CVE-2026-89903: LoongArch: Do not save/restore percpu base register in rethook trampoline

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

LoongArch: Do not save/restore percpu base register in rethook trampoline

The rethook trampoline saves $r21 ($u0), the percpu base, into its frame at entry and restores it at exit. Inbetween rethooktrampolinehandler() may schedule via preemptenablenotrace().

If the task migrates to another CPU, the frame's $r21 holds the old CPU's percpu base, and restoring it poisons $r21 on the new CPU. Until the next user->kernel transition heals $r21, all thiscpu() accesses (runqueues, RCU per-CPU data, timer tick programming, FPU ownership) hit the wrong CPU's percpu area.

Under kretprobe-heavy preemptible load this can corrupt scheduler and timer state: scheduling-while-atomic splats, wrong-CPU RCU warnings, WARNONONCE(rq != thisrq()) in nohzbalanceexitidle(), and CPUs parking in the idle loop with the constant timer never re-armed (hard lockup). Reproduces on a Loongson-3A6000 with kretprobes on VFS paths plus heavy file churn (OS install / unsquashfs).

By convention $r21 always holds the current CPU's percpu base in kernel mode: SAVESOME() at exception entry reloads it only when coming from user mode, and RESTORESOME() restores it only when returning to user mode; the context-switch path never writes it. Therefore the live $r21 at trampoline exit is already correct, and nothing inbetween can change it legitimately (kernel C code cannot write a global register variable). The same flaw existed even in the pre-rethook kretprobe trampoline since v6.3; it was carried over when rethook replaced it. Drop both the save and the restore here. Drop the restore is enough to solve the issue, and drop the save is to keep the code tidy and no need to clear it.