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Date: Wed, 15 Apr 2015 12:17:36 -0400
From: Justin Keller <justincompsci@...il.com>
To: Miroslav Lichvar <mlichvar@...hat.com>
Cc: linux-kernel@...r.kernel.org, John Stultz <john.stultz@...aro.org>,
Thomas Gleixner <tglx@...utronix.de>,
Prarit Bhargava <prarit@...hat.com>,
Richard Cochran <richardcochran@...il.com>,
Arnd Bergmann <arnd@...db.de>
Subject: Re: [RFC][PATCH] timekeeping: Limit system time to prevent 32-bit
time_t overflow
Is there a reason for "step = leap"?
On Wed, Apr 15, 2015 at 11:41 AM, Miroslav Lichvar <mlichvar@...hat.com> wrote:
> On systems with 32-bit time_t, it seems there are quite a few problems
> that applications may have with time overflowing in year 2038. Beside
> getting in an unexpected state by not checking integer operations with
> time_t variables, some system calls have unexpected behavior, e.g. the
> system time can't be set back to the current time (negative value),
> timers with the ABSTIME flag can't be set (negative value) or they
> expire immediately (current time is always larger).
>
> It would be unrealistic to expect all applications to be able to handle
> all these problems. Year 2038 is still many years away, but this can be
> a problem even now. The clock can be set close to the overflow
> accidentally or maliciously, e.g. when the clock is synchronized over
> network by NTP or PTP.
>
> This patch sets a maximum value of the system time to prevent the system
> time from getting too close to the overflow. The time can't be set to a
> larger value. When the maximum is reached in normal time accumulation,
> the clock will be stepped back by one week.
>
> A new kernel sysctl time_max is added to select the maximum time. It can
> be set to 0 for no limit, 1 for one week before 32-bit time_t overflow,
> and 2 for one week before ktime_t overflow. The default value is 1 with
> 32-bit time_t and 2 with 64-bit time_t. This can be changed later to be
> always 2 when 64-bit versions of system calls using 32-bit time_t are
> available.
>
> Signed-off-by: Miroslav Lichvar <mlichvar@...hat.com>
> ---
> Documentation/sysctl/kernel.txt | 19 +++++++++++++
> include/linux/timekeeping.h | 5 ++++
> include/uapi/linux/sysctl.h | 1 +
> kernel/sysctl.c | 9 ++++++
> kernel/sysctl_binary.c | 1 +
> kernel/time/timekeeping.c | 62 +++++++++++++++++++++++++++++++++++++----
> 6 files changed, 91 insertions(+), 6 deletions(-)
>
> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> index 83ab256..08e4f28 100644
> --- a/Documentation/sysctl/kernel.txt
> +++ b/Documentation/sysctl/kernel.txt
> @@ -82,6 +82,7 @@ show up in /proc/sys/kernel:
> - sysctl_writes_strict
> - tainted
> - threads-max
> +- time_max
> - unknown_nmi_panic
> - watchdog_thresh
> - version
> @@ -847,6 +848,24 @@ can be ORed together:
>
> ==============================================================
>
> +time_max:
> +
> +Select the maximum allowed value of the system time. The system clock cannot be
> +set to a larger value and when it reaches the maximum on its own, it will be
> +stepped back by one week.
> +
> +0: No limit.
> +
> +1: One week before 32-bit time_t overflows, i.e. Jan 12 03:14:07 UTC 2038.
> + This is currently the default value with 32-bit time_t, but it will likely
> + change when 64-bit versions of system calls using time_t are available.
> +
> +2: One week before time in the internal kernel representation (ktime_t)
> + overflows, i.e. Apr 4 23:47:16 UTC 2262. This is the default value with
> + 64-bit time_t.
> +
> +==============================================================
> +
> unknown_nmi_panic:
>
> The value in this file affects behavior of handling NMI. When the
> diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
> index 99176af..51fc970 100644
> --- a/include/linux/timekeeping.h
> +++ b/include/linux/timekeeping.h
> @@ -5,6 +5,11 @@
>
> void timekeeping_init(void);
> extern int timekeeping_suspended;
> +extern int sysctl_time_max;
> +
> +struct ctl_table;
> +extern int sysctl_time_max_handler(struct ctl_table *table, int write,
> + void __user *buffer, size_t *length, loff_t *ppos);
>
> /*
> * Get and set timeofday
> diff --git a/include/uapi/linux/sysctl.h b/include/uapi/linux/sysctl.h
> index 0956373..8fd2aab 100644
> --- a/include/uapi/linux/sysctl.h
> +++ b/include/uapi/linux/sysctl.h
> @@ -154,6 +154,7 @@ enum
> KERN_NMI_WATCHDOG=75, /* int: enable/disable nmi watchdog */
> KERN_PANIC_ON_NMI=76, /* int: whether we will panic on an unrecovered */
> KERN_PANIC_ON_WARN=77, /* int: call panic() in WARN() functions */
> + KERN_TIMEMAX=78, /* int: select maximum allowed system time */
> };
>
>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index ce410bb..6713a5b 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1120,6 +1120,15 @@ static struct ctl_table kern_table[] = {
> .extra1 = &zero,
> .extra2 = &one,
> },
> + {
> + .procname = "time_max",
> + .data = &sysctl_time_max,
> + .maxlen = sizeof(int),
> + .mode = 0644,
> + .proc_handler = sysctl_time_max_handler,
> + .extra1 = &zero,
> + .extra2 = &two,
> + },
> { }
> };
>
> diff --git a/kernel/sysctl_binary.c b/kernel/sysctl_binary.c
> index 7e7746a..66c0946 100644
> --- a/kernel/sysctl_binary.c
> +++ b/kernel/sysctl_binary.c
> @@ -138,6 +138,7 @@ static const struct bin_table bin_kern_table[] = {
> { CTL_INT, KERN_MAX_LOCK_DEPTH, "max_lock_depth" },
> { CTL_INT, KERN_PANIC_ON_NMI, "panic_on_unrecovered_nmi" },
> { CTL_INT, KERN_PANIC_ON_WARN, "panic_on_warn" },
> + { CTL_INT, KERN_TIMEMAX, "time_max" },
> {}
> };
>
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 946acb7..112932f 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -32,6 +32,9 @@
> #define TK_MIRROR (1 << 1)
> #define TK_CLOCK_WAS_SET (1 << 2)
>
> +#define SEC_PER_DAY (24 * 3600)
> +#define SEC_PER_WEEK (7 * SEC_PER_DAY)
> +
> /*
> * The most important data for readout fits into a single 64 byte
> * cache line.
> @@ -884,6 +887,37 @@ EXPORT_SYMBOL(getnstime_raw_and_real);
>
> #endif /* CONFIG_NTP_PPS */
>
> +/* Maximum allowed system time as a sysctl setting and in seconds */
> +int sysctl_time_max __read_mostly;
> +static u64 time_max_sec __read_mostly;
> +
> +static void update_time_max_sec(int tm)
> +{
> + if (tm > 1) {
> + /* One week before ktime_t overflow */
> + time_max_sec = KTIME_SEC_MAX - SEC_PER_WEEK;
> + } else if (tm == 1) {
> + /* One week before 32-bit time_t overflow */
> + time_max_sec = 0x7fffffff - SEC_PER_WEEK;
> + } else {
> + /* No limit */
> + time_max_sec = -1;
> + }
> +}
> +
> +int sysctl_time_max_handler(struct ctl_table *table, int write,
> + void __user *buffer, size_t *length, loff_t *ppos)
> +{
> + int rc;
> +
> + rc = proc_dointvec_minmax(table, write, buffer, length, ppos);
> + if (rc)
> + return rc;
> +
> + update_time_max_sec(sysctl_time_max);
> + return 0;
> +}
> +
> /**
> * do_gettimeofday - Returns the time of day in a timeval
> * @tv: pointer to the timeval to be set
> @@ -912,7 +946,7 @@ int do_settimeofday64(const struct timespec64 *ts)
> struct timespec64 ts_delta, xt;
> unsigned long flags;
>
> - if (!timespec64_valid_strict(ts))
> + if (!timespec64_valid_strict(ts) || ts->tv_sec >= time_max_sec)
> return -EINVAL;
>
> raw_spin_lock_irqsave(&timekeeper_lock, flags);
> @@ -965,7 +999,7 @@ int timekeeping_inject_offset(struct timespec *ts)
>
> /* Make sure the proposed value is valid */
> tmp = timespec64_add(tk_xtime(tk), ts64);
> - if (!timespec64_valid_strict(&tmp)) {
> + if (!timespec64_valid_strict(&tmp) || tmp.tv_sec >= time_max_sec) {
> ret = -EINVAL;
> goto error;
> }
> @@ -1238,6 +1272,10 @@ void __init timekeeping_init(void)
> write_seqcount_begin(&tk_core.seq);
> ntp_init();
>
> + /* For now, prevent 32-bit time_t overflow by default */
> + sysctl_time_max = sizeof(time_t) > 4 ? 2 : 1;
> + update_time_max_sec(sysctl_time_max);
> +
> clock = clocksource_default_clock();
> if (clock->enable)
> clock->enable(clock);
> @@ -1687,23 +1725,35 @@ static inline unsigned int accumulate_nsecs_to_secs(struct timekeeper *tk)
>
> while (tk->tkr_mono.xtime_nsec >= nsecps) {
> int leap;
> + s64 step;
>
> tk->tkr_mono.xtime_nsec -= nsecps;
> tk->xtime_sec++;
>
> /* Figure out if its a leap sec and apply if needed */
> leap = second_overflow(tk->xtime_sec);
> - if (unlikely(leap)) {
> + step = leap;
> +
> + /* If the system time reached the maximum, step it back */
> + if (unlikely(tk->xtime_sec >= time_max_sec)) {
> + step = time_max_sec - tk->xtime_sec - SEC_PER_WEEK;
> + printk(KERN_NOTICE
> + "Clock: maximum time reached, stepping back\n");
> + }
> +
> + if (unlikely(step)) {
> struct timespec64 ts;
>
> - tk->xtime_sec += leap;
> + tk->xtime_sec += step;
>
> - ts.tv_sec = leap;
> + ts.tv_sec = step;
> ts.tv_nsec = 0;
> tk_set_wall_to_mono(tk,
> timespec64_sub(tk->wall_to_monotonic, ts));
>
> - __timekeeping_set_tai_offset(tk, tk->tai_offset - leap);
> + if (leap)
> + __timekeeping_set_tai_offset(tk,
> + tk->tai_offset - leap);
>
> clock_set = TK_CLOCK_WAS_SET;
> }
> --
> 2.1.0
>
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