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

update_rlimit_cpu() converts the RLIMIT_CPU value to nanoseconds with

        u64 nsecs = rlim_new * NSEC_PER_SEC;

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

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

        u64 softns = (u64)soft * NSEC_PER_SEC;

As a result, the truncated value is installed into the CPUCLOCK_PROF
expiry cache (nextevt), causing the process CPU timer to be programmed
to fire prematurely for any RLIMIT_CPU soft limit >= 5 seconds. The
actual SIGXCPU/SIGKILL decision in check_process_timers() 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 rlim_cur identically.

64-bit kernels are unaffected.

Fixes: 858cf3a8c599 ("timers/itimer: Convert internal cputime_t units to nsec")
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260616112017.1681372-1-zhanxusheng@xiaomi.com

diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 74775b94d11bd38a8f5daabdf8501df191045640..5e633d8750d1f6e91c23be8fcc0377f9dcb7ee90 100644 (file)
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -41,7 +41,7 @@ void posix_cputimers_group_init(struct posix_cputimers *pct, u64 cpu_limit)
  */
 int update_rlimit_cpu(struct task_struct *task, unsigned long rlim_new)
 {
-       u64 nsecs = rlim_new * NSEC_PER_SEC;
+       u64 nsecs = (u64)rlim_new * NSEC_PER_SEC;
        unsigned long irq_fl;
 
        if (!lock_task_sighand(task, &irq_fl))