linux-kernel - Re: [PATCH 2/3] x86/boot: Fix another __read

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Message-ID: <20160929114853.GC10578@nazgul.tnic>
Date:   Thu, 29 Sep 2016 13:48:53 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, david@...giorato.net,
        Brian Gerst <brgerst@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] x86/boot: Fix another __read_cr4() case on 486

On Wed, Sep 28, 2016 at 12:34:15PM -0700, Andy Lutomirski wrote:
> The condition for reading CR4 was wrong: there are some CPUs with
> CPUID but not CR4.  Rather than trying to make the condition exact,
> using __read_cr4_safe().
> 
> Reported-by: david@...giorato.net
> Fixes: 18bc7bd523e0 ("x86/boot: Synchronize trampoline_cr4_features and mmu_cr4_features directly")
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/kernel/setup.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 0fa60f5f5a16..5930a4d191b4 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -1137,9 +1137,13 @@ void __init setup_arch(char **cmdline_p)
>  	 * auditing all the early-boot CR4 manipulation would be needed to
>  	 * rule it out.
>  	 */
> -	if (boot_cpu_data.cpuid_level >= 0)
> -		/* A CPU has %cr4 if and only if it has CPUID. */
> -		mmu_cr4_features = __read_cr4();
> +	if (boot_cpu_data.cpuid_level >= 0) {
> +		/*
> +		 * CPUs without CPUID don't have CR4.  CPUs with CPUID
> +		 * usually have CR4.
> +		 */
> +		mmu_cr4_features = __read_cr4_safe();
> +	}

Why are we even doing the CPUID check instead of unconditionally doing
__read_cr4_safe()?

The safe variant will give 0 on !CR4 machines.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
--