include/linux/nospec.h

   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 #include <asm/barrier.h>
   9
  10 /**
  11  * array_index_mask_nospec() - generate a ~0 mask when index < size, 0 otherwise
  12  * @index: array element index
  13  * @size: number of elements in array
  14  *
  15  * When @index is out of bounds (@index >= @size), the sign bit will be
  16  * set.  Extend the sign bit to all bits and invert, giving a result of
  17  * zero for an out of bounds index, or ~0 if within bounds [0, @size).
  18  */
  19 #ifndef array_index_mask_nospec
  20 static inline unsigned long array_index_mask_nospec(unsigned long index,
  21                                                     unsigned long size)
  22 {
  23         /*
  24          * Always calculate and emit the mask even if the compiler
  25          * thinks the mask is not needed. The compiler does not take
  26          * into account the value of @index under speculation.
  27          */
  28         OPTIMIZER_HIDE_VAR(index);
  29         return ~(long)(index | (size - 1UL - index)) >> (BITS_PER_LONG - 1);
  30 }
  31 #endif
  32
  33 /*
  34  * array_index_nospec - sanitize an array index after a bounds check
  35  *
  36  * For a code sequence like:
  37  *
  38  *     if (index < size) {
  39  *         index = array_index_nospec(index, size);
  40  *         val = array[index];
  41  *     }
  42  *
  43  * ...if the CPU speculates past the bounds check then
  44  * array_index_nospec() will clamp the index within the range of [0,
  45  * size).
  46  */
  47 #define array_index_nospec(index, size)                                 \
  48 ({                                                                      \
  49         typeof(index) _i = (index);                                     \
  50         typeof(size) _s = (size);                                       \
  51         unsigned long _mask = array_index_mask_nospec(_i, _s);          \
  52                                                                         \
  53         BUILD_BUG_ON(sizeof(_i) > sizeof(long));                        \
  54         BUILD_BUG_ON(sizeof(_s) > sizeof(long));                        \
  55                                                                         \
  56         (typeof(_i)) (_i & _mask);                                      \
  57 })
  58 #endif /* _LINUX_NOSPEC_H */