linux-kernel - Re: [PATCH 2/3] KVM: nVMX: restore L1's EFER prior to setting the nested state

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Date:   Wed, 10 Nov 2021 16:01:49 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     Wanpeng Li <wanpengli@...cent.com>, Borislav Petkov <bp@...en8.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH 2/3] KVM: nVMX: restore L1's EFER prior to setting the
 nested state

On 11/10/21 11:00, Maxim Levitsky wrote:
> +	/*
> +	 * The vcpu might currently contain L2's IA32_EFER, due to the way
> +	 * some userspace kvm users (e.g qemu) restore nested state.
> +	 *
> +	 * To fix this, restore its IA32_EFER to the value it would have
> +	 * after VM exit from the nested guest.
> +	 *
> +	 */
> +
> +	vcpu->arch.efer = nested_vmx_get_vmcs12_host_efer(vcpu, vmcs12);
> +

In principle the value of LOAD_HOST_EFER on exit need not be the same as 
on entry.  But you don't need all of EFER, only EFER.LME/EFER.LMA, and 
those two bits must match ("the values of the LMA and LME bits in the 
field must each be that of the “host address-space size” VM-exit 
control" from the "Checks on Host Control Registers, MSRs, and SSP"; 
plus the "Checks Related to Address-Space Size").

At least it's worth adjusting the comment to explain that.  But the root 
cause of the issue is just nested_vmx_check_* accessing vcpu->arch.  So 
you can instead:

- split out of nested_vmx_check_host_state a new function 
nested_vmx_check_address_state_size that does

#ifdef CONFIG_X86_64
	if (CC(!!(vmcs12->vm_exit_controls & VM_EXIT_HOST_ADDR_SPACE_SIZE) !=
	       !!(vcpu->arch.efer & EFER_LMA)))
		return -EINVAL;
#endif
	return 0;

- call it from vmentry but not from migration

- in nested_vmx_check_host_state, assign ia32e from 
vmcs12->vm_exit_controls instead of vcpu->arch.efer

Paolo