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

[Josh Poimboeuf]

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)?

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

(And this makes me realize my uaccess pointer masking patch [1] never
got merged...)

[1] https://lore.kernel.org/lkml/1d06ed6485b66b9f674900368b63d7ef79f666ca.1599756789.git.jpoimboe@xxxxxxxxxx/

-- 
Josh