Re: [PATCH 2/3] x86/boot: Fix another __read_cr4() case on 486

[Borislav Petkov]

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@xxxxxxxxxxxxxx
> Fixes: 18bc7bd523e0 ("x86/boot: Synchronize trampoline_cr4_features and mmu_cr4_features directly")
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
>  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.
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