patch-0001-nospec-introduce-evaluate_nospec.patch

From db591d6e76ed33750039e44a75108809681ed672 Mon Sep 17 00:00:00 2001
From: Norbert Manthey <nmanthey@amazon.de>
Date: Thu, 14 Mar 2019 13:55:00 +0100
Subject: [PATCH] nospec: introduce evaluate_nospec

Since the L1TF vulnerability of Intel CPUs, loading hypervisor data into
L1 cache is problematic, because when hyperthreading is used as well, a
guest running on the sibling core can leak this potentially secret data.

To prevent these speculative accesses, we block speculation after
accessing the domain property field by adding lfence instructions. This
way, the CPU continues executing and loading data only once the condition
is actually evaluated.

As this protection is typically used in if statements, the lfence has to
come in a compatible way. Therefore, a function that returns true after an
lfence instruction is introduced. To protect both branches after a
conditional, an lfence instruction has to be added for the two branches.
To be able to block speculation after several evaluations, the generic
barrier macro block_speculation is also introduced.

As the L1TF vulnerability is only present on the x86 architecture, there is
no need to add protection for other architectures. Hence, the introduced
functions are defined but empty.

On the x86 architecture, by default, the lfence instruction is not present
either. Only when a L1TF vulnerable platform is detected, the lfence
instruction is patched in via alternative patching. Similarly, PV guests
are protected wrt L1TF by default, so that the protection is furthermore
disabled in case HVM is exclueded via the build configuration.

Introducing the lfence instructions catches a lot of potential leaks with
a simple unintrusive code change. During performance testing, we did not
notice performance effects.

This is part of the speculative hardening effort.

Signed-off-by: Norbert Manthey <nmanthey@amazon.de>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
---
 xen/include/asm-arm/nospec.h | 25 +++++++++++++++++++++++
 xen/include/asm-x86/nospec.h | 39 ++++++++++++++++++++++++++++++++++++
 xen/include/xen/nospec.h     |  1 +
 3 files changed, 65 insertions(+)
 create mode 100644 xen/include/asm-arm/nospec.h
 create mode 100644 xen/include/asm-x86/nospec.h

diff --git a/xen/include/asm-arm/nospec.h b/xen/include/asm-arm/nospec.h
new file mode 100644
index 000000000000..51c7aea4f447
--- /dev/null
+++ b/xen/include/asm-arm/nospec.h
@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright 2018 Amazon.com, Inc. or its affiliates. All Rights Reserved. */
+
+#ifndef _ASM_ARM_NOSPEC_H
+#define _ASM_ARM_NOSPEC_H
+
+static inline bool evaluate_nospec(bool condition)
+{
+    return condition;
+}
+
+static inline void block_speculation(void)
+{
+}
+
+#endif /* _ASM_ARM_NOSPEC_H */
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * indent-tabs-mode: nil
+ * End:
+ */
diff --git a/xen/include/asm-x86/nospec.h b/xen/include/asm-x86/nospec.h
new file mode 100644
index 000000000000..2aa47b3455a6
--- /dev/null
+++ b/xen/include/asm-x86/nospec.h
@@ -0,0 +1,39 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright 2018 Amazon.com, Inc. or its affiliates. All Rights Reserved. */
+
+#ifndef _ASM_X86_NOSPEC_H
+#define _ASM_X86_NOSPEC_H
+
+#include <asm/alternative.h>
+
+/* Allow to insert a read memory barrier into conditionals */
+static always_inline bool barrier_nospec_true(void)
+{
+#ifdef CONFIG_HVM
+    alternative("", "lfence", X86_FEATURE_SC_L1TF_VULN);
+#endif
+    return true;
+}
+
+/* Allow to protect evaluation of conditionasl with respect to speculation */
+static always_inline bool evaluate_nospec(bool condition)
+{
+    return condition ? barrier_nospec_true() : !barrier_nospec_true();
+}
+
+/* Allow to block speculative execution in generic code */
+static always_inline void block_speculation(void)
+{
+    barrier_nospec_true();
+}
+
+#endif /* _ASM_X86_NOSPEC_H */
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * indent-tabs-mode: nil
+ * End:
+ */
diff --git a/xen/include/xen/nospec.h b/xen/include/xen/nospec.h
index 8acfa60f1b71..2ac8feccc213 100644
--- a/xen/include/xen/nospec.h
+++ b/xen/include/xen/nospec.h
@@ -8,6 +8,7 @@
 #define XEN_NOSPEC_H
 
 #include <asm/system.h>
+#include <asm/nospec.h>
 
 /**
  * array_index_mask_nospec() - generate a ~0 mask when index < size, 0 otherwise
-- 
2.26.3