linux-kernel - Re: [PATCH v2 01/18] KVM: x86: host-initiated EFER.LME write affects the MMU

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Message-ID: <c337d280825c1e95d9181ab8aeb505a0b074c8d1.camel@redhat.com>
Date:   Wed, 23 Feb 2022 15:40:30 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com, stable@...r.kernel.org
Subject: Re: [PATCH v2 01/18] KVM: x86: host-initiated EFER.LME write
 affects the MMU

On Thu, 2022-02-17 at 16:03 -0500, Paolo Bonzini wrote:
> While the guest runs, EFER.LME cannot change unless CR0.PG is clear, and therefore
> EFER.NX is the only bit that can affect the MMU role.  However, set_efer accepts
> a host-initiated change to EFER.LME even with CR0.PG=1.  In that case, the
> MMU has to be reset.
> 
> Fixes: 11988499e62b ("KVM: x86: Skip EFER vs. guest CPUID checks for host-initiated writes")
> Cc: stable@...r.kernel.org
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/mmu.h | 1 +
>  arch/x86/kvm/x86.c | 2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 51faa2c76ca5..a5a50cfeffff 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -48,6 +48,7 @@
>  			       X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_PKE)
>  
>  #define KVM_MMU_CR0_ROLE_BITS (X86_CR0_PG | X86_CR0_WP)
> +#define KVM_MMU_EFER_ROLE_BITS (EFER_LME | EFER_NX)
>  
>  static __always_inline u64 rsvd_bits(int s, int e)
>  {
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d3da64106685..99a58c25f5c2 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1647,7 +1647,7 @@ static int set_efer(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  	}
>  
>  	/* Update reserved bits */
> -	if ((efer ^ old_efer) & EFER_NX)
> +	if ((efer ^ old_efer) & KVM_MMU_EFER_ROLE_BITS)
>  		kvm_mmu_reset_context(vcpu);
>  
>  	return 0;

It makes sense.

I am just curios, is there a report of failure
due to this issue? I can imagine something like this breaking
nested migration of 32 bit guests and such and/or smm and such.

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky