[Xen-devel] [PATCH] xen: Port the array_index_nospec() infrastructure from Linux

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

This is as the infrastructure appeared in Linux 4.17, adapted slightly for
Xen.  The architecture independent array_index_mask_nospec() is ported for
ARMs benefit, as it currently lacks the optimised version.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>

Julien/Stefano: The ARM side of things was beyond my expertise to port to Xen.
A different option would be avoid porting the generic implementation would
result in a hard compiler error if someone tried to use array_index_nospec()
in ARM code.
---
 xen/include/asm-x86/system.h | 24 +++++++++++++++
 xen/include/xen/compiler.h   |  4 +++
 xen/include/xen/nospec.h     | 70 ++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 98 insertions(+)
 create mode 100644 xen/include/xen/nospec.h

diff --git a/xen/include/asm-x86/system.h b/xen/include/asm-x86/system.h
index 43fb6fe..483cd20 100644
--- a/xen/include/asm-x86/system.h
+++ b/xen/include/asm-x86/system.h
@@ -221,6 +221,30 @@ static always_inline unsigned long __xadd(
 #define set_mb(var, value) do { xchg(&var, value); } while (0)
 #define set_wmb(var, value) do { var = value; smp_wmb(); } while (0)
 
+/**
+ * array_index_mask_nospec() - generate a mask that is ~0UL when the
+ *      bounds check succeeds and 0 otherwise
+ * @index: array element index
+ * @size: number of elements in array
+ *
+ * Returns:
+ *     0 - (index < size)
+ */
+static inline unsigned long array_index_mask_nospec(unsigned long index,
+                                                    unsigned long size)
+{
+    unsigned long mask;
+
+    asm volatile ( "cmp %[size], %[index]; sbb %[mask], %[mask];"
+                   : [mask] "=r" (mask)
+                   : [size] "g" (size), [index] "r" (index) );
+
+    return mask;
+}
+
+/* Override default implementation in nospec.h. */
+#define array_index_mask_nospec array_index_mask_nospec
+
 #define local_irq_disable()     asm volatile ( "cli" : : : "memory" )
 #define local_irq_enable()      asm volatile ( "sti" : : : "memory" )
 
diff --git a/xen/include/xen/compiler.h b/xen/include/xen/compiler.h
index 533a8ea..887500e 100644
--- a/xen/include/xen/compiler.h
+++ b/xen/include/xen/compiler.h
@@ -81,6 +81,10 @@
 #pragma GCC visibility push(hidden)
 #endif
 
+/* Make the optimizer believe the variable can be manipulated arbitrarily. */
+#define OPTIMIZER_HIDE_VAR(var)                 \
+    __asm__ ("" : "=g" (var) : "0" (var))
+
 /* This macro obfuscates arithmetic on a variable address so that gcc
    shouldn't recognize the original var, and make assumptions about it */
 /*
diff --git a/xen/include/xen/nospec.h b/xen/include/xen/nospec.h
new file mode 100644
index 0000000..4879399
--- /dev/null
+++ b/xen/include/xen/nospec.h
@@ -0,0 +1,70 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright(c) 2018 Linus Torvalds. All rights reserved. */
+/* Copyright(c) 2018 Alexei Starovoitov. All rights reserved. */
+/* Copyright(c) 2018 Intel Corporation. All rights reserved. */
+/* Copyright(c) 2018 Citrix Systems R&D Ltd. All rights reserved. */
+
+#ifndef XEN_NOSPEC_H
+#define XEN_NOSPEC_H
+
+#include <asm/system.h>
+
+/**
+ * array_index_mask_nospec() - generate a ~0 mask when index < size, 0 
otherwise
+ * @index: array element index
+ * @size: number of elements in array
+ *
+ * When @index is out of bounds (@index >= @size), the sign bit will be
+ * set.  Extend the sign bit to all bits and invert, giving a result of
+ * zero for an out of bounds index, or ~0 if within bounds [0, @size).
+ */
+#ifndef array_index_mask_nospec
+static inline unsigned long array_index_mask_nospec(unsigned long index,
+                                                    unsigned long size)
+{
+    /*
+     * Always calculate and emit the mask even if the compiler
+     * thinks the mask is not needed. The compiler does not take
+     * into account the value of @index under speculation.
+     */
+    OPTIMIZER_HIDE_VAR(index);
+    return ~(long)(index | (size - 1UL - index)) >> (BITS_PER_LONG - 1);
+}
+#endif
+
+/*
+ * array_index_nospec - sanitize an array index after a bounds check
+ *
+ * For a code sequence like:
+ *
+ *     if (index < size) {
+ *         index = array_index_nospec(index, size);
+ *         val = array[index];
+ *     }
+ *
+ * ...if the CPU speculates past the bounds check then
+ * array_index_nospec() will clamp the index within the range of [0,
+ * size).
+ */
+#define array_index_nospec(index, size)                                 \
+({                                                                      \
+    typeof(index) _i = (index);                                         \
+    typeof(size) _s = (size);                                           \
+    unsigned long _mask = array_index_mask_nospec(_i, _s);              \
+                                                                        \
+    BUILD_BUG_ON(sizeof(_i) > sizeof(long));                            \
+    BUILD_BUG_ON(sizeof(_s) > sizeof(long));                            \
+                                                                        \
+    (typeof(_i)) (_i & _mask);                                          \
+})
+
+#endif /* XEN_NOSPEC_H */
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * indent-tabs-mode: nil
+ * End:
+ */
-- 
2.1.4


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