KVM: x86: Protect kvm_lapic_reg_write() from Spectre-v1/L1TF attacks

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@google.com>
Signed-off-by: Marios Pomonis <pomonis@google.com>
Reviewed-by: Andrew Honig <ahonig@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[bwh: Backported to 3.16:
 - Add #include <linux/nospec.h>
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index d03a88cb31299f797a3ecf9b1b2c6c7a2949abd6..e38d6a7dfbd604848b0a63bd2581c86e1c2b7f2e 100644 (file)
--- 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_lapic *apic, u32 reg, u32 val)
        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) &