L1D Flushing
============

With an increasing number of vulnerabilities being reported around data
leaks from the Level 1 Data cache (L1D) the kernel provides an opt-in
mechanism to flush the L1D cache on context switch.

This mechanism can be used to address e.g. CVE-2020-0550. For applications
the mechanism keeps them safe from vulnerabilities, related to leaks
(snooping of) from the L1D cache.


Related CVEs
------------
The following CVEs can be addressed by this
mechanism

    =============       ========================     ==================
    CVE-2020-0550       Improper Data Forwarding     OS related aspects
    =============       ========================     ==================

Usage Guidelines
----------------

Please see document: :ref:`Documentation/userspace-api/spec_ctrl.rst
<set_spec_ctrl>` for details.

**NOTE**: The feature is disabled by default, applications need to
specifically opt into the feature to enable it.

Mitigation
----------

When PR_SPEC_L1D_FLUSH is enabled for a task a flush of the L1D cache is
performed when the task is scheduled out and the incoming task belongs to a
different process and therefore to a different address space.

If the underlying CPU supports L1D flushing in hardware, the hardware
mechanism is used, software fallback for the mitigation, is not supported.

Mitigation control on the kernel command line
---------------------------------------------

The kernel command line allows to control the L1D flush mitigations at boot
time with the option "l1d_flush=". The valid arguments for this option are:

  ============  =============================================================
  on            Enables the prctl interface, applications trying to use
                the prctl() will fail with an error if l1d_flush is not
                enabled
  ============  =============================================================

By default the mechanism is disabled.

Limitations
-----------

The mechanism does not mitigate L1D data leaks between tasks belonging to
different processes which are concurrently executing on sibling threads of
a physical CPU core when SMT is enabled on the system.

This can be addressed by controlled placement of processes on physical CPU
cores or by disabling SMT. See the relevant chapter in the L1TF mitigation
document: :ref:`Documentation/admin-guide/hw-vuln/l1tf.rst <smt_control>`.

**NOTE** : The opt-in of a task for L1D flushing works only when the task's
affinity is limited to cores running in non-SMT mode. If a task which
requested L1D flushing is scheduled on a SMT-enabled core the kernel sends
a SIGBUS to the task.