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
   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          * Warn developers about inappropriate array_index_nospec() usage.
  24          *
  25          * Even if the CPU speculates past the WARN_ONCE branch, the
  26          * sign bit of @index is taken into account when generating the
  27          * mask.
  28          *
  29          * This warning is compiled out when the compiler can infer that
  30          * @index and @size are less than LONG_MAX.
  31          */
  32         if (WARN_ONCE(index > LONG_MAX || size > LONG_MAX,
  33                         "array_index_nospec() limited to range of [0, LONG_MAX]\n"))
  34                 return 0;
  35
  36         /*
  37          * Always calculate and emit the mask even if the compiler
  38          * thinks the mask is not needed. The compiler does not take
  39          * into account the value of @index under speculation.
  40          */
  41         OPTIMIZER_HIDE_VAR(index);
  42         return ~(long)(index | (size - 1UL - index)) >> (BITS_PER_LONG - 1);
  43 }
  44 #endif
  45
  46 /*
  47  * array_index_nospec - sanitize an array index after a bounds check
  48  *
  49  * For a code sequence like:
  50  *
  51  *     if (index < size) {
  52  *         index = array_index_nospec(index, size);
  53  *         val = array[index];
  54  *     }
  55  *
  56  * ...if the CPU speculates past the bounds check then
  57  * array_index_nospec() will clamp the index within the range of [0,
  58  * size).
  59  */
  60 #define array_index_nospec(index, size)                                 \
  61 ({                                                                      \
  62         typeof(index) _i = (index);                                     \
  63         typeof(size) _s = (size);                                       \
  64         unsigned long _mask = array_index_mask_nospec(_i, _s);          \
  65                                                                         \
  66         BUILD_BUG_ON(sizeof(_i) > sizeof(long));                        \
  67         BUILD_BUG_ON(sizeof(_s) > sizeof(long));                        \
  68                                                                         \
  69         _i &= _mask;                                                    \
  70         _i;                                                             \
  71 })
  72 #endif /* _LINUX_NOSPEC_H */