[PATCH v7 3/3] Documentation: Add L1D flushing Documentation

[Balbir Singh <sblbir@amazon.com>]

Add documentation of l1d flushing, explain the need for the
feature and how it can be used.

[tglx: Reword the documentation]
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Balbir Singh <sblbir@amazon.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
 Documentation/admin-guide/hw-vuln/index.rst   |  1 +
 .../admin-guide/hw-vuln/l1d_flush.rst         | 51 +++++++++++++++++++
 Documentation/userspace-api/spec_ctrl.rst     |  7 +++
 3 files changed, 59 insertions(+)
 create mode 100644 Documentation/admin-guide/hw-vuln/l1d_flush.rst

diff --git a/Documentation/admin-guide/hw-vuln/index.rst b/Documentation/admin-guide/hw-vuln/index.rst
index 0795e3c2643f..35633b299d45 100644
--- a/Documentation/admin-guide/hw-vuln/index.rst
+++ b/Documentation/admin-guide/hw-vuln/index.rst
@@ -14,3 +14,4 @@ are configurable at compile, boot or run time.
    mds
    tsx_async_abort
    multihit.rst
+   l1d_flush
diff --git a/Documentation/admin-guide/hw-vuln/l1d_flush.rst b/Documentation/admin-guide/hw-vuln/l1d_flush.rst
new file mode 100644
index 000000000000..530a1e0ffbd3
--- /dev/null
+++ b/Documentation/admin-guide/hw-vuln/l1d_flush.rst
@@ -0,0 +1,51 @@
+L1D Flushing for the paranoid
+=============================
+
+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 paranoid
+applications the mechanism keeps them safe from any yet to be discovered
+vulnerabilities, related to leaks from the L1D cache.
+
+
+Related CVEs
+------------
+At the present moment, 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` for
+details.
+
+**NOTE**: The feature is disabled by default, applications need to
+specifically opt into the feature to enable it.
+
+Mitigation
+----------
+
+When PR_SET_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, otherwise a software fallback, similar to the L1TF
+mitigation, is invoked.
+
+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>`.
diff --git a/Documentation/userspace-api/spec_ctrl.rst b/Documentation/userspace-api/spec_ctrl.rst
index 7ddd8f667459..b40afe97dbfb 100644
--- a/Documentation/userspace-api/spec_ctrl.rst
+++ b/Documentation/userspace-api/spec_ctrl.rst
@@ -106,3 +106,10 @@ Speculation misfeature controls
    * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_ENABLE, 0, 0);
    * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_DISABLE, 0, 0);
    * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_FORCE_DISABLE, 0, 0);
+
+- PR_SPEC_L1D_FLUSH_OUT: Flush L1D Cache on context switch out of the task
+
+  Invocations:
+   * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH_OUT, 0, 0, 0);
+   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH_OUT, PR_SPEC_ENABLE, 0, 0);
+   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH_OUT, PR_SPEC_DISABLE, 0, 0);
-- 
2.17.1