Re: [PATCH v2 1/2] arm64: kprobes: disable preemption across XOL single-step

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Feb 17, 2026 at 07:08:54PM +0530, Khaja Hussain Shaik Khaji wrote:
> On arm64, non-emulatable kprobes instructions execute out-of-line (XOL)
> after returning from the initial debug exception. The XOL instruction
> runs in normal kernel context, while kprobe state is maintained per-CPU.

The XOL instruction runs in a context with all DAIF bits set (see
kprobes_save_local_irqflag() and kprobes_restore_local_irqflag()), so
not quite a regular kernel context.

> If the task is preempted or migrates during the XOL window, the subsequent
> SS-BRK exception may be handled on a different CPU, corrupting per-CPU
> kprobe state and preventing correct recovery.

I think we need a better explanation of this.

Since DAIF is masked, we won't take an IRQ to preempt during the actual
XOL execution.

AFAICT we *could* explicitly preempt/schedule in C code around the XOL
execution. However, AFAICT that'd equally apply to other architectures,
and on x86 they *removed* the preempt count manipulation in commit:

  2bbda764d720aaca ("kprobes/x86: Do not disable preempt on int3 path")

... so it looks like there's a wider potential problem here.

Can you please share an example failure that you have seen? .. and how
you triggered it (e.g. is this a plain kprobe, something with bpf, etc).

I reckon you could hack a warning something into schedule() (or
cond_resched(), etc) that detects when there's an active XOL slot, so
that we can get the full backtrace.

> Disable preemption across the XOL instruction and re-enable it in the
> SS-BRK handler to prevent migration until control returns to the kprobe
> handler.

This might work, but without some more detail I'm not certain this is
sufficient, and I believe other architectures are likely affected by the
same problem.

Thanks,
Mark.

> 
> Signed-off-by: Khaja Hussain Shaik Khaji <khaja.khaji@oss.qualcomm.com>
> ---
>  arch/arm64/kernel/probes/kprobes.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index 43a0361a8bf0..d8a70c456543 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -227,6 +227,14 @@ static void __kprobes setup_singlestep(struct kprobe *p,
>  
>  		kprobes_save_local_irqflag(kcb, regs);
>  		instruction_pointer_set(regs, slot);
> +
> +		/*
> +		 * Disable preemption across the out-of-line (XOL) instruction.
> +		 * The XOL instruction executes in normal kernel context and
> +		 * kprobe state is per-CPU.
> +		 */
> +		preempt_disable();
> +
>  	} else {
>  		/* insn simulation */
>  		arch_simulate_insn(p, regs);
> @@ -363,6 +371,11 @@ kprobe_ss_brk_handler(struct pt_regs *regs, unsigned long esr)
>  		kprobes_restore_local_irqflag(kcb, regs);
>  		post_kprobe_handler(cur, kcb, regs);
>  
> +		/*
> +		 * Re-enable preemption after completing the XOL instruction.
> +		 */
> +		preempt_enable_no_resched();
> +
>  		return DBG_HOOK_HANDLED;
>  	}
>  
> -- 
> 2.34.1
>