linux-kernel - [PATCH v5 10/12] x86/retpoline: Add boot time option to disable retpoline

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Date:   Sat,  6 Jan 2018 11:49:32 +0000
From:   David Woodhouse <dwmw@...zon.co.uk>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Paul Turner <pjt@...gle.com>, LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, tglx@...utronix.de,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk
Subject: [PATCH v5 10/12] x86/retpoline: Add boot time option to disable retpoline

From: Andi Kleen <ak@...ux.intel.com>

Add a noretpoline option boot to disable retpoline and patch out the
extra sequences. It cannot patch out the jumps to the thunk functions
from code generated by the compiler, but those thunks turn into a single
indirect branch now.

Signed-off-by: Andi Kleen <ak@...ux.intel.com>
Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
---
 Documentation/admin-guide/kernel-parameters.txt |  3 +++
 arch/x86/kernel/cpu/intel.c                     | 11 +++++++++++
 2 files changed, 14 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 520fdec..f30f9b4 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2596,6 +2596,9 @@
 
 	nohugeiomap	[KNL,x86] Disable kernel huge I/O mappings.
 
+	noretpoline	[X86] Disable the retpoline kernel indirect branch speculation
+			workarounds. System may allow data leaks with this option.
+
 	nosmt		[KNL,S390] Disable symmetric multithreading (SMT).
 			Equivalent to smt=1.
 
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index b720dac..35e123e 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -31,6 +31,17 @@
 #include <asm/apic.h>
 #endif
 
+#ifdef RETPOLINE
+static int __init noretpoline_setup(char *__unused)
+{
+	pr_info("Retpoline runtime disabled\n");
+	setup_clear_cpu_cap(X86_FEATURE_RETPOLINE);
+	setup_clear_cpu_cap(X86_FEATURE_RETPOLINE_AMD);
+	return 1;
+}
+__setup("noretpoline", noretpoline_setup);
+#endif
+
 /*
  * Just in case our CPU detection goes bad, or you have a weird system,
  * allow a way to override the automatic disabling of MPX.
-- 
2.7.4