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<topic xml:lang="en-us" id="buffer-overwriting-mitigation">
  <title>Buffer Overwriting and Disabling Hyper-Threading</title>
  
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    <p>First, up-to-date CPU microcode is a prerequisite for the buffer overwriting (clearing) mitigations. Some host OSes may install these automatically, though it has traditionally been a task best performed by the system firmware. Please check with your system or mainboard manufacturer for the latest firmware update.</p>
    <p>This mitigation aims at removing potentially sensitive data from the affected buffers before running guest code. Since this means additional work each time the guest is scheduled, there might be some performance side effects.</p>
    <p>We recommend disabling hyper-threading (HT) on hosts affected by CVE-2018-12126 and CVE-2018-12127, because the affected sets of buffers are normally shared between thread pairs and therefore cause leaks between the threads. This is traditionally done from the firmware setup, but some OSes also offers ways disable HT. In some cases it may be disabled by default, but please verify as the effectiveness of the mitigation depends on it.</p>
    <p>The default action taken by <ph conkeyref="vbox-conkeyref-phrases/product-name"/> is to clear the affected buffers when a thread is scheduled to execute guest code, rather than on each VM entry. This reduces the performance impact, while making the assumption that the host OS will not handle security sensitive data from interrupt handlers and similar without taking precautions.</p>
    <p>The <userinput>VBoxManage modifyvm</userinput> command provides a more aggressive flushing option is provided by means of the <codeph>--mds-clear-on-vm-entry</codeph> option. When enabled the affected buffers will be cleared on every VM entry. The performance impact is greater than with the default option, though this of course depends on the workload. Workloads producing a lot of VM exits (like networking, VGA access, and similiar) will probably be most impacted.</p>
    <p>For users not concerned by this security issue, the default mitigation can be disabled using the <userinput>VBoxManage modifyvm name --mds-clear-on-sched off</userinput> command.</p>
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