[librecmc/librecmc.git] /

From 6e34f618d37ddbb5854c42e2ad4fca83492fa7b7 Mon Sep 17 00:00:00 2001
From: Jouni Malinen <jouni@codeaurora.org>
Date: Wed, 27 Feb 2019 18:38:30 +0200
Subject: [PATCH 02/14] Add helper functions for constant time operations

These functions can be used to help implement constant time operations
for various cryptographic operations that must minimize externally
observable differences in processing (both in timing and also in
internal cache use, etc.).

This is related to CVE-2019-9494 and CVE-2019-9495.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
---
 src/utils/const_time.h | 191 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 191 insertions(+)
 create mode 100644 src/utils/const_time.h

--- /dev/null
+++ b/src/utils/const_time.h
@@ -0,0 +1,191 @@
+/*
+ * Helper functions for constant time operations
+ * Copyright (c) 2019, The Linux Foundation
+ *
+ * This software may be distributed under the terms of the BSD license.
+ * See README for more details.
+ *
+ * These helper functions can be used to implement logic that needs to minimize
+ * externally visible differences in execution path by avoiding use of branches,
+ * avoiding early termination or other time differences, and forcing same memory
+ * access pattern regardless of values.
+ */
+
+#ifndef CONST_TIME_H
+#define CONST_TIME_H
+
+
+#if defined(__clang__)
+#define NO_UBSAN_UINT_OVERFLOW \
+       __attribute__((no_sanitize("unsigned-integer-overflow")))
+#else
+#define NO_UBSAN_UINT_OVERFLOW
+#endif
+
+
+/**
+ * const_time_fill_msb - Fill all bits with MSB value
+ * @val: Input value
+ * Returns: Value with all the bits set to the MSB of the input val
+ */
+static inline unsigned int const_time_fill_msb(unsigned int val)
+{
+       /* Move the MSB to LSB and multiple by -1 to fill in all bits. */
+       return (val >> (sizeof(val) * 8 - 1)) * ~0U;
+}
+
+
+/* Returns: -1 if val is zero; 0 if val is not zero */
+static inline unsigned int const_time_is_zero(unsigned int val)
+       NO_UBSAN_UINT_OVERFLOW
+{
+       /* Set MSB to 1 for 0 and fill rest of bits with the MSB value */
+       return const_time_fill_msb(~val & (val - 1));
+}
+
+
+/* Returns: -1 if a == b; 0 if a != b */
+static inline unsigned int const_time_eq(unsigned int a, unsigned int b)
+{
+       return const_time_is_zero(a ^ b);
+}
+
+
+/* Returns: -1 if a == b; 0 if a != b */
+static inline u8 const_time_eq_u8(unsigned int a, unsigned int b)
+{
+       return (u8) const_time_eq(a, b);
+}
+
+
+/**
+ * const_time_eq_bin - Constant time memory comparison
+ * @a: First buffer to compare
+ * @b: Second buffer to compare
+ * @len: Number of octets to compare
+ * Returns: -1 if buffers are equal, 0 if not
+ *
+ * This function is meant for comparing passwords or hash values where
+ * difference in execution time or memory access pattern could provide external
+ * observer information about the location of the difference in the memory
+ * buffers. The return value does not behave like memcmp(), i.e.,
+ * const_time_eq_bin() cannot be used to sort items into a defined order. Unlike
+ * memcmp(), the execution time of const_time_eq_bin() does not depend on the
+ * contents of the compared memory buffers, but only on the total compared
+ * length.
+ */
+static inline unsigned int const_time_eq_bin(const void *a, const void *b,
+                                            size_t len)
+{
+       const u8 *aa = a;
+       const u8 *bb = b;
+       size_t i;
+       u8 res = 0;
+
+       for (i = 0; i < len; i++)
+               res |= aa[i] ^ bb[i];
+
+       return const_time_is_zero(res);
+}
+
+
+/**
+ * const_time_select - Constant time unsigned int selection
+ * @mask: 0 (false) or -1 (true) to identify which value to select
+ * @true_val: Value to select for the true case
+ * @false_val: Value to select for the false case
+ * Returns: true_val if mask == -1, false_val if mask == 0
+ */
+static inline unsigned int const_time_select(unsigned int mask,
+                                            unsigned int true_val,
+                                            unsigned int false_val)
+{
+       return (mask & true_val) | (~mask & false_val);
+}
+
+
+/**
+ * const_time_select_int - Constant time int selection
+ * @mask: 0 (false) or -1 (true) to identify which value to select
+ * @true_val: Value to select for the true case
+ * @false_val: Value to select for the false case
+ * Returns: true_val if mask == -1, false_val if mask == 0
+ */
+static inline int const_time_select_int(unsigned int mask, int true_val,
+                                       int false_val)
+{
+       return (int) const_time_select(mask, (unsigned int) true_val,
+                                      (unsigned int) false_val);
+}
+
+
+/**
+ * const_time_select_u8 - Constant time u8 selection
+ * @mask: 0 (false) or -1 (true) to identify which value to select
+ * @true_val: Value to select for the true case
+ * @false_val: Value to select for the false case
+ * Returns: true_val if mask == -1, false_val if mask == 0
+ */
+static inline u8 const_time_select_u8(u8 mask, u8 true_val, u8 false_val)
+{
+       return (u8) const_time_select(mask, true_val, false_val);
+}
+
+
+/**
+ * const_time_select_s8 - Constant time s8 selection
+ * @mask: 0 (false) or -1 (true) to identify which value to select
+ * @true_val: Value to select for the true case
+ * @false_val: Value to select for the false case
+ * Returns: true_val if mask == -1, false_val if mask == 0
+ */
+static inline s8 const_time_select_s8(u8 mask, s8 true_val, s8 false_val)
+{
+       return (s8) const_time_select(mask, (unsigned int) true_val,
+                                     (unsigned int) false_val);
+}
+
+
+/**
+ * const_time_select_bin - Constant time binary buffer selection copy
+ * @mask: 0 (false) or -1 (true) to identify which value to copy
+ * @true_val: Buffer to copy for the true case
+ * @false_val: Buffer to copy for the false case
+ * @len: Number of octets to copy
+ * @dst: Destination buffer for the copy
+ *
+ * This function copies the specified buffer into the destination buffer using
+ * operations with identical memory access pattern regardless of which buffer
+ * is being copied.
+ */
+static inline void const_time_select_bin(u8 mask, const u8 *true_val,
+                                        const u8 *false_val, size_t len,
+                                        u8 *dst)
+{
+       size_t i;
+
+       for (i = 0; i < len; i++)
+               dst[i] = const_time_select_u8(mask, true_val[i], false_val[i]);
+}
+
+
+static inline int const_time_memcmp(const void *a, const void *b, size_t len)
+{
+       const u8 *aa = a;
+       const u8 *bb = b;
+       int diff, res = 0;
+       unsigned int mask;
+
+       if (len == 0)
+               return 0;
+       do {
+               len--;
+               diff = (int) aa[len] - (int) bb[len];
+               mask = const_time_is_zero((unsigned int) diff);
+               res = const_time_select_int(mask, res, diff);
+       } while (len);
+
+       return res;
+}
+
+#endif /* CONST_TIME_H */