linux-kernel - Re: [PATCH v2 1/7] x86: refactor kprobes_fault() like kprobe_exceptions

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Message-ID: <20180828113611.GC32231@nazgul.tnic>
Date:   Tue, 28 Aug 2018 13:36:11 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Jann Horn <jannh@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        kernel-hardening@...ts.openwall.com,
        Tony Luck <tony.luck@...el.com>, linux-kernel@...r.kernel.org,
        dvyukov@...gle.com, linux-edac@...r.kernel.org
Subject: Re: [PATCH v2 1/7] x86: refactor kprobes_fault() like
 kprobe_exceptions_notify()

On Mon, Aug 27, 2018 at 08:56:25PM +0200, Jann Horn wrote:
> This is an extension of commit b506a9d08bae ("x86: code clarification patch
> to Kprobes arch code"). As that commit explains, even though
> kprobe_running() can't be called with preemption enabled, you don't have to
> disable preemption - if preemption is on, you can't be in a kprobe.
> 
> Also, use X86_TRAP_PF instead of 14.
> 
> Signed-off-by: Jann Horn <jannh@...gle.com>
> ---
>  arch/x86/mm/fault.c | 19 ++++++++-----------
>  1 file changed, 8 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index b9123c497e0a..2254a30533b9 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -44,17 +44,14 @@ kmmio_fault(struct pt_regs *regs, unsigned long addr)
>  
>  static nokprobe_inline int kprobes_fault(struct pt_regs *regs)
>  {
> -	int ret = 0;
> -
> -	/* kprobe_running() needs smp_processor_id() */
> -	if (kprobes_built_in() && !user_mode(regs)) {
> -		preempt_disable();
> -		if (kprobe_running() && kprobe_fault_handler(regs, 14))
> -			ret = 1;
> -		preempt_enable();
> -	}
> -
> -	return ret;
> +	/*
> +	 * To be potentially processing a kprobe fault and to be allowed to call
> +	 * kprobe_running(), we have to be non-preemptible.
> +	 */
> +	if (kprobes_built_in() && !user_mode(regs) && !preemptible() &&
> +	    kprobe_running() && kprobe_fault_handler(regs, X86_TRAP_PF))
> +		return 1;
> +	return 0;

Maybe even:

	return (kprobes_built_in() &&
		!user_mode(regs) &&
		!preemptible() &&
		kprobe_running() &&
		kprobe_fault_handler(regs, X86_TRAP_PF));

Although I'd do it a bit more readable by flipping the checks and
splitting them:

	if (!kprobes_built_in())
		return 0;

	if (user_mode(regs))
		return 0;

	...

	return kprobe_fault_handler(regs, X86_TRAP_PF);
}

-- 
Regards/Gruss,
    Boris.

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