linux-kernel - Re: [patch 2/2] x86/mm/pti: Do not invoke PTI functions when PTI is disabled

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Message-ID: <20190828190006.GB77809@gmail.com>
Date:   Wed, 28 Aug 2019 21:00:06 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Song Liu <songliubraving@...com>,
        Joerg Roedel <jroedel@...e.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [patch 2/2] x86/mm/pti: Do not invoke PTI functions when PTI is
 disabled


* Thomas Gleixner <tglx@...utronix.de> wrote:

> When PTI is disabled at boot time either because the CPU is not affected or
> PTI has been disabled on the command line, the boot code still calls into
> pti_finalize() which then unconditionally invokes:
> 
>      pti_clone_entry_text()
>      pti_clone_kernel_text()
> 
> pti_clone_kernel_text() was called unconditionally before the 32bit support
> was added and 32bit added the call to pti_clone_entry_text().
> 
> The call has no side effects as cloning the page tables into the available
> second one, which was allocated for PTI does not create damage. But it does
> not make sense either and in case that this functionality would be extended
> later this might actually lead to hard to diagnose issue.

s/issue/issues

> Neither function should be called when PTI is runtime disabled. Make the
> invocation conditional.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  arch/x86/mm/pti.c |    2 ++
>  1 file changed, 2 insertions(+)
> 
> --- a/arch/x86/mm/pti.c
> +++ b/arch/x86/mm/pti.c
> @@ -668,6 +668,8 @@ void __init pti_init(void)
>   */
>  void pti_finalize(void)
>  {
> +	if (!boot_cpu_has(X86_FEATURE_PTI))
> +		return;
>  	/*
>  	 * We need to clone everything (again) that maps parts of the
>  	 * kernel image.

Acked-by: Ingo Molnar <mingo@...nel.org>

Thanks,

	Ingo