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


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