Re: [PATCH] LoongArch: Fix calling smp_processor_id() in preemptible code

[Huacai Chen]

Hi, Ruoyao,

On Wed, Feb 25, 2026 at 10:15 PM Xi Ruoyao <xry111@xxxxxxxxxxx> wrote:
>
> Fix the warning recently exploited by commit 4ab17e762b34 ("LoongArch:
> BPF: Use BPF prog pack allocator"):
>
>     BUG: using smp_processor_id() in preemptible [00000000] code: systemd/1
>     caller is larch_insn_text_copy+0x40/0xf0
>
> Simply changing it to raw_smp_processor_id() is not enough: if preempt
> and CPU hotplug happens after raw_smp_processor_id() but before
> stop_machine(), the CPU where raw_smp_processor_id() has run may be
> offline when stop_machine() and no CPU will run copy_to_kernel_nofault()
> in text_copy_cb().  Thus guard the larch_insn_text_copy() calls with
> cpus_read_lock() and change stop_machine() to stop_machine_cpuslocked()
> to prevent this.
>
> Fixes: 9fbd18cf4c69 ("LoongArch: BPF: Add dynamic code modification support")
> Signed-off-by: Xi Ruoyao <xry111@xxxxxxxxxxx>
> ---
>
> P.S. I've considered moving the locks inside larch_insn_text_copy() but
> doing so seems not an easy hack.  In bpf_arch_text_poke() obviously the
> memcpy() call must be guarded by text_mutex, so we have to leave the
> acquire of text_mutex out of larch_insn_text_copy.  But in the entire
> kernel the acquire of mutexes is always after cpus_read_lock(), so we
> cannot put cpus_read_lock() into larch_insn_text_copy() while leaving
> the text_mutex acquire out (or we risk a deadlock due to inconsistent
> lock acquire order).  So I'd just submit this as-is to fix the bug and
> leave the posssible refactor for others.
>
>  arch/loongarch/kernel/inst.c | 4 ++--
>  arch/loongarch/net/bpf_jit.c | 6 ++++++
>  2 files changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/arch/loongarch/kernel/inst.c b/arch/loongarch/kernel/inst.c
> index bf037f0c6b26..db236e2b34a7 100644
> --- a/arch/loongarch/kernel/inst.c
> +++ b/arch/loongarch/kernel/inst.c
> @@ -263,14 +263,14 @@ int larch_insn_text_copy(void *dst, void *src, size_t len)
>                 .dst = dst,
>                 .src = src,
>                 .len = len,
> -               .cpu = smp_processor_id(),
> +               .cpu = raw_smp_processor_id(),
I think we should keep the old function here, because it can help
others to catch new bugs if the code is refactored in future.

Huacai

>         };
>
>         start = round_down((size_t)dst, PAGE_SIZE);
>         end   = round_up((size_t)dst + len, PAGE_SIZE);
>
>         set_memory_rw(start, (end - start) / PAGE_SIZE);
> -       ret = stop_machine(text_copy_cb, &copy, cpu_online_mask);
> +       ret = stop_machine_cpuslocked(text_copy_cb, &copy, cpu_online_mask);
>         set_memory_rox(start, (end - start) / PAGE_SIZE);
>
>         return ret;
> diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
> index 3bd89f55960d..e8e0ad34928c 100644
> --- a/arch/loongarch/net/bpf_jit.c
> +++ b/arch/loongarch/net/bpf_jit.c
> @@ -1379,9 +1379,11 @@ void *bpf_arch_text_copy(void *dst, void *src, size_t len)
>  {
>         int ret;
>
> +       cpus_read_lock();
>         mutex_lock(&text_mutex);
>         ret = larch_insn_text_copy(dst, src, len);
>         mutex_unlock(&text_mutex);
> +       cpus_read_unlock();
>
>         return ret ? ERR_PTR(-EINVAL) : dst;
>  }
> @@ -1429,10 +1431,12 @@ int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type old_t,
>         if (ret)
>                 return ret;
>
> +       cpus_read_lock();
>         mutex_lock(&text_mutex);
>         if (memcmp(ip, new_insns, LOONGARCH_LONG_JUMP_NBYTES))
>                 ret = larch_insn_text_copy(ip, new_insns, LOONGARCH_LONG_JUMP_NBYTES);
>         mutex_unlock(&text_mutex);
> +       cpus_read_unlock();
>
>         return ret;
>  }
> @@ -1450,10 +1454,12 @@ int bpf_arch_text_invalidate(void *dst, size_t len)
>         for (i = 0; i < (len / sizeof(u32)); i++)
>                 inst[i] = INSN_BREAK;
>
> +       cpus_read_lock();
>         mutex_lock(&text_mutex);
>         if (larch_insn_text_copy(dst, inst, len))
>                 ret = -EINVAL;
>         mutex_unlock(&text_mutex);
> +       cpus_read_unlock();
>
>         kvfree(inst);
>
> --
> 2.53.0
>