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          * 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 */