linux-kernel - Re: Avoid speculative indirect calls in kernel

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Date:   Thu, 04 Jan 2018 20:05:31 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Tom Lendacky <thomas.lendacky@....com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "pavel@....cz" <pavel@....cz>
Cc:     "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "andi@...stfloor.org" <andi@...stfloor.org>,
        "gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: Avoid speculative indirect calls in kernel

On Thu, 2018-01-04 at 14:00 -0600, Tom Lendacky wrote:
> Yes, lfence is sufficient.  As long as the target is in the register
> before the lfence and we jump through the register all is good, i.e.:

Thanks. Can I have a Reviewed-by: for this then please:

http://git.infradead.org/users/dwmw2/linux-retpoline.git/commitdiff/08d9eda03

From: David Woodhouse <dwmw@...zon.co.uk>
Date: Thu, 4 Jan 2018 20:01:53 +0000
Subject: [PATCH] x86/retpoline: Simplify AMD variant of retpoline thunk

On AMD (which is X86_FEATURE_K8), just the lfence is sufficient.

Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
---
 arch/x86/lib/retpoline.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index bbdda5cc136e..26070976bff0 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -11,7 +11,7 @@
 
 ENTRY(__x86.indirect_thunk.\reg)
 	CFI_STARTPROC
-	ALTERNATIVE "call 2f", __stringify(jmp *%\reg), X86_BUG_NO_RETPOLINE
+	ALTERNATIVE_2 "call 2f", __stringify(lfence;jmp *%\reg), X86_FEATURE_K8, __stringify(jmp *%\reg), X86_BUG_NO_RETPOLINE
 1:
 	lfence
 	jmp	1b
-- 
2.14.3
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