linux-kernel - Re: [PATCH] arm64: kprobes: Restore local irqflag if kprobes is cancelled

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Date:   Mon, 12 Apr 2021 22:11:44 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Jisheng Zhang <Jisheng.Zhang@...aptics.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Liao Chang <liaochang1@...wei.com>
Subject: Re: [PATCH] arm64: kprobes: Restore local irqflag if kprobes is
 cancelled

Hi Jisheng,

On Mon, 12 Apr 2021 17:41:01 +0800
Jisheng Zhang <Jisheng.Zhang@...aptics.com> wrote:

> If instruction being single stepped caused a page fault, the kprobes
> is cancelled to let the page fault handler continue as a normal page
> fault. But the local irqflags are disabled so cpu will restore pstate
> with DAIF masked. After pagefault is serviced, the kprobes is
> triggerred again, we overwrite the saved_irqflag by calling
> kprobes_save_local_irqflag(). NOTE, DAIF is masked in this new saved
> irqflag. After kprobes is serviced, the cpu pstate is retored with
> DAIF masked.
> 
> This patch is inspired by one patch for riscv from Liao Chang.

Thanks for pointing it out. But I think kprobes_restore_local_irqflag()
is also needed for kcb->kprobe_status == KPROBE_REENTER case...no.
This is more complicated. In the reenter case, I think we have to retry
the kpreprobe_fault_handler() with recovered previous kprobes so that
it can handle page fault in its handler.

Hmm, RISC-V and other code also needs same fix.

Thank you,

> 
> Signed-off-by: Jisheng Zhang <Jisheng.Zhang@...aptics.com>
> ---
>  arch/arm64/kernel/probes/kprobes.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index 66aac2881ba8..85645b2b0c7a 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -267,10 +267,12 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
>  		if (!instruction_pointer(regs))
>  			BUG();
>  
> -		if (kcb->kprobe_status == KPROBE_REENTER)
> +		if (kcb->kprobe_status == KPROBE_REENTER) {
>  			restore_previous_kprobe(kcb);
> -		else
> +		} else {
> +			kprobes_restore_local_irqflag(kcb, regs);
>  			reset_current_kprobe();
> +		}
>  
>  		break;
>  	case KPROBE_HIT_ACTIVE:
> -- 
> 2.31.0
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>