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

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Message-ID: <875yuu9goa.fsf@vitty.brq.redhat.com>
Date:   Tue, 21 Sep 2021 15:55:33 +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

Vitaly Kuznetsov <vkuznets@...hat.com> writes:

> 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.

Scratch that, kvm_vcpu_reset() and kvm_arch_vcpu_create() got mixed up
in my head :-(

>
> 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