From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Date: Mon, 5 Feb 2018 19:34:55 +0000 (-0500)
Subject: x86/spectre_v2: Add spectre_v2_heuristics=
X-Git-Tag: v4.1.12-124.31.3~1144
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=2768230b60c362512f3837db2742706859cd79e5;p=users%2Fjedix%2Flinux-maple.git

x86/spectre_v2: Add spectre_v2_heuristics=

As we have one already in the tree:
 x86/spectre: Favor IBRS on Skylake over retpoline

But we may want a knob to turn that off altgoether. The
hard to remember spectre_v2_heuristics=off or
spectre_v2_heuristics=skylake=off

will take care of disabling that optimization.

Orabug: 27477743
CVE: CVE-2017-5715
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
---

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index f01453c4a342..7f3662353206 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3515,6 +3515,14 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			Not specifying this option is equivalent to
 			spectre_v2=auto.
 
+	spectre_v2_heuristics=
+			[X86] Control Spectre_v2 variant heuristics.
+
+			off		- disable all heuristics (see below)
+			skylake=off	- do not use IBRS if present on Skylake
+					  instead of retpoline (this is equivalant
+				          to spectre_v2=ibrs).
+
 	spia_io_base=	[HW,MTD]
 	spia_fio_base=
 	spia_pedr=
diff --git a/arch/x86/kernel/cpu/bugs_64.c b/arch/x86/kernel/cpu/bugs_64.c
index d56b3d462f19..198bc1a552af 100644
--- a/arch/x86/kernel/cpu/bugs_64.c
+++ b/arch/x86/kernel/cpu/bugs_64.c
@@ -41,6 +41,40 @@ EXPORT_SYMBOL(use_ibpb);
 DEFINE_MUTEX(spec_ctrl_mutex);
 EXPORT_SYMBOL(spec_ctrl_mutex);
 
+bool use_ibrs_on_skylake = true;
+EXPORT_SYMBOL(use_ibrs_on_skylake);
+
+
+int __init spectre_v2_heuristics_setup(char *p)
+{
+	ssize_t len;
+
+	while (*p) {
+		/* Disable all heuristics. */
+		if (!strncmp(p, "off", 3)) {
+			use_ibrs_on_skylake = false;
+			break;
+		}
+		len = strlen("skylake");
+		if (!strncmp(p, "skylake", len)) {
+			p += len;
+			if (*p == '=')
+				++p;
+			if (*p == '\0')
+				break;
+			if (!strncmp(p, "off", 3))
+				use_ibrs_on_skylake = false;
+		}
+
+		p = strpbrk(p, ",");
+		if (!p)
+			break;
+		p++; /* skip ',' */
+	}
+	return 1;
+}
+__setup("spectre_v2_heuristics=", spectre_v2_heuristics_setup);
+
 static void __init spectre_v2_select_mitigation(void);
 
 void __init check_bugs(void)
@@ -357,7 +391,8 @@ retpoline_auto:
 			 * are forced to use retpoline on Skylake then use that.
 			 */
 			if (!retp_compiler() /* prefer IBRS over minimal ASM */ ||
-			    (retp_compiler() && !retpoline_selected(cmd) && is_skylake_era())) {
+			    (retp_compiler() && !retpoline_selected(cmd) &&
+			     is_skylake_era() && use_ibrs_on_skylake)) {
 				mode = SPECTRE_V2_IBRS;
 				/* OK, some form of IBRS is enabled, lets see if we need to STUFF_RSB */
 				if (!boot_cpu_has(X86_FEATURE_SMEP))