linux-kernel - Re: [PATCH 1/6] KVM: Explicitly verify target vCPU is online in kvm_get

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Message-ID: <a653b3e3-a75b-cdd4-bf58-1bfc9691299c@amd.com>
Date: Thu, 10 Oct 2024 18:33:24 +0200
From: "Gupta, Pankaj" <pankaj.gupta@....com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
 Michal Luczaj <mhal@...x.co>, Alexander Potapenko <glider@...gle.com>,
 Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>
Subject: Re: [PATCH 1/6] KVM: Explicitly verify target vCPU is online in
 kvm_get_vcpu()

On 10/10/2024 5:54 PM, Sean Christopherson wrote:
> On Thu, Oct 10, 2024, Pankaj Gupta wrote:
>> On 10/9/2024 5:04 PM, Sean Christopherson wrote:
>>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>>> index db567d26f7b9..450dd0444a92 100644
>>> --- a/include/linux/kvm_host.h
>>> +++ b/include/linux/kvm_host.h
>>> @@ -969,6 +969,15 @@ static inline struct kvm_io_bus *kvm_get_bus(struct kvm *kvm, enum kvm_bus idx)
>>>    static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
>>>    {
>>>    	int num_vcpus = atomic_read(&kvm->online_vcpus);
>>> +
>>> +	/*
>>> +	 * Explicitly verify the target vCPU is online, as the anti-speculation
>>> +	 * logic only limits the CPU's ability to speculate, e.g. given a "bad"
>>> +	 * index, clamping the index to 0 would return vCPU0, not NULL.
>>> +	 */
>>> +	if (i >= num_vcpus)
>>> +		return NULL;
>>
>> Would sev.c needs a NULL check for?
>>
>> sev_migrate_from()
>> ...
>> src_vcpu = kvm_get_vcpu(src_kvm, i);
>> src_svm = to_svm(src_vcpu);
>> ...
> 
> Nope, sev_check_source_vcpus() verifies the source and destination have the same
> number of online vCPUs before calling sev_migrate_from(), and it's all done with
> both VMs locked.
> 
> static int sev_check_source_vcpus(struct kvm *dst, struct kvm *src)
> {
> 	struct kvm_vcpu *src_vcpu;
> 	unsigned long i;
> 
> 	if (!sev_es_guest(src))
> 		return 0;
> 
> 	if (atomic_read(&src->online_vcpus) != atomic_read(&dst->online_vcpus))
> 		return -EINVAL;
> 
> 	kvm_for_each_vcpu(i, src_vcpu, src) {
> 		if (!src_vcpu->arch.guest_state_protected)
> 			return -EINVAL;
> 	}
> 
> 	return 0;
> }

Yes, right!

Thanks,
Pankaj