KVM: x86: Convert vapic synchronization to _cached functions (CVE-2013-6368)

In kvm_lapic_sync_from_vapic and kvm_lapic_sync_to_vapic there is the
potential to corrupt kernel memory if userspace provides an address that
is at the end of a page.  This patches concerts those functions to use
kvm_write_guest_cached and kvm_read_guest_cached.  It also checks the
vapic_address specified by userspace during ioctl processing and returns
an error to userspace if the address is not a valid GPA.

This is generally not guest triggerable, because the required write is
done by firmware that runs before the guest.  Also, it only affects AMD
processors and oldish Intel that do not have the FlexPriority feature
(unless you disable FlexPriority, of course; then newer processors are
also affected).

Fixes: b93463aa59d6 ('KVM: Accelerated apic support')

Reported-by: Andrew Honig <ahonig@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Andrew Honig <ahonig@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 15 insertions, 12 deletions

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 89b52ec7d09c..b8bec45c1610 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1692,7 +1692,6 @@ static void apic_sync_pv_eoi_from_guest(struct kvm_vcpu *vcpu,
 void kvm_lapic_sync_from_vapic(struct kvm_vcpu *vcpu)
 {
 	u32 data;
-	void *vapic;
 
 	if (test_bit(KVM_APIC_PV_EOI_PENDING, &vcpu->arch.apic_attention))
 		apic_sync_pv_eoi_from_guest(vcpu, vcpu->arch.apic);
@@ -1700,9 +1699,8 @@ void kvm_lapic_sync_from_vapic(struct kvm_vcpu *vcpu)
 	if (!test_bit(KVM_APIC_CHECK_VAPIC, &vcpu->arch.apic_attention))
 		return;
 
-	vapic = kmap_atomic(vcpu->arch.apic->vapic_page);
-	data = *(u32 *)(vapic + offset_in_page(vcpu->arch.apic->vapic_addr));
-	kunmap_atomic(vapic);
+	kvm_read_guest_cached(vcpu->kvm, &vcpu->arch.apic->vapic_cache, &data,
+				sizeof(u32));
 
 	apic_set_tpr(vcpu->arch.apic, data & 0xff);
 }
@@ -1738,7 +1736,6 @@ void kvm_lapic_sync_to_vapic(struct kvm_vcpu *vcpu)
 	u32 data, tpr;
 	int max_irr, max_isr;
 	struct kvm_lapic *apic = vcpu->arch.apic;
-	void *vapic;
 
 	apic_sync_pv_eoi_to_guest(vcpu, apic);
 
@@ -1754,18 +1751,24 @@ void kvm_lapic_sync_to_vapic(struct kvm_vcpu *vcpu)
 		max_isr = 0;
 	data = (tpr & 0xff) | ((max_isr & 0xf0) << 8) | (max_irr << 24);
 
-	vapic = kmap_atomic(vcpu->arch.apic->vapic_page);
-	*(u32 *)(vapic + offset_in_page(vcpu->arch.apic->vapic_addr)) = data;
-	kunmap_atomic(vapic);
+	kvm_write_guest_cached(vcpu->kvm, &vcpu->arch.apic->vapic_cache, &data,
+				sizeof(u32));
 }
 
-void kvm_lapic_set_vapic_addr(struct kvm_vcpu *vcpu, gpa_t vapic_addr)
+int kvm_lapic_set_vapic_addr(struct kvm_vcpu *vcpu, gpa_t vapic_addr)
 {
-	vcpu->arch.apic->vapic_addr = vapic_addr;
-	if (vapic_addr)
+	if (vapic_addr) {
+		if (kvm_gfn_to_hva_cache_init(vcpu->kvm,
+					&vcpu->arch.apic->vapic_cache,
+					vapic_addr, sizeof(u32)))
+			return -EINVAL;
 		__set_bit(KVM_APIC_CHECK_VAPIC, &vcpu->arch.apic_attention);
-	else
+	} else {
 		__clear_bit(KVM_APIC_CHECK_VAPIC, &vcpu->arch.apic_attention);
+	}
+
+	vcpu->arch.apic->vapic_addr = vapic_addr;
+	return 0;
 }
 
 int kvm_x2apic_msr_write(struct kvm_vcpu *vcpu, u32 msr, u64 data)