linux-kernel - Re: [PATCH v3 04/37] KVM: nVMX: Invalidate all roots when emulating INVVPID without EPT

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Message-ID: <87v9mv84qu.fsf@vitty.brq.redhat.com>
Date:   Mon, 23 Mar 2020 16:34:17 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Liran Alon <liran.alon@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        John Haxby <john.haxby@...cle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v3 04/37] KVM: nVMX: Invalidate all roots when emulating INVVPID without EPT

Sean Christopherson <sean.j.christopherson@...el.com> writes:

> From: Junaid Shahid <junaids@...gle.com>
>
> Free all roots when emulating INVVPID for L1 and EPT is disabled, as
> outstanding changes to the page tables managed by L1 need to be
> recognized.  Because L1 and L2 share an MMU when EPT is disabled, and
> because VPID is not tracked by the MMU role, all roots in the current
> MMU (root_mmu) need to be freed, otherwise a future nested VM-Enter or
> VM-Exit could do a fast CR3 switch (without a flush/sync) and consume
> stale SPTEs.
>
> Fixes: 5c614b3583e7b ("KVM: nVMX: nested VPID emulation")
> Signed-off-by: Junaid Shahid <junaids@...gle.com>
> [sean: ported to upstream KVM, reworded the comment and changelog]
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 9624cea4ed9f..bc74fbbf33c6 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -5250,6 +5250,20 @@ static int handle_invvpid(struct kvm_vcpu *vcpu)
>  		return kvm_skip_emulated_instruction(vcpu);
>  	}
>  
> +	/*
> +	 * Sync the shadow page tables if EPT is disabled, L1 is invalidating
> +	 * linear mappings for L2 (tagged with L2's VPID).  Free all roots as
> +	 * VPIDs are not tracked in the MMU role.
> +	 *
> +	 * Note, this operates on root_mmu, not guest_mmu, as L1 and L2 share
> +	 * an MMU when EPT is disabled.
> +	 *
> +	 * TODO: sync only the affected SPTEs for INVDIVIDUAL_ADDR.
> +	 */
> +	if (!enable_ept)
> +		kvm_mmu_free_roots(vcpu, &vcpu->arch.root_mmu,
> +				   KVM_MMU_ROOTS_ALL);
> +

This is related to my remark on the previous patch; the comment above
makes me think I'm missing something obvious, enlighten me please)

My understanding is that L1 and L2 will share arch.root_mmu not only
when EPT is globally disabled, we seem to switch between
root_mmu/guest_mmu only when nested_cpu_has_ept(vmcs12) but different L2
guests may be different on this. Do we need to handle this somehow?

>  	return nested_vmx_succeed(vcpu);
>  }

-- 
Vitaly