++++++---------
>  3 files changed, 23 insertions(+), 9 deletions(-)
> 
> --- a/include/linux/timekeeper_internal.h
> +++ b/include/linux/timekeeper_internal.h
> @@ -57,6 +57,7 @@ struct tk_read_base {
>   * @cs_was_changed_seq:	The sequence number of clocksource change events
>   * @next_leap_ktime:	CLOCK_MONOTONIC time value of a pending leap-second
>   * @raw_sec:		CLOCK_MONOTONIC_RAW  time in seconds
> + * @monotonic_to_boot:	CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
>   * @cycle_interval:	Number of clock cycles in one NTP interval
>   * @xtime_interval:	Number of clock shifted nano seconds in one NTP
>   *			interval.
> @@ -84,6 +85,9 @@ struct tk_read_base {
>   *
>   * wall_to_monotonic is no longer the boot time, getboottime must be
>   * used instead.
> + *
> + * @monotonic_to_boottime is a timespec64 representation of @offs_boot to
> + * accelerate the VDSO update for CLOCK_BOOTTIME.
>   */
>  struct timekeeper {
>  	struct tk_read_base	tkr_mono;
> @@ -99,6 +103,7 @@ struct timekeeper {
>  	u8			cs_was_changed_seq;
>  	ktime_t			next_leap_ktime;
>  	u64			raw_sec;
> +	struct timespec64	monotonic_to_boot;
>  
>  	/* The following members are for timekeeping internal use */
>  	u64			cycle_interval;
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -146,6 +146,11 @@ static void tk_set_wall_to_mono(struct t
>  static inline void tk_update_sleep_time(struct timekeeper *tk, ktime_t delta)
>  {
>  	tk->offs_boot = ktime_add(tk->offs_boot, delta);
> +	/*
> +	 * Timespec representation for VDSO update to avoid 64bit division
> +	 * on every update.
> +	 */
> +	tk->monotonic_to_boot = ktime_to_timespec64(tk->offs_boot);
>  }
>  
>  /*
> --- a/kernel/time/vsyscall.c
> +++ b/kernel/time/vsyscall.c
> @@ -17,7 +17,7 @@ static inline void update_vdso_data(stru
>  				    struct timekeeper *tk)
>  {
>  	struct vdso_timestamp *vdso_ts;
> -	u64 nsec;
> +	u64 nsec, sec;
>  
>  	vdata[CS_HRES_COARSE].cycle_last	= tk->tkr_mono.cycle_last;
>  	vdata[CS_HRES_COARSE].mask		= tk->tkr_mono.mask;
> @@ -45,23 +45,27 @@ static inline void update_vdso_data(stru
>  	}
>  	vdso_ts->nsec	= nsec;
>  
> -	/* CLOCK_MONOTONIC_RAW */
> -	vdso_ts		= &vdata[CS_RAW].basetime[CLOCK_MONOTONIC_RAW];
> -	vdso_ts->sec	= tk->raw_sec;
> -	vdso_ts->nsec	= tk->tkr_raw.xtime_nsec;
> +	/* Copy MONOTONIC time for BOOTTIME */
> +	sec	= vdso_ts->sec;
> +	/* Add the boot offset */
> +	sec	+= tk->monotonic_to_boot.tv_sec;
> +	nsec	+= (u64)tk->monotonic_to_boot.tv_nsec << tk->tkr_mono.shift;
>  
>  	/* CLOCK_BOOTTIME */
>  	vdso_ts		= &vdata[CS_HRES_COARSE].basetime[CLOCK_BOOTTIME];
> -	vdso_ts->sec	= tk->xtime_sec + tk->wall_to_monotonic.tv_sec;
> -	nsec = tk->tkr_mono.xtime_nsec;
> -	nsec += ((u64)(tk->wall_to_monotonic.tv_nsec +
> -		       ktime_to_ns(tk->offs_boot)) << tk->tkr_mono.shift);
> +	vdso_ts->sec	= sec;
> +
>  	while (nsec >= (((u64)NSEC_PER_SEC) << tk->tkr_mono.shift)) {
>  		nsec -= (((u64)NSEC_PER_SEC) << tk->tkr_mono.shift);
>  		vdso_ts->sec++;
>  	}
>  	vdso_ts->nsec	= nsec;
>  
> +	/* CLOCK_MONOTONIC_RAW */
> +	vdso_ts		= &vdata[CS_RAW].basetime[CLOCK_MONOTONIC_RAW];
> +	vdso_ts->sec	= tk->raw_sec;
> +	vdso_ts->nsec	= tk->tkr_raw.xtime_nsec;
> +
>  	/* CLOCK_TAI */
>  	vdso_ts		= &vdata[CS_HRES_COARSE].basetime[CLOCK_TAI];
>  	vdso_ts->sec	= tk->xtime_sec + (s64)tk->tai_offset;
>