Re: [PATCH stable 4.9] posix-timers: Sanitize overrun handling

[Florian Fainelli]

On 11/1/18 1:02 PM, Florian Fainelli wrote:
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> 
> [ Upstream commit 78c9c4dfbf8c04883941445a195276bb4bb92c76 ]
> 
> The posix timer overrun handling is broken because the forwarding functions
> can return a huge number of overruns which does not fit in an int. As a
> consequence timer_getoverrun(2) and siginfo::si_overrun can turn into
> random number generators.
> 
> The k_clock::timer_forward() callbacks return a 64 bit value now. Make
> k_itimer::ti_overrun[_last] 64bit as well, so the kernel internal
> accounting is correct. 3Remove the temporary (int) casts.
> 
> Add a helper function which clamps the overrun value returned to user space
> via timer_getoverrun(2) or siginfo::si_overrun limited to a positive value
> between 0 and INT_MAX. INT_MAX is an indicator for user space that the
> overrun value has been clamped.
> 
> Reported-by: Team OWL337 <icytxw@xxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Acked-by: John Stultz <john.stultz@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> Link: https://lkml.kernel.org/r/20180626132705.018623573@xxxxxxxxxxxxx
> [florian: Make patch apply to v4.9.135]
> Signed-off-by: Florian Fainelli <f.fainelli@xxxxxxxxx>
> ---
> Thomas, can you review for correctness? Thanks!

Thomas, John, does that look like a reasonable backport for 4.9?

> 
>  include/linux/posix-timers.h   |  4 ++--
>  kernel/time/posix-cpu-timers.c |  2 +-
>  kernel/time/posix-timers.c     | 29 +++++++++++++++++++----------
>  3 files changed, 22 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/posix-timers.h b/include/linux/posix-timers.h
> index 62d44c176071..e4b8678183f5 100644
> --- a/include/linux/posix-timers.h
> +++ b/include/linux/posix-timers.h
> @@ -65,8 +65,8 @@ struct k_itimer {
>  	spinlock_t it_lock;
>  	clockid_t it_clock;		/* which timer type */
>  	timer_t it_id;			/* timer id */
> -	int it_overrun;			/* overrun on pending signal  */
> -	int it_overrun_last;		/* overrun on last delivered signal */
> +	s64 it_overrun;			/* overrun on pending signal  */
> +	s64 it_overrun_last;		/* overrun on last delivered signal */
>  	int it_requeue_pending;		/* waiting to requeue this timer */
>  #define REQUEUE_PENDING 1
>  	int it_sigev_notify;		/* notify word of sigevent struct */
> diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
> index 39008d78927a..21a27bb73587 100644
> --- a/kernel/time/posix-cpu-timers.c
> +++ b/kernel/time/posix-cpu-timers.c
> @@ -103,7 +103,7 @@ static void bump_cpu_timer(struct k_itimer *timer,
>  			continue;
>  
>  		timer->it.cpu.expires += incr;
> -		timer->it_overrun += 1 << i;
> +		timer->it_overrun += 1LL << i;
>  		delta -= incr;
>  	}
>  }
> diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
> index fc7c37ad90a0..0e6ed2e7d066 100644
> --- a/kernel/time/posix-timers.c
> +++ b/kernel/time/posix-timers.c
> @@ -355,6 +355,17 @@ static __init int init_posix_timers(void)
>  
>  __initcall(init_posix_timers);
>  
> +/*
> + * The siginfo si_overrun field and the return value of timer_getoverrun(2)
> + * are of type int. Clamp the overrun value to INT_MAX
> + */
> +static inline int timer_overrun_to_int(struct k_itimer *timr, int baseval)
> +{
> +	s64 sum = timr->it_overrun_last + (s64)baseval;
> +
> +	return sum > (s64)INT_MAX ? INT_MAX : (int)sum;
> +}
> +
>  static void schedule_next_timer(struct k_itimer *timr)
>  {
>  	struct hrtimer *timer = &timr->it.real.timer;
> @@ -362,12 +373,11 @@ static void schedule_next_timer(struct k_itimer *timr)
>  	if (timr->it.real.interval.tv64 == 0)
>  		return;
>  
> -	timr->it_overrun += (unsigned int) hrtimer_forward(timer,
> -						timer->base->get_time(),
> -						timr->it.real.interval);
> +	timr->it_overrun += hrtimer_forward(timer, timer->base->get_time(),
> +					    timr->it.real.interval);
>  
>  	timr->it_overrun_last = timr->it_overrun;
> -	timr->it_overrun = -1;
> +	timr->it_overrun = -1LL;
>  	++timr->it_requeue_pending;
>  	hrtimer_restart(timer);
>  }
> @@ -396,7 +406,7 @@ void do_schedule_next_timer(struct siginfo *info)
>  		else
>  			schedule_next_timer(timr);
>  
> -		info->si_overrun += timr->it_overrun_last;
> +		info->si_overrun = timer_overrun_to_int(timr, info->si_overrun);
>  	}
>  
>  	if (timr)
> @@ -491,8 +501,7 @@ static enum hrtimer_restart posix_timer_fn(struct hrtimer *timer)
>  					now = ktime_add(now, kj);
>  			}
>  #endif
> -			timr->it_overrun += (unsigned int)
> -				hrtimer_forward(timer, now,
> +			timr->it_overrun += hrtimer_forward(timer, now,
>  						timr->it.real.interval);
>  			ret = HRTIMER_RESTART;
>  			++timr->it_requeue_pending;
> @@ -633,7 +642,7 @@ SYSCALL_DEFINE3(timer_create, const clockid_t, which_clock,
>  	it_id_set = IT_ID_SET;
>  	new_timer->it_id = (timer_t) new_timer_id;
>  	new_timer->it_clock = which_clock;
> -	new_timer->it_overrun = -1;
> +	new_timer->it_overrun = -1LL;
>  
>  	if (timer_event_spec) {
>  		if (copy_from_user(&event, timer_event_spec, sizeof (event))) {
> @@ -762,7 +771,7 @@ common_timer_get(struct k_itimer *timr, struct itimerspec *cur_setting)
>  	 */
>  	if (iv.tv64 && (timr->it_requeue_pending & REQUEUE_PENDING ||
>  			timr->it_sigev_notify == SIGEV_NONE))
> -		timr->it_overrun += (unsigned int) hrtimer_forward(timer, now, iv);
> +		timr->it_overrun += hrtimer_forward(timer, now, iv);
>  
>  	remaining = __hrtimer_expires_remaining_adjusted(timer, now);
>  	/* Return 0 only, when the timer is expired and not pending */
> @@ -824,7 +833,7 @@ SYSCALL_DEFINE1(timer_getoverrun, timer_t, timer_id)
>  	if (!timr)
>  		return -EINVAL;
>  
> -	overrun = timr->it_overrun_last;
> +	overrun = timer_overrun_to_int(timr, 0);
>  	unlock_timer(timr, flags);
>  
>  	return overrun;
> 


-- 
Florian