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

[Andy Lutomirski <luto@kernel.org>]

On Mon, May 3, 2021 at 4:30 PM Josh Poimboeuf <jpoimboe@redhat.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();

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.  That being said, it's probably still worth
investigating the gadgets.

>
> Also, in section V.A., they identified 37 gadgets.  Has anybody looked
> at those yet?

Not I.