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

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Date:   Tue, 4 May 2021 08:24:49 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Andy Lutomirski <luto@...nel.org>, 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 Tue, May 04, 2021 at 01:06:06PM +0000, David Laight wrote:
> From: Josh Poimboeuf
> > Sent: 04 May 2021 04:16
> ...
> > 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.
> 
> Isn't that one of the very boring standard spectre cases?

Classic v1 as described in the Spectre paper was a data-dependent
load/store, not a data-dependent branch.

-- 
Josh