Re: [PATCH 1/6] KVM: x86: Fix tracing of CPUID.function when function is out-of-range

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 3/3/2020 4:49 AM, Sean Christopherson wrote:
> On Mon, Mar 02, 2020 at 09:26:54PM +0100, Jan Kiszka wrote:
>> On 02.03.20 20:57, Sean Christopherson wrote:
>>> Rework kvm_cpuid() to query entry->function when adjusting the output
>>> values so that the original function (in the aptly named "function") is
>>> preserved for tracing.  This fixes a bug where trace_kvm_cpuid() will
>>> trace the max function for a range instead of the requested function if
>>> the requested function is out-of-range and an entry for the max function
>>> exists.
>>>
>>> Fixes: 43561123ab37 ("kvm: x86: Improve emulation of CPUID leaves 0BH and 1FH")
>>> Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
>>> Cc: Jim Mattson <jmattson@google.com>
>>> Cc: Xiaoyao Li <xiaoyao.li@intel.com>
>>> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
>>> ---
>>>   arch/x86/kvm/cpuid.c | 15 +++++++--------
>>>   1 file changed, 7 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>>> index b1c469446b07..6be012937eba 100644
>>> --- a/arch/x86/kvm/cpuid.c
>>> +++ b/arch/x86/kvm/cpuid.c
>>> @@ -997,12 +997,12 @@ static bool cpuid_function_in_range(struct kvm_vcpu *vcpu, u32 function)
>>>   	return max && function <= max->eax;
>>>   }
>>> +/* Returns true if the requested leaf/function exists in guest CPUID. */
>>>   bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
>>>   	       u32 *ecx, u32 *edx, bool check_limit)
>>>   {
>>> -	u32 function = *eax, index = *ecx;
>>> +	const u32 function = *eax, index = *ecx;
>>>   	struct kvm_cpuid_entry2 *entry;
>>> -	struct kvm_cpuid_entry2 *max;
>>>   	bool found;
>>>   	entry = kvm_find_cpuid_entry(vcpu, function, index);
>>> @@ -1015,18 +1015,17 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
>>>   	 */
>>>   	if (!entry && check_limit && !guest_cpuid_is_amd(vcpu) &&
>>>   	    !cpuid_function_in_range(vcpu, function)) {
>>> -		max = kvm_find_cpuid_entry(vcpu, 0, 0);
>>> -		if (max) {
>>> -			function = max->eax;
>>> -			entry = kvm_find_cpuid_entry(vcpu, function, index);
>>> -		}
>>> +		entry = kvm_find_cpuid_entry(vcpu, 0, 0);
>>> +		if (entry)
>>> +			entry = kvm_find_cpuid_entry(vcpu, entry->eax, index);
>>>   	}
>>>   	if (entry) {
>>>   		*eax = entry->eax;
>>>   		*ebx = entry->ebx;
>>>   		*ecx = entry->ecx;
>>>   		*edx = entry->edx;
>>> -		if (function == 7 && index == 0) {
>>> +
>>> +		if (entry->function == 7 && index == 0) {
>>>   			u64 data;
>>>   		        if (!__kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data, true) &&
>>>   			    (data & TSX_CTRL_CPUID_CLEAR))
>>>
>>
>> What about the !entry case below this? It was impacted by the function
>> capping so far, not it's no longer.
> 
> Hmm, the only way the output would be different is in a really contrived
> scenario where userspace doesn't provide an entry for the max basic leaf.
> 
> The !entry path can only be reached with "orig_function != function" if
> orig_function is out of range and there is no entry for the max basic leaf.

> The adjustments for 0xb/0x1f require the max basic leaf to be 0xb or 0x1f,
> and to take effect with !entry would require there to be a CPUID.max.1 but
> not a CPUID.max.0.  That'd be a violation of Intel's SDM, i.e. it's bogus
> userspace input and IMO can be ignored.
> 

Sorry I cannot catch you. Why it's a violation of Intel's SDM?

Supposing the max basic is 0x1f, and it queries cpuid(0x20, 0x5),
it should return cpuid(0x1f, 0x5).

But based on this patch, it returns all zeros.