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 #include <linux/compiler.h>
  10 #include <asm/barrier.h>
  11
  12 struct task_struct;
  13
  14 /**
  15  * array_index_mask_nospec() - generate a ~0 mask when index < size, 0 otherwise
  16  * @index: array element index
  17  * @size: number of elements in array
  18  *
  19  * When @index is out of bounds (@index >= @size), the sign bit will be
  20  * set.  Extend the sign bit to all bits and invert, giving a result of
  21  * zero for an out of bounds index, or ~0 if within bounds [0, @size).
  22  */
  23 #ifndef array_index_mask_nospec
  24 static inline unsigned long array_index_mask_nospec(unsigned long index,
  25                                                     unsigned long size)
  26 {
  27         /*
  28          * Always calculate and emit the mask even if the compiler
  29          * thinks the mask is not needed. The compiler does not take
  30          * into account the value of @index under speculation.
  31          */
  32         OPTIMIZER_HIDE_VAR(index);
  33         return ~(long)(index | (size - 1UL - index)) >> (BITS_PER_LONG - 1);
  34 }
  35 #endif
  36
  37 /*
  38  * array_index_nospec - sanitize an array index after a bounds check
  39  *
  40  * For a code sequence like:
  41  *
  42  *     if (index < size) {
  43  *         index = array_index_nospec(index, size);
  44  *         val = array[index];
  45  *     }
  46  *
  47  * ...if the CPU speculates past the bounds check then
  48  * array_index_nospec() will clamp the index within the range of [0,
  49  * size).
  50  */
  51 #define array_index_nospec(index, size)                                 \
  52 ({                                                                      \
  53         typeof(index) _i = (index);                                     \
  54         typeof(size) _s = (size);                                       \
  55         unsigned long _mask = array_index_mask_nospec(_i, _s);          \
  56                                                                         \
  57         BUILD_BUG_ON(sizeof(_i) > sizeof(long));                        \
  58         BUILD_BUG_ON(sizeof(_s) > sizeof(long));                        \
  59                                                                         \
  60         (typeof(_i)) (_i & _mask);                                      \
  61 })
  62
  63 /* Speculation control prctl */
  64 int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which);
  65 int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
  66                              unsigned long ctrl);
  67 /* Speculation control for seccomp enforced mitigation */
  68 void arch_seccomp_spec_mitigate(struct task_struct *task);
  69
  70 #endif /* _LINUX_NOSPEC_H */