Re: [Question] int3 instruction generates a #UD in SEV VM

[Tom Lendacky <thomas.lendacky@amd.com>]

On 8/2/23 09:33, Tom Lendacky wrote:
> On 8/2/23 09:25, Tom Lendacky wrote:
>> On 8/2/23 09:01, Sean Christopherson wrote:
>>> On Wed, Aug 02, 2023, Wu Zongyo wrote:
>>>> On Mon, Jul 31, 2023 at 11:45:29PM +0800, wuzongyong wrote:
>>>>>
>>>>> On 2023/7/31 23:03, Tom Lendacky wrote:
>>>>>> On 7/31/23 09:30, Sean Christopherson wrote:
>>>>>>> On Sat, Jul 29, 2023, wuzongyong wrote:
>>>>>>>> Hi,
>>>>>>>> I am writing a firmware in Rust to support SEV based on project 
>>>>>>>> td-shim[1].
>>>>>>>> But when I create a SEV VM (just SEV, no SEV-ES and no SEV-SNP) 
>>>>>>>> with the firmware,
>>>>>>>> the linux kernel crashed because the int3 instruction in 
>>>>>>>> int3_selftest() cause a
>>>>>>>> #UD.
>>>>>>>
>>>>>>> ...
>>>>>>>
>>>>>>>> BTW, if a create a normal VM without SEV by qemu & OVMF, the int3 
>>>>>>>> instruction always generates a
>>>>>>>> #BP.
>>>>>>>> So I am confused now about the behaviour of int3 instruction, 
>>>>>>>> could anyone help to explain the behaviour?
>>>>>>>> Any suggestion is appreciated!
>>>>>>>
>>>>>>> Have you tried my suggestions from the other thread[*]?
>>>>> Firstly, I'm sorry for sending muliple mails with the same content. I 
>>>>> thought the mails I sent previously
>>>>> didn't be sent successfully.
>>>>> And let's talk the problem here.
>>>>>>>
>>>>>>>     : > > I'm curious how this happend. I cannot find any condition 
>>>>>>> that would
>>>>>>>     : > > cause the int3 instruction generate a #UD according to 
>>>>>>> the AMD's spec.
>>>>>>>     :
>>>>>>>     : One possibility is that the value from memory that gets 
>>>>>>> executed diverges from the
>>>>>>>     : value that is read out be the #UD handler, e.g. due to 
>>>>>>> patching (doesn't seem to
>>>>>>>     : be the case in this test), stale cache/tlb entries, etc.
>>>>>>>     :
>>>>>>>     : > > BTW, it worked nomarlly with qemu and ovmf.
>>>>>>>     : >
>>>>>>>     : > Does this happen every time you boot the guest with your 
>>>>>>> firmware? What
>>>>>>>     : > processor are you running on?
>>>>>>>     :
>>>>> Yes, every time.
>>>>> The processor I used is EPYC 7T83.
>>>>>>>     : And have you ruled out KVM as the culprit?  I.e. verified 
>>>>>>> that KVM is NOT injecting
>>>>>>>     : a #UD.  That obviously shouldn't happen, but it should be 
>>>>>>> easy to check via KVM
>>>>>>>     : tracepoints.
>>>>>>
>>>>>> I have a feeling that KVM is injecting the #UD, but it will take 
>>>>>> instrumenting KVM to see which path the #UD is being injected from.
>>>>>>
>>>>>> Wu Zongyo, can you add some instrumentation to figure that out if 
>>>>>> the trace points towards KVM injecting the #UD?
>>>>> Ok, I will try to do that.
>>>> You're right. The #UD is injected by KVM.
>>>>
>>>> The path I found is:
>>>>      svm_vcpu_run
>>>>          svm_complete_interrupts
>>>>         kvm_requeue_exception // vector = 3
>>>>             kvm_make_request
>>>>
>>>>      vcpu_enter_guest
>>>>          kvm_check_and_inject_events
>>>>         svm_inject_exception
>>>>             svm_update_soft_interrupt_rip
>>>>             __svm_skip_emulated_instruction
>>>>                 x86_emulate_instruction
>>>>                 svm_can_emulate_instruction
>>>>                     kvm_queue_exception(vcpu, UD_VECTOR)
>>>>
>>>> Does this mean a #PF intercept occur when the guest try to deliver a
>>>> #BP through the IDT? But why?
>>>
>>> I doubt it's a #PF.  A #NPF is much more likely, though it could be 
>>> something
>>> else entirely, but I'm pretty sure that would require bugs in both the 
>>> host and
>>> guest.
>>>
>>> What is the last exit recorded by trace_kvm_exit() before the #UD is 
>>> injected?
>>
>> I'm guessing it was a #NPF, too. Could it be related to the changes that
>> went in around svm_update_soft_interrupt_rip()?
>>
>> 6ef88d6e36c2 ("KVM: SVM: Re-inject INT3/INTO instead of retrying the 
>> instruction")
> 
> Sorry, that should have been:
> 
> 7e5b5ef8dca3 ("KVM: SVM: Re-inject INTn instead of retrying the insn on 
> "failure"")

Doh! I was right the first time... sigh

6ef88d6e36c2 ("KVM: SVM: Re-inject INT3/INTO instead of retrying the instruction")

Thanks,
Tom

> 
>>
>> Before this the !nrips check would prevent the call into
>> svm_skip_emulated_instruction(). But now, there is a call to:
>>
>>    svm_update_soft_interrupt_rip()
>>      __svm_skip_emulated_instruction()
>>        kvm_emulate_instruction()
>>          x86_emulate_instruction() (passed a NULL insn pointer)
>>            kvm_can_emulate_insn() (passed a NULL insn pointer)
>>              svm_can_emulate_instruction() (passed NULL insn pointer)
>>
>> Because it is an SEV guest, it ends up in the "if (unlikely(!insn))" path
>> and injects the #UD.
>>
>> Thanks,
>> Tom
>>