[PATCH 3.16 80/99] KVM: x86: Protect kvm_lapic_reg_write() from Spectre-v1/L1TF attacks

[Ben Hutchings]

3.16.84-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Marios Pomonis <pomonis@xxxxxxxxxx>

commit 4bf79cb089f6b1c6c632492c0271054ce52ad766 upstream.

This fixes a Spectre-v1/L1TF vulnerability in kvm_lapic_reg_write().
This function contains index computations based on the
(attacker-controlled) MSR number.

Fixes: 0105d1a52640 ("KVM: x2apic interface to lapic")

Signed-off-by: Nick Finco <nifi@xxxxxxxxxx>
Signed-off-by: Marios Pomonis <pomonis@xxxxxxxxxx>
Reviewed-by: Andrew Honig <ahonig@xxxxxxxxxx>
Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
[bwh: Backported to 3.16:
 - Add #include <linux/nospec.h>
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -35,6 +35,7 @@
 #include <asm/apicdef.h>
 #include <linux/atomic.h>
 #include <linux/jump_label.h>
+#include <linux/nospec.h>
 #include "kvm_cache_regs.h"
 #include "irq.h"
 #include "trace.h"
@@ -1196,15 +1197,20 @@ static int apic_reg_write(struct kvm_lap
 	case APIC_LVTTHMR:
 	case APIC_LVTPC:
 	case APIC_LVT1:
-	case APIC_LVTERR:
+	case APIC_LVTERR: {
 		/* TODO: Check vector */
+		size_t size;
+		u32 index;
+
 		if (!kvm_apic_sw_enabled(apic))
 			val |= APIC_LVT_MASKED;
-
-		val &= apic_lvt_mask[(reg - APIC_LVTT) >> 4];
+		size = ARRAY_SIZE(apic_lvt_mask);
+		index = array_index_nospec(
+				(reg - APIC_LVTT) >> 4, size);
+		val &= apic_lvt_mask[index];
 		apic_set_reg(apic, reg, val);
-
 		break;
+	}
 
 	case APIC_LVTT:
 		if ((kvm_apic_get_reg(apic, APIC_LVTT) &