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

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Message-ID: <5937e5a2f1014e2da4a07e249745ceb1@AcuMS.aculab.com>
Date:   Tue, 4 May 2021 13:06:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Josh Poimboeuf' <jpoimboe@...hat.com>,
        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?"

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?

	David

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