linux-kernel - Re: [PATCH v2 02/10] KVM: x86: Clear KVM's cached guest CR3 at RESET/INIT

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Message-ID: <878rzq9gte.fsf@vitty.brq.redhat.com>
Date:   Tue, 21 Sep 2021 15:52:29 +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 02/10] KVM: x86: Clear KVM's cached guest CR3 at
 RESET/INIT

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

> Explicitly zero the guest's CR3 and mark it available+dirty at RESET/INIT.
> Per Intel's SDM and AMD's APM, CR3 is zeroed at both RESET and INIT.  For
> RESET, this is a nop as vcpu is zero-allocated.  For INIT, the bug has
> likely escaped notice because no firmware/kernel puts its page tables root
> at PA=0, let alone relies on INIT to get the desired CR3 for such page
> tables.
>
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/x86.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e77a5bf2d940..2cb38c67ed43 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10899,6 +10899,9 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  	kvm_set_rflags(vcpu, X86_EFLAGS_FIXED);
>  	kvm_rip_write(vcpu, 0xfff0);
>  
> +	vcpu->arch.cr3 = 0;
> +	kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);

kvm_register_mark_dirty() is redundant here as PATCH1 does

      vcpu->arch.regs_avail = ~0;
      vcpu->arch.regs_dirty = ~0;

just a few lines above. The dependency is, however, implicit and this
patch is marked for stable@ (well, PATCH1 has 8 Fixes: tags so I'd
expect it to get picked by everyone too, especially by robots) and
flipping two bits is cheap.

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

> +
>  	/*
>  	 * CR0.CD/NW are set on RESET, preserved on INIT.  Note, some versions
>  	 * of Intel's SDM list CD/NW as being set on INIT, but they contradict

-- 
Vitaly