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

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Date:   Mon, 3 May 2021 18:30:10 -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 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@redhat.com/

-- 
Josh