Code: static int pit_ioport_read(struct kvm_io_device *this,
			   gpa_t addr, int len, void *data)
{
	struct kvm_pit *pit = dev_to_pit(this);
	struct kvm_kpit_state *pit_state = &pit->pit_state;
	struct kvm *kvm = pit->kvm;
	int ret, count;
	struct kvm_kpit_channel_state *s;
	if (!pit_in_range(addr))
 		return -EOPNOTSUPP;
 
 	addr &= KVM_PIT_CHANNEL_MASK;
 	s = &pit_state->channels[addr];
 
 	mutex_lock(&pit_state->lock);

	if (s->status_latched) {
		s->status_latched = 0;
		ret = s->status;
	} else if (s->count_latched) {
		switch (s->count_latched) {
		default:
		case RW_STATE_LSB:
			ret = s->latched_count & 0xff;
			s->count_latched = 0;
			break;
		case RW_STATE_MSB:
			ret = s->latched_count >> 8;
			s->count_latched = 0;
			break;
		case RW_STATE_WORD0:
			ret = s->latched_count & 0xff;
			s->count_latched = RW_STATE_MSB;
			break;
		}
	} else {
		switch (s->read_state) {
		default:
		case RW_STATE_LSB:
			count = pit_get_count(kvm, addr);
			ret = count & 0xff;
			break;
		case RW_STATE_MSB:
			count = pit_get_count(kvm, addr);
			ret = (count >> 8) & 0xff;
			break;
		case RW_STATE_WORD0:
			count = pit_get_count(kvm, addr);
			ret = count & 0xff;
			s->read_state = RW_STATE_WORD1;
			break;
		case RW_STATE_WORD1:
			count = pit_get_count(kvm, addr);
			ret = (count >> 8) & 0xff;
			s->read_state = RW_STATE_WORD0;
			break;
		}
	}

	if (len > sizeof(ret))
		len = sizeof(ret);
	memcpy(data, (char *)&ret, len);

	mutex_unlock(&pit_state->lock);
	return 0;
}

Vulnerability Type: Exec Code Overflow 
CWE ID: CWE-119
Summary: The pit_ioport_read in i8254.c in the Linux kernel before 2.6.33 and QEMU before 2.3.1 does not distinguish between read lengths and write lengths, which might allow guest OS users to execute arbitrary code on the host OS by triggering use of an invalid index.
Commit Message: KVM: PIT: control word is write-only

PIT control word (address 0x43) is write-only, reads are undefined.

Cc: stable@kernel.org
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>