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

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Date:   Thu, 21 Jul 2022 10:56:45 +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: David Laight
> Sent: 21 July 2022 09:22
> 
> 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.

Actually, can't you use %eax instead of %r10d?
IIRC it is only used for the number of FP registers in a varargs
call - and that isn't used in the kernel.
That removes the 3 'REG' prefixes and lets you use the
2-byte 04-xx instruction to add to %al.

Although I'm sure I remember something about a penalty for
accessing %al just after the full register.
So the 3-byte sign extending add may be better.

	David

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