linux-kernel - [PATCH 3/4] x86/microcode/AMD: Do not load when running on a hypervisor

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Date:   Sun, 18 Dec 2016 17:44:13 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     X86 ML <x86@...nel.org>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Jürgen Gross <jgross@...e.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH 3/4] x86/microcode/AMD: Do not load when running on a hypervisor

From: Borislav Petkov <bp@...e.de>

Doing so is completely void of sense for multiple reasons so prevent
it. Set dis_ucode_ldr to true and thus disable the microcode loader by
default to address xen pv guests which execute the AP path but not the
BSP path.

By having it turned off by default, the APs won't run into the loader
either.

Also, check CPUID(1).ECX[31] which hypervisors set. Well almost, not the
xen pv one. That one gets the aforementioned "fix".

Also, improve the detection method by caching the final decision whether
to continue loading in dis_ucode_ldr and do it once on the BSP. The APs
then simply test that value.

Signed-off-by: Borislav Petkov <bp@...e.de>
Tested-by: Juergen Gross <jgross@...e.com>
Acked-by: Juergen Gross <jgross@...e.com>
Tested-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
---
 arch/x86/kernel/cpu/microcode/core.c | 28 +++++++++++++++++++---------
 1 file changed, 19 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 6996413c78c3..c4bb2f7169f6 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -44,7 +44,7 @@
 #define DRIVER_VERSION	"2.2"
 
 static struct microcode_ops	*microcode_ops;
-static bool dis_ucode_ldr;
+static bool dis_ucode_ldr = true;
 
 LIST_HEAD(microcode_cache);
 
@@ -76,6 +76,7 @@ struct cpu_info_ctx {
 static bool __init check_loader_disabled_bsp(void)
 {
 	static const char *__dis_opt_str = "dis_ucode_ldr";
+	u32 a, b, c, d;
 
 #ifdef CONFIG_X86_32
 	const char *cmdline = (const char *)__pa_nodebug(boot_command_line);
@@ -88,8 +89,23 @@ static bool __init check_loader_disabled_bsp(void)
 	bool *res = &dis_ucode_ldr;
 #endif
 
-	if (cmdline_find_option_bool(cmdline, option))
-		*res = true;
+	if (!have_cpuid_p())
+		return *res;
+
+	a = 1;
+	c = 0;
+	native_cpuid(&a, &b, &c, &d);
+
+	/*
+	 * CPUID(1).ECX[31]: reserved for hypervisor use. This is still not
+	 * completely accurate as xen pv guests don't see that CPUID bit set but
+	 * that's good enough as they don't land on the BSP path anyway.
+	 */
+	if (c & BIT(31))
+		return *res;
+
+	if (cmdline_find_option_bool(cmdline, option) <= 0)
+		*res = false;
 
 	return *res;
 }
@@ -121,9 +137,6 @@ void __init load_ucode_bsp(void)
 	if (check_loader_disabled_bsp())
 		return;
 
-	if (!have_cpuid_p())
-		return;
-
 	vendor = x86_cpuid_vendor();
 	family = x86_cpuid_family();
 
@@ -157,9 +170,6 @@ void load_ucode_ap(void)
 	if (check_loader_disabled_ap())
 		return;
 
-	if (!have_cpuid_p())
-		return;
-
 	vendor = x86_cpuid_vendor();
 	family = x86_cpuid_family();
 
-- 
2.11.0