Re: [PATCH 3.17 100/141] x86, microcode: Fix accessing dis_ucode_ldr on 32-bit

[Borislav Petkov]

Adding x86 people.

On Tue, Nov 25, 2014 at 04:59:34PM -0500, Boris Ostrovsky wrote:
> This should be cpuid(0x1, &eax, &ebx, &ecx, &edx). Otherwise we are not
> getting bits that the hypervisor wants the guest to see (on Xen cpuid()
> turns into hypercall, on baremetal it's native).
> 
> With that change it works and
> 
> Tested-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

Thanks for testing.

> (May be worth adding a comment as to what is_guest() is checking for since
> 31 is a magic number).

See below.

> BTW, the crash had nothing to do with accessing dis_ucode_ldr, we are
> crashing much later, in load_ucode_intel_ap(), trying to access
> *initrd_start_p. And the reason we didn't crash before was because compiler
> optimized out whole load_ucode_ap() since check_loader_disabled_ap() was
> always true.

Right, and my fix actually uncovered the original issue :-\

Ok, here's a v2 which adds the check to the late loader too, for
completeness. I'll write a proper commit message tomorrow.

---
diff --git a/arch/x86/include/asm/microcode.h b/arch/x86/include/asm/microcode.h
index 64dc362506b7..654907db5f09 100644
--- a/arch/x86/include/asm/microcode.h
+++ b/arch/x86/include/asm/microcode.h
@@ -87,4 +87,9 @@ static inline int __init save_microcode_in_initrd(void)
 }
 #endif
 
+/* Check whether we're running as a guest on a hypervisor. */
+static inline bool x86_guest(void)
+{
+	return !!(cpuid_ecx(1) & BIT(31));
+}
 #endif /* _ASM_X86_MICROCODE_H */
diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 2ce9051174e6..0b6db2a97f61 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -557,6 +557,9 @@ static int __init microcode_init(void)
 	struct cpuinfo_x86 *c = &cpu_data(0);
 	int error;
 
+	if (x86_guest())
+		return 0;
+
 	if (dis_ucode_ldr)
 		return 0;
 
diff --git a/arch/x86/kernel/cpu/microcode/core_early.c b/arch/x86/kernel/cpu/microcode/core_early.c
index 2c017f242a78..dfa93e74c370 100644
--- a/arch/x86/kernel/cpu/microcode/core_early.c
+++ b/arch/x86/kernel/cpu/microcode/core_early.c
@@ -98,6 +98,9 @@ void __init load_ucode_bsp(void)
 {
 	int vendor, x86;
 
+	if (x86_guest())
+		return;
+
 	if (check_loader_disabled_bsp())
 		return;
 
@@ -134,6 +137,9 @@ void load_ucode_ap(void)
 {
 	int vendor, x86;
 
+	if (x86_guest())
+		return;
+
 	if (check_loader_disabled_ap())
 		return;
 

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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