linux-kernel - Re: [PATCH v2 10/10] KVM: x86: WARN on non-zero CRs at RESET to detect improper initalization

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Message-ID: <8735py9gi5.fsf@vitty.brq.redhat.com>
Date:   Tue, 21 Sep 2021 15:59:14 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Reiji Watanabe <reijiw@...gle.com>
Subject: Re: [PATCH v2 10/10] KVM: x86: WARN on non-zero CRs at RESET to
 detect improper initalization

Sean Christopherson <seanjc@...gle.com> writes:

> WARN if CR0, CR3, or CR4 are non-zero at RESET, which given the current
> KVM implementation, really means WARN if they're not zeroed at vCPU
> creation.  VMX in particular has several ->set_*() flows that read other
> registers to handle side effects, and because those flows are common to
> RESET and INIT, KVM subtly relies on emulated/virtualized registers to be
> zeroed at vCPU creation in order to do the right thing at RESET.
>
> Use CRs as a sentinel because they are most likely to be written as side
> effects, and because KVM specifically needs CR0.PG and CR0.PE to be '0'
> to correctly reflect the state of the vCPU's MMU.  CRs are also loaded
> and stored from/to the VMCS, and so adds some level of coverage to verify
> that KVM doesn't conflate zero-allocating the VMCS with properly
> initializing the VMCS with VMWRITEs.
>
> Note, '0' is somewhat arbitrary, vCPU creation can technically stuff any
> value for a register so long as it's coherent with respect to the current
> vCPU state.  In practice, '0' works for all registers and is convenient.
>
> Suggested-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/x86.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ec61b90d9b73..4e25baac3977 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10800,6 +10800,16 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  	unsigned long new_cr0;
>  	u32 eax, dummy;
>  
> +	/*
> +	 * Several of the "set" flows, e.g. ->set_cr0(), read other registers
> +	 * to handle side effects.  RESET emulation hits those flows and relies
> +	 * on emulated/virtualized registers, including those that are loaded
> +	 * into hardware, to be zeroed at vCPU creation.  Use CRs as a sentinel
> +	 * to detect improper or missing initialization.
> +	 */
> +	WARN_ON_ONCE(!init_event &&
> +		     (old_cr0 || kvm_read_cr3(vcpu) || kvm_read_cr4(vcpu)));
> +
>  	kvm_lapic_reset(vcpu, init_event);
>  
>  	vcpu->arch.hflags = 0;

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly