Re: [PATCH v3 2/5] KVM: MMU: check guest CR3 reserved bits based on its physical address width.

Paolo Bonzini
2017-09-18T08:41:28Z

On 18/09/2017 10:15, Yu Zhang wrote:
>>
>> static bool emulator_get_cpuid(struct x86_emulate_ctxt *ctxt,
>> u32 *eax, u32 *ebx, u32 *ecx, u32 *edx, bool
>> check_limit)
>> {
>> return kvm_cpuid(emul_to_vcpu(ctxt), eax, ebx, ecx, edx,
>> check_limit);
>> }
>>
>> And:
>>
>> bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
>> u32 *ecx, u32 *edx, bool check_limit)
>> {
>> u32 function = *eax, index = *ecx;
>> struct kvm_cpuid_entry2 *best;
>> bool entry_found = true;
>> ...
>>
>> Doesn't this immediately try to dereference a NULL pointer? How much
>> testing have you done of this code?
>
> Thanks Jim.
> I have tested this code in a simulator to successfully boot a VM in
> shadow mode. Seems this code is not covered(but I am now still
> perplexed why this is not covered). Any possibility that the
> check_cr_write() is not triggered when emulating the cr operations?

CR moves usually don't go through the emulator (the main exception is
emulation of invalid guest state when the processor doesn't support
unrestricted_guest=1, but even that is unlikely to happen with
EFER.LMA=1). This explains why you didn't see the failure.

> Anyway, this should be a bug and thanks for pointing this out, and
> I'll send out the fix later.
Thanks,

Paolo