linux-kernel - Re: Do we need to do anything about "dead µops?"

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Message-ID: <20210504031616.covixup7rhdil3yq@treble>
Date:   Mon, 3 May 2021 22:16:16 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        David Kaplan <David.Kaplan@....com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Kees Cook <keescook@...omium.org>, Jann Horn <jannh@...gle.com>
Subject: Re: Do we need to do anything about "dead µops?"

On Mon, May 03, 2021 at 06:31:21PM -0700, Andy Lutomirski wrote:
> On Mon, May 3, 2021 at 4:30 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >
> > On Sat, May 01, 2021 at 09:26:33AM -0700, Andy Lutomirski wrote:
> > > Hi all-
> > >
> > > The "I See Dead µops" paper that is all over the Internet right now is
> > > interesting, and I think we should discuss the extent to which we
> > > should do anything about it.  I think there are two separate issues:
> > >
> > > First, should we (try to) flush the µop cache across privilege
> > > boundaries?  I suspect we could find ways to do this, but I don't
> > > really see the point.  A sufficiently capable attacker (i.e. one who
> > > can execute their own code in the dangerous speculative window or one
> > > who can find a capable enough string of gadgets) can put secrets into
> > > the TLB, various cache levels, etc.  The µop cache is a nice piece of
> > > analysis, but I don't think it's qualitatively different from anything
> > > else that we don't flush.  Am I wrong?
> >
> > Wouldn't this type of gadget (half-v1 gadget + value-dependent-branch)
> > would be much more likely to occur than a traditional Spectre v1
> > (half-v1 gadget + value-addressed-load)?
> 
> I don't fully believe this.  It's certainly the case that:
> 
> if (mispredicted as false)
>   return;
> secret = some_secret();
> if (secret == 42)
>   do_something();

Well, obviously we should never write secret-protecting code in that
manner.

I was actually thinking more along the lines of

	val = 0;

	if (user_supplied_idx < ARRAY_SIZE) // trained to speculatively be 'true'
		val = boring_non_secret_array[user_supplied_idx];

	if (val & 1)
		do_something();

In other words, the victim code wouldn't be accessing the secret
intentionally.  So there's no reason for it to avoid doing
data-dependent branches.

> will leak the fact that the secret is 42 into the µop cache, but it
> will also leak it into the icache and lots of other things.  I see
> nothing new here.

Hm, I suppose.  I don't think I'd ever considered that vector though.

All the more reason to mask usercopy addresses...

-- 
Josh