1 files changed, 72 insertions, 0 deletions

diff --git a/include/linux/nospec.h b/include/linux/nospec.h
new file mode 100644
index 000000000000..b99bced39ac2
--- /dev/null
+++ b/include/linux/nospec.h
@@ -0,0 +1,72 @@
+// 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.
+
+#ifndef _LINUX_NOSPEC_H
+#define _LINUX_NOSPEC_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)
+{
+        /*
+         * Warn developers about inappropriate array_index_nospec() usage.
+         *
+         * Even if the CPU speculates past the WARN_ONCE branch, the
+         * sign bit of @index is taken into account when generating the
+         * mask.
+         *
+         * This warning is compiled out when the compiler can infer that
+         * @index and @size are less than LONG_MAX.
+         */
+        if (WARN_ONCE(index > LONG_MAX || size > LONG_MAX,
+                        "array_index_nospec() limited to range of [0, LONG_MAX]\n"))
+                return 0;
+
+        /*
+         * 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));                        \
+                                                                        \
+        _i &= _mask;                                                    \
+        _i;                                                             \
+})
+#endif /* _LINUX_NOSPEC_H */