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[Xen-devel] [PATCH SpectreV1+L1TF v5 5/9] nospec: introduce evaluate_nospec

Since the L1TF vulnerability of Intel CPUs, loading hypervisor data into
L1 cache is problemetic, 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 the macros are 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 evalauations, the generic
barrier macro block_speculation is also introduced.

As the L1TF vulnerability is only present on the x86 architecture, the
macros will not use the lfence instruction on other architectures and the
protection is disabled during compilation. By default, the lfence
instruction is not present either. Only when a L1TF vulnerable platform
is detected, the lfence instruction is patched in via alterantive patching.

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.

Signed-off-by: Norbert Manthey <nmanthey@xxxxxxxxx>
 xen/include/xen/nospec.h | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/xen/include/xen/nospec.h b/xen/include/xen/nospec.h
--- a/xen/include/xen/nospec.h
+++ b/xen/include/xen/nospec.h
@@ -7,6 +7,7 @@
 #ifndef XEN_NOSPEC_H
 #define XEN_NOSPEC_H
+#include <asm/alternative.h>
 #include <asm/system.h>
@@ -64,6 +65,33 @@ static inline unsigned long array_index_mask_nospec(unsigned 
long index,
 #define array_access_nospec(array, index)                               \
     (array)[array_index_nospec(index, ARRAY_SIZE(array))]
+ * Allow to insert a read memory barrier into conditionals
+ */
+#if defined(CONFIG_X86) && defined(CONFIG_HVM)
+static inline bool arch_barrier_nospec_true(void) {
+    alternative("", "lfence", X86_FEATURE_SC_L1TF_VULN);
+    return true;
+static inline bool arch_barrier_nospec_true(void) { return true; }
+ * Allow to protect evaluation of conditional with respect to speculation on 
+ */
+#ifndef CONFIG_X86
+#define evaluate_nospec(condition) (condition)
+#define evaluate_nospec(condition)                                         \
+    ((condition) ? arch_barrier_nospec_true() : !arch_barrier_nospec_true())
+ * Allow to block speculative execution in generic code
+ */
+#define block_speculation() (void)arch_barrier_nospec_true()
 #endif /* XEN_NOSPEC_H */

Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrer: Christian Schlaeger, Ralf Herbrich
Ust-ID: DE 289 237 879
Eingetragen am Amtsgericht Charlottenburg HRB 149173 B

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