LXR linux/arch/x86/include/asm/stackprotector.h

   1/* SPDX-License-Identifier: GPL-2.0 */
   2/*
   3 * GCC stack protector support.
   4 *
   5 * Stack protector works by putting predefined pattern at the start of
   6 * the stack frame and verifying that it hasn't been overwritten when
   7 * returning from the function.  The pattern is called stack canary
   8 * and unfortunately gcc historically required it to be at a fixed offset
   9 * from the percpu segment base.  On x86_64, the offset is 40 bytes.
  10 *
  11 * The same segment is shared by percpu area and stack canary.  On
  12 * x86_64, percpu symbols are zero based and %gs (64-bit) points to the
  13 * base of percpu area.  The first occupant of the percpu area is always
  14 * fixed_percpu_data which contains stack_canary at the appropriate
  15 * offset.  On x86_32, the stack canary is just a regular percpu
  16 * variable.
  17 *
  18 * Putting percpu data in %fs on 32-bit is a minor optimization compared to
  19 * using %gs.  Since 32-bit userspace normally has %fs == 0, we are likely
  20 * to load 0 into %fs on exit to usermode, whereas with percpu data in
  21 * %gs, we are likely to load a non-null %gs on return to user mode.
  22 *
  23 * Once we are willing to require GCC 8.1 or better for 64-bit stackprotector
  24 * support, we can remove some of this complexity.
  25 */
  26
  27#ifndef _ASM_STACKPROTECTOR_H
  28#define _ASM_STACKPROTECTOR_H 1
  29
  30#ifdef CONFIG_STACKPROTECTOR
  31
  32#include <asm/tsc.h>
  33#include <asm/processor.h>
  34#include <asm/percpu.h>
  35#include <asm/desc.h>
  36
  37#include <linux/random.h>
  38#include <linux/sched.h>
  39
  40/*
  41 * Initialize the stackprotector canary value.
  42 *
  43 * NOTE: this must only be called from functions that never return
  44 * and it must always be inlined.
  45 *
  46 * In addition, it should be called from a compilation unit for which
  47 * stack protector is disabled. Alternatively, the caller should not end
  48 * with a function call which gets tail-call optimized as that would
  49 * lead to checking a modified canary value.
  50 */
  51static __always_inline void boot_init_stack_canary(void)
  52{
  53        u64 canary;
  54        u64 tsc;
  55
  56#ifdef CONFIG_X86_64
  57        BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40);
  58#endif
  59        /*
  60         * We both use the random pool and the current TSC as a source
  61         * of randomness. The TSC only matters for very early init,
  62         * there it already has some randomness on most systems. Later
  63         * on during the bootup the random pool has true entropy too.
  64         */
  65        get_random_bytes(&canary, sizeof(canary));
  66        tsc = rdtsc();
  67        canary += tsc + (tsc << 32UL);
  68        canary &= CANARY_MASK;
  69
  70        current->stack_canary = canary;
  71#ifdef CONFIG_X86_64
  72        this_cpu_write(fixed_percpu_data.stack_canary, canary);
  73#else
  74        this_cpu_write(__stack_chk_guard, canary);
  75#endif
  76}
  77
  78static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
  79{
  80#ifdef CONFIG_X86_64
  81        per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary;
  82#else
  83        per_cpu(__stack_chk_guard, cpu) = idle->stack_canary;
  84#endif
  85}
  86
  87#else   /* STACKPROTECTOR */
  88
  89/* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
  90
  91static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
  92{ }
  93
  94#endif  /* STACKPROTECTOR */
  95#endif  /* _ASM_STACKPROTECTOR_H */
  96