linux-kernel - Re: [PATCH] KVM: SVM: Assume a 64-bit hypercall for guests with protected state

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Message-ID: <87pmxg73h7.fsf@vitty.brq.redhat.com>
Date:   Mon, 24 May 2021 13:53:08 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Brijesh Singh <brijesh.singh@....com>,
        Ashish Kalra <Ashish.Kalra@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] KVM: SVM: Assume a 64-bit hypercall for guests with
 protected state

Tom Lendacky <thomas.lendacky@....com> writes:

> When processing a hypercall for a guest with protected state, currently
> SEV-ES guests, the guest CS segment register can't be checked to
> determine if the guest is in 64-bit mode. For an SEV-ES guest, it is
> expected that communication between the guest and the hypervisor is
> performed to shared memory using the GHCB. In order to use the GHCB, the
> guest must have been in long mode, otherwise writes by the guest to the
> GHCB would be encrypted and not be able to be comprehended by the
> hypervisor. Given that, assume that the guest is in 64-bit mode when
> processing a hypercall from a guest with protected state.
>
> Fixes: f1c6366e3043 ("KVM: SVM: Add required changes to support intercepts under SEV-ES")
> Reported-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
> ---
>  arch/x86/kvm/x86.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9b6bca616929..e715c69bb882 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8403,7 +8403,12 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>  
>  	trace_kvm_hypercall(nr, a0, a1, a2, a3);
>  
> -	op_64_bit = is_64_bit_mode(vcpu);
> +	/*
> +	 * If running with protected guest state, the CS register is not
> +	 * accessible. The hypercall register values will have had to been
> +	 * provided in 64-bit mode, so assume the guest is in 64-bit.
> +	 */
> +	op_64_bit = is_64_bit_mode(vcpu) || vcpu->arch.guest_state_protected;
>  	if (!op_64_bit) {
>  		nr &= 0xFFFFFFFF;
>  		a0 &= 0xFFFFFFFF;

While this is might be a very theoretical question, what about other
is_64_bit_mode() users? Namely, a very similar to the above check exists
in kvm_hv_hypercall() and kvm_xen_hypercall().

-- 
Vitaly