Re: [PATCH] hw/timer/hpet: Fix expiration time overflow

[Philippe Mathieu-Daudé]

On 30/1/23 14:50, Akihiko Odaki wrote:
> The expiration time provided for timer_mod() can overflow if a
> ridiculously large value is set to the comparator register. The
> resulting value can represent a past time after rounded, forcing the
> timer to fire immediately. If the timer is configured as periodic, it
> will rearm the timer again, and form an endless loop.
>
> Check if the expiration value will overflow, and if it will, stop the
> timer instead of rearming the timer with the overflowed time.
>
> This bug was found by Alexander Bulekov when fuzzing igb, a new
> network device emulation:
> 20230129053316.1071513-1-alxndr@bu.edu/">https://patchew.org/QEMU/20230129053316.1071513-1-alxndr@bu.edu/
>
> The fixed test case is:
> fuzz/crash_2d7036941dcda1ad4380bb8a9174ed0c949bcefd
>
> Fixes: 16b29ae180 ("Add HPET emulation to qemu (Beth Kon)")
> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> ---
>   hw/timer/hpet.c | 19 +++++++++++++------
>   1 file changed, 13 insertions(+), 6 deletions(-)
>
> diff --git a/hw/timer/hpet.c b/hw/timer/hpet.c
> index 9520471be2..3657d5f463 100644
> --- a/hw/timer/hpet.c
> +++ b/hw/timer/hpet.c
> @@ -352,6 +352,16 @@ static const VMStateDescription vmstate_hpet = {
>       }
>   };
>   
> +static void arm(HPETTimer *t, uint64_t ticks)

Could we rename as hpet_[re]arm() similarly to this file's other helpers?

> +{
> +    if (ticks < ns_to_ticks(INT64_MAX / 2)) {
> +        timer_mod(t->qemu_timer,
> +                  qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + ticks_to_ns(ticks));
> +    } else {
> +        timer_del(t->qemu_timer);
> +    }
> +}