[PATCH 3.16 83/99] KVM: x86: Protect DR-based index computations 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 ea740059ecb37807ba47b84b33d1447435a8d868 upstream.

This fixes a Spectre-v1/L1TF vulnerability in __kvm_set_dr() and
kvm_get_dr().
Both kvm_get_dr() and kvm_set_dr() (a wrapper of __kvm_set_dr()) are
exported symbols so KVM should tream them conservatively from a security
perspective.

Fixes: 020df0794f57 ("KVM: move DR register access handling into generic code")

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>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -801,9 +801,11 @@ static void kvm_update_dr7(struct kvm_vc
 
 static int __kvm_set_dr(struct kvm_vcpu *vcpu, int dr, unsigned long val)
 {
+	size_t size = ARRAY_SIZE(vcpu->arch.db);
+
 	switch (dr) {
 	case 0 ... 3:
-		vcpu->arch.db[dr] = val;
+		vcpu->arch.db[array_index_nospec(dr, size)] = val;
 		if (!(vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP))
 			vcpu->arch.eff_db[dr] = val;
 		break;
@@ -848,9 +850,11 @@ EXPORT_SYMBOL_GPL(kvm_set_dr);
 
 static int _kvm_get_dr(struct kvm_vcpu *vcpu, int dr, unsigned long *val)
 {
+	size_t size = ARRAY_SIZE(vcpu->arch.db);
+
 	switch (dr) {
 	case 0 ... 3:
-		*val = vcpu->arch.db[dr];
+		*val = vcpu->arch.db[array_index_nospec(dr, size)];
 		break;
 	case 4:
 		if (kvm_read_cr4_bits(vcpu, X86_CR4_DE))