KVM: x86: Protect MSR-based index computations in fixed_msr_to_seg_unit() from Spectre-v1/L1TF attacks

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

Fixes: de9aef5 ("KVM: MTRR: introduce fixed_mtrr_segment table")

Signed-off-by: Nick Finco <[email protected]>
Signed-off-by: Marios Pomonis <[email protected]>
Reviewed-by: Andrew Honig <[email protected]>
Cc: [email protected]
Reviewed-by: Jim Mattson <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: marpom

arch/x86/kvm/mtrr.c

@@ -192,11 +192,15 @@ static bool fixed_msr_to_seg_unit(u32 msr, int *seg, int *unit)
 		break;
 	case MSR_MTRRfix16K_80000 ... MSR_MTRRfix16K_A0000:
 		*seg = 1;
-		*unit = msr - MSR_MTRRfix16K_80000;
+		*unit = array_index_nospec(
+			msr - MSR_MTRRfix16K_80000,
+			MSR_MTRRfix16K_A0000 - MSR_MTRRfix16K_80000 + 1);
 		break;
 	case MSR_MTRRfix4K_C0000 ... MSR_MTRRfix4K_F8000:
 		*seg = 2;
-		*unit = msr - MSR_MTRRfix4K_C0000;
+		*unit = array_index_nospec(
+			msr - MSR_MTRRfix4K_C0000,
+			MSR_MTRRfix4K_F8000 - MSR_MTRRfix4K_C0000 + 1);
 		break;
 	default:
 		return false;