Re: [PATCH v6 5/5] x86/tdx: Add Quote generation support

[Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@linux.intel.com>]

On 5/17/22 4:06 PM, Kai Huang wrote:
> On Tue, 2022-05-17 at 13:08 -0700, Sathyanarayanan Kuppuswamy wrote:
>>
>> On 5/16/22 7:58 PM, Kai Huang wrote:
>>> On Fri, 2022-05-13 at 12:29 -0700, Sathyanarayanan Kuppuswamy wrote:
>>>>>
>>>>>
>>>>>> +
>>>>>> +	/* Wait for attestation completion */
>>>>>> +	ret = wait_for_completion_interruptible(&entry->compl);
>>>>>> +	if (ret < 0) {
>>>>>> +		/*
>>>>>> +		 * For premature termination, since VMM still owns the
>>>>>> +		 * shared buffer, mark the request invalid to let
>>>>>> +		 * quote_callback_handler() handle the memory cleanup
>>>>>> +		 * function.
>>>>>> +		 */
>>>>>> +		invalidate_quote_request(entry);
>>>>>
>>>>> Interrupt can arrive after signal interrupt.  So invalidate_quote_request()
>>>>> should check if the request is already processed, and return 0 or -EINTR.
>>>>> Probably check the state always and del_list under single lock/unlock pair.
>>>>
>>>> Agree. But I think we should return -EINTR for the interrupted case
>>>> irrespective of the processed status (so no return 0).
>>>>
>>>> I will hold the lock and handle the cleanup for the processed
>>>> status.
>>>
>>> Even if we check the buffer status in invalidate_quote_request(), there's no
>>> guarantee the VMM won't change the buffer status _after_ we do the check, so
>>> looks such check isn't necessary.
>>>
>>
>> Consider the case where we get a callback interrupt, and before we
>> complete the processing for it, user terminates the request. In this
>> scenario,  quote_callback_handler() will consider the request is
>> still valid and no do the memory cleanup. To handle this case,
>> we need to check the status in invalidate_quote_request() and do
>> the cleanup if required.
>>
>> /* Handles early termination of GetQuote requests */
>> void invalidate_quote_request(struct quote_entry *entry)
>> {
>>           struct tdx_quote_hdr *quote_hdr;
>>
>>           /*
>>            * For early termination, if the request is not yet
>>            * processed by VMM (GET_QUOTE_IN_FLIGHT), the VMM
>>            * still owns the shared buffer, so mark the request
>>            * invalid to let quote_callback_handler() handle the
>>            * memory cleanup function. If the request is already
>>            * processed, then do the cleanup and return.
>>            */
>>
>>           mutex_lock(&quote_lock);
>>           quote_hdr = (struct tdx_quote_hdr *)entry->buf->vmaddr;
>>           if (quote_hdr->status == GET_QUOTE_IN_FLIGHT) {
> 
> What prevents VMM from updating quote_hdr->status from IN_FLIGHT to DONE _after_
> this check?

If IN_FLIGHT -> DONE happens at this point, you will get a separate
callback interrupt for it after this change. Since we hold quote_lock
here, processing of that callback work will wait till we make the
request invalid. For invalid requests, quote_callback_handler() will
handle the cleanup.

This logic is mainly for the case where quote_callback_handler() in
process and user terminates the request. In this case,
quote_callback_handler() will not do the cleanup, and we should add
logic here to handle it.

> 
> If you want to add such check, you should check against GET_QUOTE_DONE, but not
> IN_FLIGHT.  Only after status is DONE,  VMM will not update the buffer.  Perhaps
> something like below:
> 
> 	mutex_lock(&quote_lock);
> 	/* Skip invalidate the buffer if VMM has done with the buffer */
> 	if (quote_hdr->status == GET_QUOTE_DONE) {
> 		mutex_unlock(&quote_lock);
> 		return 0;
> 	}

There is no DONE status. Status can be either one of the following values.

GET_QUOTE_ERROR
GET_QUOTE_SUCCESS
GET_QUOTE_SERVICE_UNAVAILABLE
GET_QUOTE_IN_FLIGHT

> 
> And in the IOCTL, you can perhaps to choose to return 0, instead of -EINTR in
> this case, as the Quote has been finished already.

I think your logic above is to return 0 for the completed request even
if it is interrupted. But IMO, since the user already interrupted the
request, then I think it is more appropriate to just return -EINTR for
this case.

> 
> But I am not sure whether this is necessary.  The worst case is one finished
> Quote is wasted I guess.

> 
>>                   entry->valid = false;
>>                   mutex_unlock(&quote_lock);
>>                   return;
>>           }
>>           _del_quote_entry(entry);
>>           mutex_unlock(&quote_lock);
>> }
>>
>>
> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer