Patch "x86/microcode/AMD: Do not load when running on a hypervisor" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

x86/microcode/AMD: Do not load when running on a hypervisor

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-microcode-amd-do-not-load-when-running-on-a-hypervisor.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Borislav Petkov <[email protected]>
Date: Sun, 18 Dec 2016 17:44:13 +0100
Subject: x86/microcode/AMD: Do not load when running on a hypervisor

From: Borislav Petkov <[email protected]>

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.

---
 arch/x86/kernel/cpu/microcode/core.c |   28 +++++++++++++++++++---------
 1 file changed, 19 insertions(+), 9 deletions(-)

@@ -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
 	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();
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/x86-microcode-amd-do-not-load-when-running-on-a-hypervisor.patch queue-4.4/x86-microcode-do-the-family-check-first.patch