Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
IronPort-SDR: pwkin+IQri2xqXZ89mkaEHT8WLSdfYFl2IYQLIZXe8Ud9lRWWqksbweb9or9JJBLhf9n4XG2nP
 kZDVcDVYblpA==
From:   Balbir Singh <sblbir@amazon.com>
To:     <tglx@linutronix.de>, <linux-kernel@vger.kernel.org>
CC:     <jpoimboe@redhat.com>, <tony.luck@intel.com>,
        <keescook@chromium.org>, <benh@kernel.crashing.org>,
        <x86@kernel.org>, <dave.hansen@intel.com>,
        "Balbir Singh" <sblbir@amazon.com>
Subject: [PATCH v2 4/4] arch/x86: Add L1D flushing Documentation
Date:   Mon, 6 Apr 2020 13:19:46 +1000
Message-ID: <20200406031946.11815-5-sblbir@amazon.com>
In-Reply-To: <20200406031946.11815-1-sblbir@amazon.com>
References: <20200406031946.11815-1-sblbir@amazon.com>
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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

Signed-off-by: Balbir Singh <sblbir@amazon.com>
---
 Documentation/admin-guide/hw-vuln/index.rst   |  1 +
 .../admin-guide/hw-vuln/l1d_flush.rst         | 40 +++++++++++++++++++
 2 files changed, 41 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..73ee9e491a74
--- /dev/null
+++ b/Documentation/admin-guide/hw-vuln/l1d_flush.rst
@@ -0,0 +1,40 @@
+L1D Flushing for the paranoid
+=============================
+
+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
+CVE-2020-0550 and for paranoid applications, keep them safe from any
+yet to be discovered vulnerabilities, related to leaks from the L1D
+cache.
+
+Tasks can opt in to this mechanism by using an architecture specific
+prctl (x86 only at the moment).
+
+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 ``arch_prctl(2)`` with one of these two arguments
+
+1. ARCH_SET_L1D_FLUSH - flush the L1D cache on context switch (out)
+2. ARCH_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 to need to specifically
+opt into the feature to enable it.
+
+Mitigation
+----------
+When ARCH_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.
-- 
2.17.1