1 // SPDX-License-Identifier: GPL-2.0 2 // Copyright(c) 2018 Linus Torvalds. All rights reserved. 3 // Copyright(c) 2018 Alexei Starovoitov. All rights reserved. 4 // Copyright(c) 2018 Intel Corporation. All rights reserved. 5 6 #ifndef _LINUX_NOSPEC_H 7 #define _LINUX_NOSPEC_H 8 9 /** 10 * array_index_mask_nospec() - generate a ~0 mask when index < size, 0 otherwise 11 * @index: array element index 12 * @size: number of elements in array 13 * 14 * When @index is out of bounds (@index >= @size), the sign bit will be 15 * set. Extend the sign bit to all bits and invert, giving a result of 16 * zero for an out of bounds index, or ~0 if within bounds [0, @size). 17 */ 18 #ifndef array_index_mask_nospec 19 static inline unsigned long array_index_mask_nospec(unsigned long index, 20 unsigned long size) 21 { 22 /* 23 * Always calculate and emit the mask even if the compiler 24 * thinks the mask is not needed. The compiler does not take 25 * into account the value of @index under speculation. 26 */ 27 OPTIMIZER_HIDE_VAR(index); 28 return ~(long)(index | (size - 1UL - index)) >> (BITS_PER_LONG - 1); 29 } 30 #endif 31 32 /* 33 * Warn developers about inappropriate array_index_nospec() usage. 34 * 35 * Even if the CPU speculates past the WARN_ONCE branch, the 36 * sign bit of @index is taken into account when generating the 37 * mask. 38 * 39 * This warning is compiled out when the compiler can infer that 40 * @index and @size are less than LONG_MAX. 41 */ 42 #define array_index_mask_nospec_check(index, size) \ 43 ({ \ 44 if (WARN_ONCE(index > LONG_MAX || size > LONG_MAX, \ 45 "array_index_nospec() limited to range of [0, LONG_MAX]\n")) \ 46 _mask = 0; \ 47 else \ 48 _mask = array_index_mask_nospec(index, size); \ 49 _mask; \ 50 }) 51 52 /* 53 * array_index_nospec - sanitize an array index after a bounds check 54 * 55 * For a code sequence like: 56 * 57 * if (index < size) { 58 * index = array_index_nospec(index, size); 59 * val = array[index]; 60 * } 61 * 62 * ...if the CPU speculates past the bounds check then 63 * array_index_nospec() will clamp the index within the range of [0, 64 * size). 65 */ 66 #define array_index_nospec(index, size) \ 67 ({ \ 68 typeof(index) _i = (index); \ 69 typeof(size) _s = (size); \ 70 unsigned long _mask = array_index_mask_nospec_check(_i, _s); \ 71 \ 72 BUILD_BUG_ON(sizeof(_i) > sizeof(long)); \ 73 BUILD_BUG_ON(sizeof(_s) > sizeof(long)); \ 74 \ 75 _i &= _mask; \ 76 _i; \ 77 }) 78 #endif /* _LINUX_NOSPEC_H */ 79