CVE-2026-90100: ptp: netc: fix period truncation and potential divide-by-zero in PEROUT

In the Linux kernel, the following vulnerability has been resolved:

ptp: netc: fix period truncation and potential divide-by-zero in PEROUT

The maxperiod bound in nettimerenableperout() was computed as:

maxperiod = (u64)NETCTMRDEFAULTFIPER + integralperiod;

which exceeds U32MAX when integralperiod > 0 (e.g. 0x100000002 for the default 333333333 Hz clock). A periodns that passes this check but exceeds U32MAX is then silently truncated when stored into the u32 struct netcpp::period field.

A truncated value of zero can reach netctimersetperoutalarm(), where the local u32 period variable would also be 0, causing a divide-by-zero in roundupu64(delta, period) whenever the stime < mintime branch is taken (which always happens for a start time of {0, 0}).

Additionally, netctimerenableperiodicpulse() and netctimerenablefiper() both compute:

fiper = pp->period - integralperiod;

A zero pp->period results in an unsigned wraparound to 0xFFFFFFFD, mis-programming the FIPER hardware register.

Fix all three issues by capping maxperiod at NETCTMRDEFAULTFIPER (0xFFFFFFFF). This ensures that any periodns 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 integralperiod ns (typically only a few nanoseconds), which is negligible in practice.