vhost-scsi: protect vq->log_used with vq->mutex

In the Linux kernel, the following vulnerability has been resolved:

vhost-scsi: protect vq->log_used with vq->mutex

The vhost-scsi completion path may access vq->log_base when vq->log_used is

already set to false.

vhost-thread QEMU-thread

vhost_scsi_complete_cmd_work()

-> vhost_add_used()

-> vhost_add_used_n()

if (unlikely(vq->log_used))

QEMU disables vq->log_used

via VHOST_SET_VRING_ADDR.

mutex_lock(&vq->mutex);

vq->log_used = false now!

mutex_unlock(&vq->mutex);

QEMU gfree(vq->log_base)

log_used()

-> log_write(vq->log_base)

Assuming the VMM is QEMU. The vq->log_base is from QEMU userpace and can be

reclaimed via gfree(). As a result, this causes invalid memory writes to

QEMU userspace.

The control queue path has the same issue.