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 struct task_struct;
  11
  12 /**
  13  * array_index_mask_nospec() - generate a ~0 mask when index < size, 0 otherwise
  14  * @index: array element index
  15  * @size: number of elements in array
  16  *
  17  * When @index is out of bounds (@index >= @size), the sign bit will be
  18  * set.  Extend the sign bit to all bits and invert, giving a result of
  19  * zero for an out of bounds index, or ~0 if within bounds [0, @size).
  20  */
  21 #ifndef array_index_mask_nospec
  22 static inline unsigned long array_index_mask_nospec(unsigned long index,
  23                                                     unsigned long size)
  24 {
  25         /*
  26          * Always calculate and emit the mask even if the compiler
  27          * thinks the mask is not needed. The compiler does not take
  28          * into account the value of @index under speculation.
  29          */
  30         OPTIMIZER_HIDE_VAR(index);
  31         return ~(long)(index | (size - 1UL - index)) >> (BITS_PER_LONG - 1);
  32 }
  33 #endif
  34
  35 /*
  36  * array_index_nospec - sanitize an array index after a bounds check
  37  *
  38  * For a code sequence like:
  39  *
  40  *     if (index < size) {
  41  *         index = array_index_nospec(index, size);
  42  *         val = array[index];
  43  *     }
  44  *
  45  * ...if the CPU speculates past the bounds check then
  46  * array_index_nospec() will clamp the index within the range of [0,
  47  * size).
  48  */
  49 #define array_index_nospec(index, size)                                 \
  50 ({                                                                      \
  51         typeof(index) _i = (index);                                     \
  52         typeof(size) _s = (size);                                       \
  53         unsigned long _mask = array_index_mask_nospec(_i, _s);          \
  54                                                                         \
  55         BUILD_BUG_ON(sizeof(_i) > sizeof(long));                        \
  56         BUILD_BUG_ON(sizeof(_s) > sizeof(long));                        \
  57                                                                         \
  58         (typeof(_i)) (_i & _mask);                                      \
  59 })
  60
  61 /* Speculation control prctl */
  62 int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which);
  63 int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
  64                              unsigned long ctrl);
  65 /* Speculation control for seccomp enforced mitigation */
  66 void arch_seccomp_spec_mitigate(struct task_struct *task);
  67
  68 #endif /* _LINUX_NOSPEC_H */