Re: [PATCH v6 6/6] Documentation: Add L1D flushing Documentation

[Thomas Gleixner]

Balbir Singh <sblbir@xxxxxxxxxx> writes:
> +With an increasing number of vulnerabilities being reported around data
> +leaks from L1D, a new user space mechanism to flush the L1D cache on
> +context switch is added to the kernel. This should help address

is added to the kernel? This is documentation of an existing feature...

> +Mitigation
> +----------
> +When PR_SET_L1D_FLUSH is enabled for a task, on switching tasks (when
> +the address space changes), a flush of the L1D cache is performed for
> +the task when it leaves the CPU. If the underlying CPU supports L1D
> +flushing in hardware, the hardware mechanism is used, otherwise a software
> +fallback, similar to the mechanism used by L1TF is used.

This lacks documentation of the limitations, especially that this does
not help against cross Hyperthread attacks.

I've massaged the whole thing a bit. See below.

Thanks,

        tglx
8<-----------------

--- 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
    mds
    tsx_async_abort
    multihit.rst
+   l1d_flush
--- /dev/null
+++ b/Documentation/admin-guide/hw-vuln/l1d_flush.rst
@@ -0,0 +1,53 @@
+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
+----------------
+Applications can call ``prctl(2)`` with one of these two arguments
+
+1. PR_SET_L1D_FLUSH - flush the L1D cache on context switch (out)
+2. PR_GET_L1D_FLUSH - get the current state of the L1D cache flush, returns 1
+   if set and 0 if not set.
+
+**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>`.