include/linux/randomize_kstack.h

   1 /* SPDX-License-Identifier: GPL-2.0-only */
   2 #ifndef _LINUX_RANDOMIZE_KSTACK_H
   3 #define _LINUX_RANDOMIZE_KSTACK_H
   4
   5 #include <linux/kernel.h>
   6 #include <linux/jump_label.h>
   7 #include <linux/percpu-defs.h>
   8
   9 DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
  10                          randomize_kstack_offset);
  11 DECLARE_PER_CPU(u32, kstack_offset);
  12
  13 /*
  14  * Do not use this anywhere else in the kernel. This is used here because
  15  * it provides an arch-agnostic way to grow the stack with correct
  16  * alignment. Also, since this use is being explicitly masked to a max of
  17  * 10 bits, stack-clash style attacks are unlikely. For more details see
  18  * "VLAs" in Documentation/process/deprecated.rst
  19  */
  20 void *__builtin_alloca(size_t size);
  21 /*
  22  * Use, at most, 10 bits of entropy. We explicitly cap this to keep the
  23  * "VLA" from being unbounded (see above). 10 bits leaves enough room for
  24  * per-arch offset masks to reduce entropy (by removing higher bits, since
  25  * high entropy may overly constrain usable stack space), and for
  26  * compiler/arch-specific stack alignment to remove the lower bits.
  27  */
  28 #define KSTACK_OFFSET_MAX(x)    ((x) & 0x3FF)
  29
  30 /*
  31  * These macros must be used during syscall entry when interrupts and
  32  * preempt are disabled, and after user registers have been stored to
  33  * the stack.
  34  */
  35 #define add_random_kstack_offset() do {                                 \
  36         if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
  37                                 &randomize_kstack_offset)) {            \
  38                 u32 offset = raw_cpu_read(kstack_offset);               \
  39                 u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset));  \
  40                 /* Keep allocation even after "ptr" loses scope. */     \
  41                 asm volatile("" :: "r"(ptr) : "memory");                \
  42         }                                                               \
  43 } while (0)
  44
  45 #define choose_random_kstack_offset(rand) do {                          \
  46         if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
  47                                 &randomize_kstack_offset)) {            \
  48                 u32 offset = raw_cpu_read(kstack_offset);               \
  49                 offset ^= (rand);                                       \
  50                 raw_cpu_write(kstack_offset, offset);                   \
  51         }                                                               \
  52 } while (0)
  53
  54 #endif