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

Summary

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 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.

Timeline

1. Sep 16, 2026, 10:32 AM UTCCVE published
2. Oct 19, 2026, 8:54 AM UTCRecord last updated

Affected versions

• Linux Linux

  From 3f5536860086d906b01ec5ed68cf50c7edcc40af · fixed in a03b5e7483ad2c33ba5dd2adb8552e84bfee8fe2; From 3f5536860086d906b01ec5ed68cf50c7edcc40af · fixed in 8f15e95b438bc6b3c9f23a33c9a0d0678ebd1dc8; From 3f5536860086d906b01ec5ed68cf50c7edcc40af · fixed in 266ffc92e68593759adfe3d58f188773d32782c3; From 3f5536860086d906b01ec5ed68cf50c7edcc40af · fixed in c048b0a3c0836c4b1bef4b51d355d27090bf6790; From 3f5536860086d906b01ec5ed68cf50c7edcc40af · fixed in c3f2feace5e4f4b01b68b9f947b19adb4155c32e

• Linux Linux

  6.3

• KernelLinux

  From 6.3.0 · fixed in 6.6.157; From 6.7.0 · fixed in 6.12.110; From 6.13.0 · fixed in 6.18.51; From 6.19.0 · fixed in 7.2.5

References 5