Re: [PATCH v5 3/3] x86/tdx: Add Quote generation support

[Kai Huang <kai.huang@intel.com>]

> 
> > 
> > Also, the  buffer may be still used by VMM when timeout (IN_FLIGHT), how can
> > this even work?
> 
> We will never reach here for IN_FLIGHT case. We will block in
> wait_for_completion_interruptible() till the status changes to success
> or failure.

But you still have 'timeout' in userspace ABI?

> 
> > 
> > > +	tdquote = NULL;
> > > +	mutex_unlock(&quote_lock);
> > > +	return ret;
> > > +}
> > > +
> > > +static void attestation_callback_handler(void)
> > > +{
> > > +	struct tdx_quote_hdr *quote_hdr;
> > > +
> > > +	quote_hdr = (struct tdx_quote_hdr *) tdquote;
> > > +
> > > +	/* Check for spurious callback IRQ case */
> > > +	if (!tdquote || quote_hdr->status == GET_QUOTE_IN_FLIGHT)
> > > +		return;
> > 
> > I don't get the logic.  Please explain.
> 
> I am trying to handle spurious IRQ case here. If we receive a callback
> IRQ from VMM before even we allocate tdquote or post the GetQuote
> request, accessing quote_hdr->status will lead to NULL pointer
> exception. So I have added check for valid quote buffer (tdquote !=
> NULL)

tdquote here isn't protected mutex, so even after you check tdquote != NULL, you
cannot guarantee it is still valid when you check quote_hdr. 

For instance, you receive two interrupts in very short time.  The first
interrupt gets you out from wait_for_completion_interruptible().  Then second
interrupt comes, and you can potentially have situation like below:

	cpu 0					cpu 1

	tdx_get_quote				attestation_callback_hander
						
						!tdquote
		...
		tdquote = NULL;

						quote_hdr->status == ...


And the 'quote_hdr' is even initialized at the beginning, and when it is used,
the actual tdquote may already been freed.

Or did I get it wrong?

> 
> Second condition (quote_hdr->status == GET_QUOTE_IN_FLIGHT)) makes
> sure we don't mark the current quote request complete until the
> Quote buffer status changes to GET_QUOTE_SUCCESS, GET_QUOTE_ERROR or
> GET_QUOTE_SERVICE_UNAVAILABLE.

I thought the GHCI spec says VMM will always inject intererupt after the Quote
is ready, so we can have assumption it won't inject when buffer is still
IN_FLIGHT.  But I am also not so sure.


Anyway, to me it seems there's ambiguities around GetQuote and
SetupEventNotifyInterrupt.  This is the reason that I suggested  you to split
the basic driver out and you can even upstream it first w/o GetQuote support.  I
think you may want to actually start to consider to upstream the basic driver
first.


-- 
Thanks,
-Kai