ACK: [SRU][Bionic][PATCH 1/1] posix-timers: Sanitize overrun handling

Kleber Souza kleber.souza at canonical.com
Wed Nov 28 15:19:10 UTC 2018


On 11/27/18 6:23 AM, Khalid Elmously wrote:
> From: Thomas Gleixner <tglx at linutronix.de>
>
> CVE-2018-12896
>
> 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 at gmail.com>
> Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
> Acked-by: John Stultz <john.stultz at linaro.org>
> Cc: Peter Zijlstra <peterz at infradead.org>
> Cc: Michael Kerrisk <mtk.manpages at gmail.com>
> Link: https://lkml.kernel.org/r/20180626132705.018623573@linutronix.de
> (cherry picked from commit 78c9c4dfbf8c04883941445a195276bb4bb92c76)
> Signed-off-by: Khalid Elmously <khalid.elmously at canonical.com>


Acked-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>


> ---
>  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 672c4f32311e..437a539898ae 100644
> --- a/include/linux/posix-timers.h
> +++ b/include/linux/posix-timers.h
> @@ -82,8 +82,8 @@ struct k_itimer {
>  	clockid_t		it_clock;
>  	timer_t			it_id;
>  	int			it_active;
> -	int			it_overrun;
> -	int			it_overrun_last;
> +	s64			it_overrun;
> +	s64			it_overrun_last;
>  	int			it_requeue_pending;
>  	int			it_sigev_notify;
>  	ktime_t			it_interval;
> diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
> index 1f27887aa194..0f5f019c6645 100644
> --- a/kernel/time/posix-cpu-timers.c
> +++ b/kernel/time/posix-cpu-timers.c
> @@ -84,7 +84,7 @@ static void bump_cpu_timer(struct k_itimer *timer, u64 now)
>  			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 708992708332..3192a986a483 100644
> --- a/kernel/time/posix-timers.c
> +++ b/kernel/time/posix-timers.c
> @@ -283,6 +283,17 @@ 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 common_hrtimer_rearm(struct k_itimer *timr)
>  {
>  	struct hrtimer *timer = &timr->it.real.timer;
> @@ -290,9 +301,8 @@ static void common_hrtimer_rearm(struct k_itimer *timr)
>  	if (!timr->it_interval)
>  		return;
>  
> -	timr->it_overrun += (unsigned int) hrtimer_forward(timer,
> -						timer->base->get_time(),
> -						timr->it_interval);
> +	timr->it_overrun += hrtimer_forward(timer, timer->base->get_time(),
> +					    timr->it_interval);
>  	hrtimer_restart(timer);
>  }
>  
> @@ -321,10 +331,10 @@ void posixtimer_rearm(struct siginfo *info)
>  
>  		timr->it_active = 1;
>  		timr->it_overrun_last = timr->it_overrun;
> -		timr->it_overrun = -1;
> +		timr->it_overrun = -1LL;
>  		++timr->it_requeue_pending;
>  
> -		info->si_overrun += timr->it_overrun_last;
> +		info->si_overrun = timer_overrun_to_int(timr, info->si_overrun);
>  	}
>  
>  	unlock_timer(timr, flags);
> @@ -418,9 +428,8 @@ 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_interval);
> +			timr->it_overrun += hrtimer_forward(timer, now,
> +							    timr->it_interval);
>  			ret = HRTIMER_RESTART;
>  			++timr->it_requeue_pending;
>  			timr->it_active = 1;
> @@ -524,7 +533,7 @@ static int do_timer_create(clockid_t which_clock, struct sigevent *event,
>  	new_timer->it_id = (timer_t) new_timer_id;
>  	new_timer->it_clock = which_clock;
>  	new_timer->kclock = kc;
> -	new_timer->it_overrun = -1;
> +	new_timer->it_overrun = -1LL;
>  
>  	if (event) {
>  		rcu_read_lock();
> @@ -789,7 +798,7 @@ 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;





More information about the kernel-team mailing list