linux-kernel - Re: [PATCH v2 10/49] KVM: x86: Drop now-redundant MAXPHYADDR and GPA rsvd bits from vCPU creation

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Message-ID: <ccbed564392478b3a5bb51b650a102ca474ba7e0.camel@redhat.com>
Date: Thu, 04 Jul 2024 21:13:59 -0400
From: Maxim Levitsky <mlevitsk@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
 <pbonzini@...hat.com>,  Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, Hou Wenlong
 <houwenlong.hwl@...group.com>, Kechen Lu <kechenl@...dia.com>, Oliver Upton
 <oliver.upton@...ux.dev>, Binbin Wu <binbin.wu@...ux.intel.com>, Yang
 Weijiang <weijiang.yang@...el.com>, Robert Hoo <robert.hoo.linux@...il.com>
Subject: Re: [PATCH v2 10/49] KVM: x86: Drop now-redundant MAXPHYADDR and
 GPA rsvd bits from vCPU creation

On Fri, 2024-05-17 at 10:38 -0700, Sean Christopherson wrote:
> Drop the manual initialization of maxphyaddr and reserved_gpa_bits during
> vCPU creation now that kvm_arch_vcpu_create() unconditionally invokes
> kvm_vcpu_after_set_cpuid(), which handles all such CPUID caching.
> 
> None of the helpers between the existing code in kvm_arch_vcpu_create()
> and the call to kvm_vcpu_after_set_cpuid() consume maxphyaddr or
> reserved_gpa_bits (though auditing vmx_vcpu_create() and svm_vcpu_create()
> isn't exactly easy).  And even if that weren't the case, KVM _must_
> refresh any affected state during kvm_vcpu_after_set_cpuid(), e.g. to
> correctly handle KVM_SET_CPUID2.  In other words, this can't introduce a
> new bug, only expose an existing bug (of which there don't appear to be
> any).


IMHO the change is not as bulletproof as claimed:

If some code does access the uninitialized state (e.g vcpu->arch.maxphyaddr which
will be zero, I assume), in between these calls, then even though
later the correct CPUID will be set and should override the incorrect state set earlier, 
the problem *is* that the mentioned code will
have to deal with non architecturally possible value (e.g maxphyaddr == 0)
which might cause a bug in it.

Of course such code currently doesn't exist, so it works but
it can fail in the future.

How about we move the call to kvm_vcpu_after_set_cpuid upward? 

Best regards,
	Maxim Levitsky

> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/x86.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2f6dda723005..bb34891d2f0a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12190,9 +12190,6 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
>  		goto free_emulate_ctxt;
>  	}
>  
> -	vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);
> -	vcpu->arch.reserved_gpa_bits = kvm_vcpu_reserved_gpa_bits_raw(vcpu);
> -
>  	vcpu->arch.pat = MSR_IA32_CR_PAT_DEFAULT;
>  
>  	kvm_async_pf_hash_reset(vcpu);