linux-kernel - Re: [PATCH 2/6] KVM: x86: Inject #GP on x2APIC WRMSR that sets reserved bits 63:32

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Date:   Sun, 08 Jan 2023 18:41:36 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Marc Orr <marcorr@...gle.com>, Ben Gardon <bgardon@...gle.com>,
        Venkatesh Srinivas <venkateshs@...omium.org>
Subject: Re: [PATCH 2/6] KVM: x86: Inject #GP on x2APIC WRMSR that sets
 reserved bits 63:32

On Sat, 2023-01-07 at 01:10 +0000, Sean Christopherson wrote:
> Reject attempts to set bits 63:32 for 32-bit x2APIC registers, i.e. all
> x2APIC registers except ICR.  Per Intel's SDM:
> 
>   Non-zero writes (by WRMSR instruction) to reserved bits to these
>   registers will raise a general protection fault exception
> 
> Opportunistically fix a typo in a nearby comment.
> 
> Reported-by: Marc Orr <marcorr@...gle.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/lapic.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index f77da92c6ea6..bf53e4752f30 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -3108,13 +3108,17 @@ static int kvm_lapic_msr_read(struct kvm_lapic *apic, u32 reg, u64 *data)
>  static int kvm_lapic_msr_write(struct kvm_lapic *apic, u32 reg, u64 data)
>  {
>  	/*
> -	 * ICR is a 64-bit register in x2APIC mode (and Hyper'v PV vAPIC) and
> +	 * ICR is a 64-bit register in x2APIC mode (and Hyper-V PV vAPIC) and
>  	 * can be written as such, all other registers remain accessible only
>  	 * through 32-bit reads/writes.
>  	 */
>  	if (reg == APIC_ICR)
>  		return kvm_x2apic_icr_write(apic, data);
>  
> +	/* Bits 63:32 are reserved in all other registers. */
> +	if (data >> 32)
> +		return 1;
> +
>  	return kvm_lapic_reg_write(apic, reg, (u32)data);
>  }
>  
Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky