[Xenial][PATCH 1/1] x86/microcode/AMD: Do not load when running on a hypervisor

[Kamal Mostafa]

From: Borislav Petkov <bp at suse.de>

BugLink: http://bugs.launchpad.net/bugs/1758869

commit a15a753539eca8ba243d576f02e7ca9c4b7d7042 upstream with minor
adjustments.

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 at suse.de>
Tested-by: Juergen Gross <jgross at suse.com>
Tested-by: Boris Ostrovsky <boris.ostrovsky at oracle.com>
Acked-by: Juergen Gross <jgross at suse.com>
Link: http://lkml.kernel.org/r/20161218164414.9649-4-bp@alien8.de
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Cc: <stable at vger.kernel.org> # 4.4.x
Signed-off-by: Rolf Neugebauer <rolf.neugebauer at docker.com>
(back-ported from commit a15a753539eca8ba243d576f02e7ca9c4b7d7042)
Reference: https://lkml.org/lkml/2018/2/8/455
Signed-off-by: Kamal Mostafa <kamal at canonical.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 57ea794..d52b5e0 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -44,7 +44,7 @@
 
 static struct microcode_ops	*microcode_ops;
 
-static bool dis_ucode_ldr;
+static bool dis_ucode_ldr = true;
 
 static int __init disable_loader(char *str)
 {
@@ -81,6 +81,7 @@ struct cpu_info_ctx {
 
 static bool __init check_loader_disabled_bsp(void)
 {
+	u32 a, b, c, d;
 #ifdef CONFIG_X86_32
 	const char *cmdline = (const char *)__pa_nodebug(boot_command_line);
 	const char *opt	    = "dis_ucode_ldr";
@@ -93,8 +94,23 @@ 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;
 }
@@ -126,9 +142,6 @@ void __init load_ucode_bsp(void)
 	if (check_loader_disabled_bsp())
 		return;
 
-	if (!have_cpuid_p())
-		return;
-
 	vendor = x86_vendor();
 	family = x86_family();
 
@@ -162,9 +175,6 @@ void load_ucode_ap(void)
 	if (check_loader_disabled_ap())
 		return;
 
-	if (!have_cpuid_p())
-		return;
-
 	vendor = x86_vendor();
 	family = x86_family();
 
-- 
2.7.4