linux-kernel - RE: [patch 00/38] x86/retbleed: Call depth tracking mitigation

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Date:   Thu, 21 Jul 2022 08:21:52 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        Sami Tolvanen <samitolvanen@...gle.com>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joao Moreira <joao@...rdrivepizza.com>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        "Cooper, Andrew" <andrew.cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        "Moreira, Joao" <joao.moreira@...el.com>,
        "Nuzman, Joseph" <joseph.nuzman@...el.com>,
        "Steven Rostedt" <rostedt@...dmis.org>,
        "Gross, Jurgen" <jgross@...e.com>,
        "Masami Hiramatsu" <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        Peter Collingbourne <pcc@...gle.com>,
        "Kees Cook" <keescook@...omium.org>
Subject: RE: [patch 00/38] x86/retbleed: Call depth tracking mitigation

From: Peter Zijlstra
> Sent: 20 July 2022 22:13
...
> The prettiest option to obscure the immediate at the callsite I could
> conjure up is something like:
> 
> kcfi_caller_linus:
> 	movl	$0x12345600, %r10d
> 	movb	$0x78, %r10b
> 	cmpl	%r10d, -OFFSET(%r11)
> 	je	1f
> 	ud2
> 1:	call	__x86_thunk_indirect_r11
> 
> Which comes to around 22 bytes (+5 over the original).

You'd be better doing:
	movl $0x12345678-0xaa, %r10d
	addl $0xaa, %r10d
so that the immediate is obscured even if the low bits are zero.

	David

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