From c3f2feace5e4f4b01b68b9f947b19adb4155c32e Mon Sep 17 00:00:00 2001 From: Wentao Guan Date: Fri, 4 Sep 2026 21:44:43 +0800 Subject: [PATCH] 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 rethook_trampoline_handler() may schedule via preempt_enable_notrace(). 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 this_cpu_*() 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, WARN_ON_ONCE(rq != this_rq()) in nohz_balance_exit_idle(), 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: SAVE_SOME() at exception entry reloads it only when coming from user mode, and RESTORE_SOME() 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. Cc: stable@vger.kernel.org # v6.3+ Fixes: 3f5536860086d ("LoongArch: Add kretprobes support") Assisted-by: Kimi:Kimi-K3 # debug and root-cause analysis Signed-off-by: Wentao Guan Signed-off-by: Huacai Chen --- arch/loongarch/kernel/rethook_trampoline.S | 2 -- 1 file changed, 2 deletions(-) diff --git a/arch/loongarch/kernel/rethook_trampoline.S b/arch/loongarch/kernel/rethook_trampoline.S index 2e009fbea53f..160189444684 100644 --- a/arch/loongarch/kernel/rethook_trampoline.S +++ b/arch/loongarch/kernel/rethook_trampoline.S @@ -24,7 +24,6 @@ cfi_st t6, PT_R18 cfi_st t7, PT_R19 cfi_st t8, PT_R20 - cfi_st u0, PT_R21 cfi_st fp, PT_R22 cfi_st s0, PT_R23 cfi_st s1, PT_R24 @@ -59,7 +58,6 @@ cfi_ld t6, PT_R18 cfi_ld t7, PT_R19 cfi_ld t8, PT_R20 - cfi_ld u0, PT_R21 cfi_ld fp, PT_R22 cfi_ld s0, PT_R23 cfi_ld s1, PT_R24