Re: [RFC] Retpoline: Binary mitigation for branch-target-injection (aka "Spectre")

From: Paul Turner <pjt@google.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: "Woodhouse, David" <dwmw@amazon.co.uk>,
	"Van De Ven, Arjan" <arjan.van.de.ven@intel.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"tim.c.chen@linux.intel.com" <tim.c.chen@linux.intel.com>,
	"peterz@infradead.org" <peterz@infradead.org>,
	"torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
	"tglx@linuxtronix.de" <tglx@linuxtronix.de>,
	"riel@redhat.com" <riel@redhat.com>,
	"keescook@google.com" <keescook@google.com>,
	"gnomes@lxorguk.ukuu.org.uk" <gnomes@lxorguk.ukuu.org.uk>,
	"dave.hansen@intel.com" <dave.hansen@intel.com>,
	"jikos@kernel.org" <jikos@kernel.org>,
	"gregkh@linux-foundation.org" <gregkh@linux-foundation.org>
Subject: Re: [RFC] Retpoline: Binary mitigation for branch-target-injection (aka "Spectre")
Date: Fri, 5 Jan 2018 02:49:44 -0800	[thread overview]
Message-ID: <20180105104944.GB253582@google.com> (raw)
In-Reply-To: <CALCETrWRca6X9AbXn2b_p_mbY9gAws3LFwsDxWw5Vk9zDLAwbg@mail.gmail.com>

On Thu, Jan 04, 2018 at 08:18:57AM -0800, Andy Lutomirski wrote:
> On Thu, Jan 4, 2018 at 1:30 AM, Woodhouse, David <dwmw@amazon.co.uk> wrote:
> > On Thu, 2018-01-04 at 01:10 -0800, Paul Turner wrote:
> >> Apologies for the discombobulation around today's disclosure.  Obviously the
> >> original goal was to communicate this a little more coherently, but the
> >> unscheduled advances in the disclosure disrupted the efforts to pull this
> >> together more cleanly.
> >>
> >> I wanted to open discussion the "retpoline" approach and and define its
> >> requirements so that we can separate the core
> >> details from questions regarding any particular implementation thereof.
> >>
> >> As a starting point, a full write-up describing the approach is available at:
> >>   https://support.google.com/faqs/answer/7625886
> >
> > Note that (ab)using 'ret' in this way is incompatible with CET on
> > upcoming processors. HJ added a -mno-indirect-branch-register option to
> > the latest round of GCC patches, which puts the branch target in a
> > register instead of on the stack. My kernel patches (which I'm about to
> > reconcile with Andi's tweaks and post) do the same.
> >
> > That means that in the cases where at runtime we want to ALTERNATIVE
> > out the retpoline, it just turns back into a bare 'jmp *\reg'.
> >
> >
> 
> I hate to say this, but I think Intel should postpone CET until the
> dust settles.  Intel should also consider a hardware-protected stack
> that is only accessible with PUSH, POP, CALL, RET, and a new MOVSTACK
> instruction.  That, by itself, would give considerable protection.
> But we still need JMP_NO_SPECULATE.  Or, better yet, get the CPU to
> stop leaking data during speculative execution.

Echoing Andy's thoughts, but from a slightly different angle:

1) BTI is worse than the current classes of return attack.  Given this,
   considered as a binary choice, it's equivalent to the current state of the
   world (e.g. no CET).
2) CET will not be "free".  I suspect in its initial revisions it will be more
   valuable for protecting end-users then enterprise workloads (cost is not
   observable for interactive workloads because there's tons of headroom in the
   first place).

While the potential incompatibility is unfortunate; I'm not sure it makes a
significant adoption to the adoption rate of CET.