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

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Message-ID: <40caabe4c708b35844c246ec6a01b4ee03cbfa63.camel@intel.com>
Date:   Wed, 18 May 2022 11:06:26 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Wander Lairson Costa <wander@...hat.com>,
        marcelo.cerri@...onical.com, tim.gardner@...onical.com,
        khalid.elmously@...onical.com, philip.cox@...onical.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 5/5] x86/tdx: Add Quote generation support

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