Re: [PATCH v2 net] ptp: netc: fix period truncation and potential divide-by-zero in PEROUT

[Abel Vesa]

On 26-08-21 11:24:49, wei.fang@xxxxxxxxxxx wrote:
> From: Wei Fang <wei.fang@xxxxxxx>
> 
> The max_period bound in net_timer_enable_perout() was computed as:
> 
>   max_period = (u64)NETC_TMR_DEFAULT_FIPER + integral_period;
> 
> which exceeds U32_MAX when integral_period > 0 (e.g. 0x100000002 for
> the default 333333333 Hz clock). A period_ns that passes this check but
> exceeds U32_MAX is then silently truncated when stored into the u32
> struct netc_pp::period field.
> 
> A truncated value of zero can reach netc_timer_set_perout_alarm(), where
> the local u32 period variable would also be 0, causing a divide-by-zero
> in roundup_u64(delta, period) whenever the stime < min_time branch is
> taken (which always happens for a start time of {0, 0}).
> 
> Additionally, netc_timer_enable_periodic_pulse() and
> netc_timer_enable_fiper() both compute:
> 
>   fiper = pp->period - integral_period;
> 
> A zero pp->period results in an unsigned wraparound to 0xFFFFFFFD,
> mis-programming the FIPER hardware register.
> 
> Fix all three issues by capping max_period at NETC_TMR_DEFAULT_FIPER
> (0xFFFFFFFF). This ensures that any period_ns passing the range check
> fits in a u32 without truncation, so the stored value is always valid
> and non-zero. The accepted range is reduced by integral_period ns
> (typically only a few nanoseconds), which is negligible in practice.
> 
> Fixes: 671e266835b8 ("ptp: netc: add periodic pulse output support")
> Signed-off-by: Wei Fang <wei.fang@xxxxxxx>

Reviewed-by: Abel Vesa <abel.vesa@xxxxxxxxxxxxxxxx>