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

Tyler Hicks tyhicks at canonical.com
Tue Nov 27 18:10:51 UTC 2018


On 2018-11-27 00:23:17, 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
> (backported from commit 78c9c4dfbf8c04883941445a195276bb4bb92c76)
> Signed-off-by: Khalid Elmously <khalid.elmously at canonical.com>
> ---
>  include/linux/posix-timers.h |  4 ++--
>  kernel/posix-cpu-timers.c    |  8 ++++----
>  kernel/posix-timers.c        | 29 +++++++++++++++++++----------
>  3 files changed, 25 insertions(+), 16 deletions(-)

This seems like some potentially hair backports to get right. I'm still
going through the Xenial and Trusty patches but I noticed that you
dropped the hunk that removes the (unsigned int) cast in
common_timer_get() for the Trusty backport. I don't think that was
intentional but let me know if it was.

Tyler

> 
> diff --git a/include/linux/posix-timers.h b/include/linux/posix-timers.h
> index 907f3fd191ac..3e28a1a8d823 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/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
> index c7f31aa272f7..0f62265a707c 100644
> --- a/kernel/posix-cpu-timers.c
> +++ b/kernel/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;
>  	}
>  }
> @@ -761,7 +761,7 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int flags,
>  	timer->it_requeue_pending = (timer->it_requeue_pending + 2) &
>  		~REQUEUE_PENDING;
>  	timer->it_overrun_last = 0;
> -	timer->it_overrun = -1;
> +	timer->it_overrun = -1LL;
>  
>  	if (new_expires != 0 && !(val < new_expires)) {
>  		/*
> @@ -1119,7 +1119,7 @@ out_unlock:
>  
>  out:
>  	timer->it_overrun_last = timer->it_overrun;
> -	timer->it_overrun = -1;
> +	timer->it_overrun = -1LL;
>  	++timer->it_requeue_pending;
>  }
>  
> @@ -1327,7 +1327,7 @@ static int do_cpu_nanosleep(const clockid_t which_clock, int flags,
>  	memset(&timer, 0, sizeof timer);
>  	spin_lock_init(&timer.it_lock);
>  	timer.it_clock = which_clock;
> -	timer.it_overrun = -1;
> +	timer.it_overrun = -1LL;
>  	error = posix_cpu_timer_create(&timer);
>  	timer.it_process = current;
>  	if (!error) {
> diff --git a/kernel/posix-timers.c b/kernel/posix-timers.c
> index 26b910462a0c..2577e33bc964 100644
> --- a/kernel/posix-timers.c
> +++ b/kernel/posix-timers.c
> @@ -346,6 +346,17 @@ 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;
> @@ -353,12 +364,11 @@ 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);
>  }
> @@ -387,7 +397,7 @@ 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)
> @@ -482,9 +492,8 @@ 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.real.interval);
> +			timr->it_overrun += hrtimer_forward(timer, now,
> +							    timr->it.real.interval);
>  			ret = HRTIMER_RESTART;
>  			++timr->it_requeue_pending;
>  		}
> @@ -624,7 +633,7 @@ 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))) {
> @@ -816,7 +825,7 @@ 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;
> -- 
> 2.17.1
> 
> 
> -- 
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> kernel-team at lists.ubuntu.com
> https://lists.ubuntu.com/mailman/listinfo/kernel-team
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