From: Thomas Gleixner tglx@linutronix.de

The tick period is aligned very early while the first clock_event_device is registered. The system runs in periodic mode and switches later to one-shot mode if possible.

The next wake-up event is programmed based on aligned value (tick_next_period) but the delta value, that is used to program the clock_event_device, is computed based on ktime_get().

With the subtracted offset, the devices fires in less than the exacted time frame. With a large enough offset the system programs the timer for the next wake-up and the remaining time left is too little to make any boot progress. The system hangs.

Move the alignment later to the setup of tick_sched timer. At this point the system switches to oneshot mode and a highres clocksource is available. It safe to update tick_next_period ktime_get() will now return accurate (not jiffies based) time.

[bigeasy: Patch description + testing].

Reported-by: Mathias Krause minipli@grsecurity.net Reported-by: "Bhatnagar, Rishabh" risbhat@amazon.com Fixes: e9523a0d81899 ("tick/common: Align tick period with the HZ tick.") Signed-off-by: Thomas Gleixner tglx@linutronix.de Signed-off-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Link: https://lore.kernel.org/5a56290d-806e-b9a5-f37c-f21958b5a8c0@grsecurity.net Link: https://lore.kernel.org/12c6f9a3-d087-b824-0d05-0d18c9bc1bf3@amazon.com --- kernel/time/tick-common.c | 11 +---------- kernel/time/tick-sched.c | 13 ++++++++++++- 2 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c index 65b8658da829e..b85f2f9c32426 100644 --- a/kernel/time/tick-common.c +++ b/kernel/time/tick-common.c @@ -218,19 +218,10 @@ static void tick_setup_device(struct tick_device *td, * this cpu: */ if (tick_do_timer_cpu == TICK_DO_TIMER_BOOT) { - ktime_t next_p; - u32 rem;

tick_do_timer_cpu = cpu;

- next_p = ktime_get(); - div_u64_rem(next_p, TICK_NSEC, &rem); - if (rem) { - next_p -= rem; - next_p += TICK_NSEC; - } - - tick_next_period = next_p; + tick_next_period = ktime_get(); #ifdef CONFIG_NO_HZ_FULL /* * The boot CPU may be nohz_full, in which case set diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 52254679ec489..42c0be3080bde 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -161,8 +161,19 @@ static ktime_t tick_init_jiffy_update(void) raw_spin_lock(&jiffies_lock); write_seqcount_begin(&jiffies_seq); /* Did we start the jiffies update yet ? */ - if (last_jiffies_update == 0) + if (last_jiffies_update == 0) { + u32 rem; + + /* + * Ensure that the tick is aligned to a multiple of + * TICK_NSEC. + */ + div_u64_rem(tick_next_period, TICK_NSEC, &rem); + if (rem) + tick_next_period += TICK_NSEC - rem; + last_jiffies_update = tick_next_period; + } period = last_jiffies_update; write_seqcount_end(&jiffies_seq); raw_spin_unlock(&jiffies_lock);

On 15.06.23 11:18, Sebastian Andrzej Siewior wrote:

From: Thomas Gleixner tglx@linutronix.de

The tick period is aligned very early while the first clock_event_device is registered. The system runs in periodic mode and switches later to one-shot mode if possible.

The next wake-up event is programmed based on aligned value (tick_next_period) but the delta value, that is used to program the clock_event_device, is computed based on ktime_get().

With the subtracted offset, the devices fires in less than the exacted time frame. With a large enough offset the system programs the timer for the next wake-up and the remaining time left is too little to make any boot progress. The system hangs.

Move the alignment later to the setup of tick_sched timer. At this point the system switches to oneshot mode and a highres clocksource is available. It safe to update tick_next_period ktime_get() will now return accurate (not jiffies based) time.

[bigeasy: Patch description + testing].

Reported-by: Mathias Krause minipli@grsecurity.net Reported-by: "Bhatnagar, Rishabh" risbhat@amazon.com Fixes: e9523a0d81899 ("tick/common: Align tick period with the HZ tick.")

Cc: stable, maybe? This commit already ended up in quite a few "stable" kernels (v6.3.2, v6.2.15, v6.1.28, v5.15.111, v5.10.180 and v5.4.243) and it might be better to list them explicitly to avoid one of them getting missed.

Signed-off-by: Thomas Gleixner tglx@linutronix.de Signed-off-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Link: https://lore.kernel.org/5a56290d-806e-b9a5-f37c-f21958b5a8c0@grsecurity.net Link: https://lore.kernel.org/12c6f9a3-d087-b824-0d05-0d18c9bc1bf3@amazon.com

---

kernel/time/tick-common.c | 11 +---------- kernel/time/tick-sched.c | 13 ++++++++++++- 2 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c index 65b8658da829e..b85f2f9c32426 100644 --- a/kernel/time/tick-common.c +++ b/kernel/time/tick-common.c @@ -218,19 +218,10 @@ static void tick_setup_device(struct tick_device *td, * this cpu: */ if (tick_do_timer_cpu == TICK_DO_TIMER_BOOT) {

• 	ktime_t next_p;
  

• 	u32 rem;
  

tick_do_timer_cpu = cpu;

• 	next_p = ktime_get();
  

• 	div_u64_rem(next_p, TICK_NSEC, &rem);
  

• 	if (rem) {
  

• 		next_p -= rem;
  

• 		next_p += TICK_NSEC;
  

• 	}
  

• 	tick_next_period = next_p;
  

• 	tick_next_period = ktime_get();
  

#ifdef CONFIG_NO_HZ_FULL /* * The boot CPU may be nohz_full, in which case set diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 52254679ec489..42c0be3080bde 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -161,8 +161,19 @@ static ktime_t tick_init_jiffy_update(void) raw_spin_lock(&jiffies_lock); write_seqcount_begin(&jiffies_seq); /* Did we start the jiffies update yet ? */

• if (last_jiffies_update == 0)

• if (last_jiffies_update == 0) {

• u32 rem;
  

• /*
  

•  * Ensure that the tick is aligned to a multiple of
  

•  * TICK_NSEC.
  

•  */
  

• div_u64_rem(tick_next_period, TICK_NSEC, &rem);
  

• if (rem)
  

• 	tick_next_period += TICK_NSEC - rem;
  

• last_jiffies_update = tick_next_period;
• } period = last_jiffies_update; write_seqcount_end(&jiffies_seq); raw_spin_unlock(&jiffies_lock);

Hah, nice spot. So you implemented what I suggested and it, indeed, works as expected, thereby:

Tested-by: Mathias Krause minipli@grsecurity.net

Thanks, Mathias

On 2023-06-15 05:18, Sebastian Andrzej Siewior wrote:

From: Thomas Gleixner tglx@linutronix.de

The tick period is aligned very early while the first clock_event_device is registered. The system runs in periodic mode and switches later to one-shot mode if possible.

The next wake-up event is programmed based on aligned value (tick_next_period) but the delta value, that is used to program the clock_event_device, is computed based on ktime_get().

With the subtracted offset, the devices fires in less than the exacted time frame. With a large enough offset the system programs the timer for the next wake-up and the remaining time left is too little to make any boot progress. The system hangs.

Move the alignment later to the setup of tick_sched timer. At this point the system switches to oneshot mode and a highres clocksource is available. It safe to update tick_next_period ktime_get() will now return accurate (not jiffies based) time.

[bigeasy: Patch description + testing].

Reported-by: Mathias Krause minipli@grsecurity.net Reported-by: "Bhatnagar, Rishabh" risbhat@amazon.com Fixes: e9523a0d81899 ("tick/common: Align tick period with the HZ tick.") Signed-off-by: Thomas Gleixner tglx@linutronix.de Signed-off-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Link: https://lore.kernel.org/5a56290d-806e-b9a5-f37c-f21958b5a8c0@grsecurity.net Link: https://lore.kernel.org/12c6f9a3-d087-b824-0d05-0d18c9bc1bf3@amazon.com

I've tested this against 5.10.184 (which is where it reproduces quick for me):

Tested-by: Luiz Capitulino luizcap@amazon.com

---

kernel/time/tick-common.c | 11 +---------- kernel/time/tick-sched.c | 13 ++++++++++++- 2 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c index 65b8658da829e..b85f2f9c32426 100644 --- a/kernel/time/tick-common.c +++ b/kernel/time/tick-common.c @@ -218,19 +218,10 @@ static void tick_setup_device(struct tick_device *td, * this cpu: */ if (tick_do_timer_cpu == TICK_DO_TIMER_BOOT) {

•                   ktime_t next_p;
  

•                   u32 rem;
  
                     tick_do_timer_cpu = cpu;
  
  

•                   next_p = ktime_get();
  

•                   div_u64_rem(next_p, TICK_NSEC, &rem);
  

•                   if (rem) {
  

•                           next_p -= rem;
  

•                           next_p += TICK_NSEC;
  

•                   }
  

•                   tick_next_period = next_p;
  

•                   tick_next_period = ktime_get();
  

  #ifdef CONFIG_NO_HZ_FULL /* * The boot CPU may be nohz_full, in which case set

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 52254679ec489..42c0be3080bde 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -161,8 +161,19 @@ static ktime_t tick_init_jiffy_update(void) raw_spin_lock(&jiffies_lock); write_seqcount_begin(&jiffies_seq); /* Did we start the jiffies update yet ? */

•   if (last_jiffies_update == 0)
  

•   if (last_jiffies_update == 0) {
  

•           u32 rem;
  

•           /*
  

•            * Ensure that the tick is aligned to a multiple of
  

•            * TICK_NSEC.
  

•            */
  

•           div_u64_rem(tick_next_period, TICK_NSEC, &rem);
  

•           if (rem)
  

•                   tick_next_period += TICK_NSEC - rem;
  

•            last_jiffies_update = tick_next_period;
  

•   }
     period = last_jiffies_update;
     write_seqcount_end(&jiffies_seq);
     raw_spin_unlock(&jiffies_lock);
  

-- 2.40.1