ACK/CMNT: [SRU][Xenial][PATCH 1/1] posix-timers: Sanitize overrun handling
Tyler Hicks
tyhicks at canonical.com
Wed Nov 28 00:12:24 UTC 2018
On 2018-11-27 00:23:18, 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>
Acked-by: Tyler Hicks <tyhicks at canonical.com>
See inline comments. Thanks!
> ---
> include/linux/posix-timers.h | 4 ++--
> kernel/time/posix-cpu-timers.c | 8 ++++----
> kernel/time/posix-timers.c | 28 ++++++++++++++++++----------
> 3 files changed, 24 insertions(+), 16 deletions(-)
>
> 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/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
> index 80016b329d94..9b6015527482 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;
> }
> }
> @@ -744,7 +744,7 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int timer_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;
You added an "LL" suffix here that wasn't in the original patch. I'm not
sure if it was intentional but I also don't think it hurts anything.
>
> if (new_expires != 0 && !(val < new_expires)) {
> /*
> @@ -1100,7 +1100,7 @@ void posix_cpu_timer_schedule(struct k_itimer *timer)
> posix_cpu_timer_kick_nohz();
> out:
> timer->it_overrun_last = timer->it_overrun;
> - timer->it_overrun = -1;
> + timer->it_overrun = -1LL;
Same here although posix_cpu_timer_schedule() and this assignment isn't
present in the current upstream kernel.
> ++timer->it_requeue_pending;
> }
>
> @@ -1305,7 +1305,7 @@ 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;
Same here.
Tyler
> error = posix_cpu_timer_create(&timer);
> timer.it_process = current;
> if (!error) {
> diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
> index fc7c37ad90a0..d34a7f0b92fe 100644
> --- a/kernel/time/posix-timers.c
> +++ b/kernel/time/posix-timers.c
> @@ -354,6 +354,16 @@ 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)
> {
> @@ -362,12 +372,11 @@ 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 +405,7 @@ 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 +500,7 @@ 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 +641,7 @@ 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 +770,7 @@ 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 +832,7 @@ 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
>
>
> --
> kernel-team mailing list
> kernel-team at lists.ubuntu.com
> https://lists.ubuntu.com/mailman/listinfo/kernel-team
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