[PATCH v2 09/13] KVM: x86: Protect MSR-based index computations from Spectre-v1/L1TF attacks in x86.c

[Marios Pomonis <pomonis@google.com>]

This fixes a Spectre-v1/L1TF vulnerability in set_msr_mce() and
get_msr_mce().
Both functions contain index computations based on the
(attacker-controlled) MSR number.

Fixes: commit 890ca9aefa78 ("KVM: Add MCE support")

Signed-off-by: Nick Finco <nifi@google.com>
Signed-off-by: Marios Pomonis <pomonis@google.com>
Reviewed-by: Andrew Honig <ahonig@google.com>
Cc: stable@vger.kernel.org
---
 arch/x86/kvm/x86.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a256e09f321a..a9e66f09422e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2496,7 +2496,10 @@ static int set_msr_mce(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	default:
 		if (msr >= MSR_IA32_MC0_CTL &&
 		    msr < MSR_IA32_MCx_CTL(bank_num)) {
-			u32 offset = msr - MSR_IA32_MC0_CTL;
+			u32 offset = array_index_nospec(
+				msr - MSR_IA32_MC0_CTL,
+				MSR_IA32_MCx_CTL(bank_num) - MSR_IA32_MC0_CTL);
+
 			/* only 0 or all 1s can be written to IA32_MCi_CTL
 			 * some Linux kernels though clear bit 10 in bank 4 to
 			 * workaround a BIOS/GART TBL issue on AMD K8s, ignore
@@ -2937,7 +2940,10 @@ static int get_msr_mce(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata, bool host)
 	default:
 		if (msr >= MSR_IA32_MC0_CTL &&
 		    msr < MSR_IA32_MCx_CTL(bank_num)) {
-			u32 offset = msr - MSR_IA32_MC0_CTL;
+			u32 offset = array_index_nospec(
+				msr - MSR_IA32_MC0_CTL,
+				MSR_IA32_MCx_CTL(bank_num) - MSR_IA32_MC0_CTL);
+
 			data = vcpu->arch.mce_banks[offset];
 			break;
 		}
-- 
2.24.0.525.g8f36a354ae-goog