linux-kernel - Re: [PATCH 3/4] KVM: x86: WARN and reject loading KVM if NX is supported but not enabled

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Message-ID: <YNUITW5fsaQe4JSo@google.com>
Date:   Thu, 24 Jun 2021 22:33:49 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] KVM: x86: WARN and reject loading KVM if NX is
 supported but not enabled

On Tue, Jun 15, 2021, Sean Christopherson wrote:
> WARN if NX is reported as supported but not enabled in EFER.  All flavors
> of the kernel, including non-PAE 32-bit kernels, set EFER.NX=1 if NX is
> supported, even if NX usage is disable via kernel command line.

Ugh, I misread .Ldefault_entry in head_32.S, it skips over the entire EFER code
if PAE=0.  Apparently I didn't test this with non-PAE paging and EPT?

Paolo, I'll send a revert since it's in kvm/next, but even better would be if
you can drop the patch :-)  Lucky for me you didn't pick up patch 4/4 that
depends on this...

I'll revisit this mess in a few weeks.

> on NX being enabled if it's supported, e.g. KVM will generate illegal NPT
> entries if nx_huge_pages is enabled and NX is supported but not enabled.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/x86.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index acc28473dec7..1f6595df45de 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10981,6 +10981,9 @@ int kvm_arch_hardware_setup(void *opaque)
>  	int r;
>  
>  	rdmsrl_safe(MSR_EFER, &host_efer);
> +	if (WARN_ON_ONCE(boot_cpu_has(X86_FEATURE_NX) &&
> +			 !(host_efer & EFER_NX)))
> +		return -EIO;
>  
>  	if (boot_cpu_has(X86_FEATURE_XSAVES))
>  		rdmsrl(MSR_IA32_XSS, host_xss);
> -- 
> 2.32.0.272.g935e593368-goog
>