Vulnerability: The recalculate_apic_map function in arch/x86/kvm/lapic.c in the KVM subsystem in the Linux kernel through 3.12.5 allows guest OS users to cause a denial of service (host OS crash) via a crafted ICR write operation in x2apic mode.
Vulnerability ID: CVE-2013-6376
CWE: CWE-189
Severity score: 6.9
Project: https://github.com/torvalds/linux
Published: 2013-12-14
File: arch/x86/kvm/lapic.c
Commit message: KVM: x86: fix guest-initiated crash with x2apic (CVE-2013-6376)

A guest can cause a BUG_ON() leading to a host kernel crash.
When the guest writes to the ICR to request an IPI, while in x2apic
mode the following things happen, the destination is read from
ICR2, which is a register that the guest can control.

kvm_irq_delivery_to_apic_fast uses the high 16 bits of ICR2 as the
cluster id.  A BUG_ON is triggered, which is a protection against
accessing map->logical_map with an out-of-bounds access and manages
to avoid that anything really unsafe occurs.

The logic in the code is correct from real HW point of view. The problem
is that KVM supports only one cluster with ID 0 in clustered mode, but
the code that has the bug does not take this into account.

Reported-by: Lars Bull <larsbull@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Commit author: Gleb Natapov

Vulnerable version (before fix):
 static inline int kvm_apic_id(struct kvm_lapic *apic) 	return (kvm_apic_get_reg(apic, APIC_ID) >> 24) & 0xff; }  static void recalculate_apic_map(struct kvm *kvm) { 	struct kvm_apic_map *new, *old = NULL; static void recalculate_apic_map(struct kvm *kvm) 		if (apic_x2apic_mode(apic)) { 			new->ldr_bits = 32; 			new->cid_shift = 16; 			new->cid_mask = new->lid_mask = 0xffff; 		} else if (kvm_apic_sw_enabled(apic) && 				!new->cid_mask /* flat mode */ && 				kvm_apic_get_reg(apic, APIC_DFR) == APIC_DFR_CLUSTER) {

Fixed version (after fix):
 static inline int kvm_apic_id(struct kvm_lapic *apic) 	return (kvm_apic_get_reg(apic, APIC_ID) >> 24) & 0xff; }  #define KVM_X2APIC_CID_BITS 0  static void recalculate_apic_map(struct kvm *kvm) { 	struct kvm_apic_map *new, *old = NULL; static void recalculate_apic_map(struct kvm *kvm) 		if (apic_x2apic_mode(apic)) { 			new->ldr_bits = 32; 			new->cid_shift = 16; 			new->cid_mask = (1 << KVM_X2APIC_CID_BITS) - 1; 			new->lid_mask = 0xffff; 		} else if (kvm_apic_sw_enabled(apic) && 				!new->cid_mask /* flat mode */ && 				kvm_apic_get_reg(apic, APIC_DFR) == APIC_DFR_CLUSTER) {

Patch:
patch: @@ -143,6 +143,8 @@ static inline int kvm_apic_id(struct kvm_lapic *apic)
 	return (kvm_apic_get_reg(apic, APIC_ID) >> 24) & 0xff;
 }
 
+#define KVM_X2APIC_CID_BITS 0
+
 static void recalculate_apic_map(struct kvm *kvm)
 {
 	struct kvm_apic_map *new, *old = NULL;
@@ -180,7 +182,8 @@ static void recalculate_apic_map(struct kvm *kvm)
 		if (apic_x2apic_mode(apic)) {
 			new->ldr_bits = 32;
 			new->cid_shift = 16;
-			new->cid_mask = new->lid_mask = 0xffff;
+			new->cid_mask = (1 << KVM_X2APIC_CID_BITS) - 1;
+			new->lid_mask = 0xffff;
 		} else if (kvm_apic_sw_enabled(apic) &&
 				!new->cid_mask /* flat mode */ &&
 				kvm_apic_get_reg(apic, APIC_DFR) == APIC_DFR_CLUSTER) {
path: arch/x86/kvm/lapic.c