Can't boot as Xen dom0 due to commit fe055896

Commit Message

Borislav Petkov Dec. 16, 2016, 9:01 a.m. UTC

On Thu, Dec 15, 2016 at 10:56:25PM -0500, Boris Ostrovsky wrote:
> You can use xen_cpuid_base(), for example. It will prevent microcode loading

Actually I want to do this at the end. CPUID(1).ECX[31] is reserved by
both vendors for hypervisor use.

> True. But I don't think it's clear that the problem we are seeing is
> Xen-specific.

If you can trigger it on baremetal, I'm all ears.

---

Comments

Andrew Cooper Dec. 16, 2016, 9:14 a.m. UTC | #1

On 16/12/2016 09:01, Borislav Petkov wrote:
> @@ -91,6 +92,17 @@ static bool __init check_loader_disabled_bsp(void)
>  	if (cmdline_find_option_bool(cmdline, option))
>  		*res = true;
>  
> +	if (!have_cpuid_p())
> +		*res = true;
> +
> +	a = 1;
> +	c = 0;
> +	native_cpuid(&a, &b, &c, &d);
> +
> +	/* CPUID(1).ECX[31]: reserved for hypervisor use */
> +	if (c & BIT(31))
> +		*res = true;

This will work for any VT-x/SVM based virt, but won't work for ring
de-privileging based virt such as Xen PV and lguest.  Without CPUID
Faulting, this will still get the real hardware CPUID without the
hypervisor bit set.

How about an additional cpl check here, which should cover all the ring
de-privileging methods in a virt-neutral way?

~Andrew

Patch

diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 6996413c78c3..54219f619205 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -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);
@@ -91,6 +92,17 @@  static bool __init check_loader_disabled_bsp(void)
 	if (cmdline_find_option_bool(cmdline, option))
 		*res = true;
 
+	if (!have_cpuid_p())
+		*res = true;
+
+	a = 1;
+	c = 0;
+	native_cpuid(&a, &b, &c, &d);
+
+	/* CPUID(1).ECX[31]: reserved for hypervisor use */
+	if (c & BIT(31))
+		*res = true;
+
 	return *res;
 }
 
@@ -121,9 +133,6 @@  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 +166,6 @@  void load_ucode_ap(void)
 	if (check_loader_disabled_ap())
 		return;
 
-	if (!have_cpuid_p())
-		return;
-
 	vendor = x86_cpuid_vendor();
 	family = x86_cpuid_family();