Line data    Source code

       1            0 : /*
       2              :  * Copyright (c) 2019 Intel Corporation.
       3              :  *
       4              :  * SPDX-License-Identifier: Apache-2.0
       5              :  */
       6              : 
       7              : #ifndef ZEPHYR_MISC_SPECULATION_H
       8              : #define ZEPHYR_MISC_SPECULATION_H
       9              : 
      10              : #include <zephyr/types.h>
      11              : 
      12              : /**
      13              :  * Sanitize an array index against bounds check bypass attacks aka the
      14              :  * Spectre V1 vulnerability.
      15              :  *
      16              :  * CPUs with speculative execution may speculate past any size checks and
      17              :  * leak confidential data due to analysis of micro-architectural properties.
      18              :  * This will unconditionally truncate any out-of-bounds indexes to
      19              :  * zero in the speculative execution path using bit twiddling instead of
      20              :  * any branch instructions.
      21              :  *
      22              :  * Example usage:
      23              :  *
      24              :  * if (index < size) {
      25              :  *     index = k_array_index_sanitize(index, size);
      26              :  *     data = array[index];
      27              :  * }
      28              :  *
      29              :  * @param index Untrusted array index which has been validated, but not used
      30              :  * @param array_size Size of the array
      31              :  * @return The original index value if < size, or 0
      32              :  */
      33            1 : static inline uint32_t k_array_index_sanitize(uint32_t index, uint32_t array_size)
      34              : {
      35              : #ifdef CONFIG_BOUNDS_CHECK_BYPASS_MITIGATION
      36              :         int32_t signed_index = index, signed_array_size = array_size;
      37              : 
      38              :         /* Take the difference between index and max.
      39              :          * A proper value will result in a negative result. We also AND in
      40              :          * the complement of index, so that we automatically reject any large
      41              :          * indexes which would wrap around the difference calculation.
      42              :          *
      43              :          * Sign-extend just the sign bit to produce a mask of all 1s (accept)
      44              :          * or all 0s (truncate).
      45              :          */
      46              :         uint32_t mask = ((signed_index - signed_array_size) & ~signed_index) >> 31;
      47              : 
      48              :         return index & mask;
      49              : #else
      50              :         ARG_UNUSED(array_size);
      51              : 
      52              :         return index;
      53              : #endif /* CONFIG_BOUNDS_CHECK_BYPASS_MITIGATION */
      54              : }
      55              : #endif /* ZEPHYR_MISC_SPECULATION_H */