CVE-2026-72068: posix-cpu-timers: Use u64 multiplication in update_rlimit_cpu()

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

posix-cpu-timers: Use u64 multiplication in updaterlimitcpu()

updaterlimitcpu() converts the RLIMITCPU value to nanoseconds with

u64 nsecs = rlimnew NSECPERSEC;

On 32-bit kernels both rlimnew (unsigned long) and NSECPERSEC (1000000000L) are 32-bit, so the multiplication is performed in unsigned long and truncated for rlimnew > 4 seconds before being widened to u64.

The same file already casts to u64 for the matching computation in checkprocesstimers():

u64 softns = (u64)soft NSECPERSEC;

As a result, the truncated value is installed into the CPUCLOCKPROF expiry cache (nextevt), causing the process CPU timer to be programmed to fire prematurely for any RLIMITCPU soft limit >= 5 seconds. The actual SIGXCPU/SIGKILL decision in checkprocesstimers() already casts to u64 and is therefore correct, so limit enforcement is not broken; only the expiry-cache programming is wrong. Apply the same cast here so both paths convert rlimcur identically.

64-bit kernels are unaffected.