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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 03 May 2022 11:02:26 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        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
Cc:     "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>,
        Isaku Yamahata <isaku.yamahata@...il.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 v5 3/3] x86/tdx: Add Quote generation support

> 
> > 
> > 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