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

[Sathyanarayanan Kuppuswamy]

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


--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer