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SubjectRe: [PATCH v6 5/5] x86/tdx: Add Quote generation support
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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 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;
}

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.

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);
> }
>
>

--
Thanks,
-Kai


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