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 11:45 AM, Sean Christopherson wrote:
> On Tue, Mar 03, 2020 at 10:27:47AM +0800, Xiaoyao Li wrote:
>> 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?
> 
> The case being discussed above would look like:
> 
> KVM CPUID Entries:
>     Function   Index Output
>     0x00000000 0x00: eax=0x0000000b ebx=0x756e6547 ecx=0x6c65746e edx=0x49656e69
>     0x00000001 0x00: eax=0x000906ea ebx=0x03000800 ecx=0xfffa3223 edx=0x0f8bfbff
>     0x00000002 0x00: eax=0x00000001 ebx=0x00000000 ecx=0x0000004d edx=0x002c307d
>     0x00000003 0x00: eax=0x00000000 ebx=0x00000000 ecx=0x00000000 edx=0x00000000
>     0x00000004 0x00: eax=0x00000121 ebx=0x01c0003f ecx=0x0000003f edx=0x00000001
>     0x00000004 0x01: eax=0x00000122 ebx=0x01c0003f ecx=0x0000003f edx=0x00000001
>     0x00000004 0x02: eax=0x00000143 ebx=0x03c0003f ecx=0x00000fff edx=0x00000001
>     0x00000004 0x03: eax=0x00000163 ebx=0x03c0003f ecx=0x00003fff edx=0x00000006
>     0x00000005 0x00: eax=0x00000000 ebx=0x00000000 ecx=0x00000003 edx=0x00000000
>     0x00000006 0x00: eax=0x00000004 ebx=0x00000000 ecx=0x00000000 edx=0x00000000
>     0x00000007 0x00: eax=0x00000000 ebx=0x009c4fbb ecx=0x00000004 edx=0x84000000
>     0x00000008 0x00: eax=0x00000000 ebx=0x00000000 ecx=0x00000000 edx=0x00000000
>     0x00000009 0x00: eax=0x00000000 ebx=0x00000000 ecx=0x00000000 edx=0x00000000
>     0x0000000a 0x00: eax=0x07300402 ebx=0x00000000 ecx=0x00000000 edx=0x00000603
> --> MISSING CPUID.0xB.0
>     0x0000000b 0x01: eax=0x00000000 ebx=0x00000001 ecx=0x00000201 edx=0x00000003
> 
> CPUID.0xB.0 does not exist, so output.ECX=0, which indicates an invalid
> level-type.
> 
> The SDM states (for CPUID.0xB):
> 
>     If an input value n in ECX returns the invalid level-type of 0 in ECX[15:8],
>     other input values with ECX > n also return 0 in ECX[15:8]
> 
> That means returning a valid level-type in CPUID.0xB.1 as above violates
> the SDM's definition of how leaf 0xB works.  I'm arguing we can ignore the
> adjustments that would be done on output.E{C,D} for an out of range leaf
> because the model is bogus.

Right.

So we'd better do something in KVM_SET_CPUID* , to avoid userspace set 
bogus cpuid.

>> 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.
> 
> Have you tested the patch, or is your comment based on the above discussion
> and/or code inspection?  Honest question, because I've thoroughly tested
> the above scenario and it works as you describe, but now I'm worried I
> completely botched my testing.
> 

No, I didn't test.

Leaf 0xB and 0x1f are special cases when they are the maximum basic 
leaf, because no matter what subleaf is, there is always a non-zero 
E[CX,DX].

If cpuid.0 returns maximum basic leaf as 0xB/0x1F, when queried leaf is 
greater, it should always return a non-zero value.