Re: [PATCH 2/2] KVM: x86: Use memcmp in kvm_cpuid_check_equal()

From: Sean Christopherson <seanjc@google.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	kvm@vger.kernel.org, Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>,
	Igor Mammedov <imammedo@redhat.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86: Use memcmp in kvm_cpuid_check_equal()
Date: Mon, 24 Jan 2022 16:52:16 +0000	[thread overview]
Message-ID: <Ye7ZQJ6NYoZqK9yk@google.com> (raw)
In-Reply-To: <87bl01i2zl.fsf@redhat.com>

On Mon, Jan 24, 2022, Vitaly Kuznetsov wrote:
> Paolo Bonzini <pbonzini@redhat.com> writes:
> >> +	if (memcmp(e2, vcpu->arch.cpuid_entries, nent * sizeof(*e2)))
> >> +		return -EINVAL;
> >
> > Hmm, not sure about that due to the padding in struct kvm_cpuid_entry2. 
> >   It might break userspace that isn't too careful about zeroing it.

Given that we already are fully committed to potentially breaking userspace by
disallowing KVM_SET_CPUID{2} after KVM_RUN, we might as well get greedy.

> FWIW, QEMU zeroes the whole thing before setting individual CPUID
> entries. Legacy KVM_SET_CPUID call is also not afffected as it copies
> entries to a newly allocated "struct kvm_cpuid_entry2[]" and explicitly
> zeroes padding.
> 
> Do we need to at least add a check for ".flags"?

Yes.