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

   1/*
   2 * GCC stack protector support.
   3 *
   4 * Stack protector works by putting predefined pattern at the start of
   5 * the stack frame and verifying that it hasn't been overwritten when
   6 * returning from the function.  The pattern is called stack canary
   7 * and unfortunately gcc requires it to be at a fixed offset from %gs.
   8 * On x86_64, the offset is 40 bytes and on x86_32 20 bytes.  x86_64
   9 * and x86_32 use segment registers differently and thus handles this
  10 * requirement differently.
  11 *
  12 * On x86_64, %gs is shared by percpu area and stack canary.  All
  13 * percpu symbols are zero based and %gs points to the base of percpu
  14 * area.  The first occupant of the percpu area is always
  15 * irq_stack_union which contains stack_canary at offset 40.  Userland
  16 * %gs is always saved and restored on kernel entry and exit using
  17 * swapgs, so stack protector doesn't add any complexity there.
  18 *
  19 * On x86_32, it's slightly more complicated.  As in x86_64, %gs is
  20 * used for userland TLS.  Unfortunately, some processors are much
  21 * slower at loading segment registers with different value when
  22 * entering and leaving the kernel, so the kernel uses %fs for percpu
  23 * area and manages %gs lazily so that %gs is switched only when
  24 * necessary, usually during task switch.
  25 *
  26 * As gcc requires the stack canary at %gs:20, %gs can't be managed
  27 * lazily if stack protector is enabled, so the kernel saves and
  28 * restores userland %gs on kernel entry and exit.  This behavior is
  29 * controlled by CONFIG_X86_32_LAZY_GS and accessors are defined in
  30 * system.h to hide the details.
  31 */
  32
  33#ifndef _ASM_STACKPROTECTOR_H
  34#define _ASM_STACKPROTECTOR_H 1
  35
  36#ifdef CONFIG_CC_STACKPROTECTOR
  37
  38#include <asm/tsc.h>
  39#include <asm/processor.h>
  40#include <asm/percpu.h>
  41#include <asm/desc.h>
  42
  43#include <linux/random.h>
  44#include <linux/sched.h>
  45
  46/*
  47 * 24 byte read-only segment initializer for stack canary.  Linker
  48 * can't handle the address bit shifting.  Address will be set in
  49 * head_32 for boot CPU and setup_per_cpu_areas() for others.
  50 */
  51#define GDT_STACK_CANARY_INIT                                           \
  52        [GDT_ENTRY_STACK_CANARY] = GDT_ENTRY_INIT(0x4090, 0, 0x18),
  53
  54/*
  55 * Initialize the stackprotector canary value.
  56 *
  57 * NOTE: this must only be called from functions that never return,
  58 * and it must always be inlined.
  59 */
  60static __always_inline void boot_init_stack_canary(void)
  61{
  62        u64 canary;
  63        u64 tsc;
  64
  65#ifdef CONFIG_X86_64
  66        BUILD_BUG_ON(offsetof(union irq_stack_union, stack_canary) != 40);
  67#endif
  68        /*
  69         * We both use the random pool and the current TSC as a source
  70         * of randomness. The TSC only matters for very early init,
  71         * there it already has some randomness on most systems. Later
  72         * on during the bootup the random pool has true entropy too.
  73         */
  74        get_random_bytes(&canary, sizeof(canary));
  75        tsc = rdtsc();
  76        canary += tsc + (tsc << 32UL);
  77        canary &= CANARY_MASK;
  78
  79        current->stack_canary = canary;
  80#ifdef CONFIG_X86_64
  81        this_cpu_write(irq_stack_union.stack_canary, canary);
  82#else
  83        this_cpu_write(stack_canary.canary, canary);
  84#endif
  85}
  86
  87static inline void setup_stack_canary_segment(int cpu)
  88{
  89#ifdef CONFIG_X86_32
  90        unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
  91        struct desc_struct *gdt_table = get_cpu_gdt_rw(cpu);
  92        struct desc_struct desc;
  93
  94        desc = gdt_table[GDT_ENTRY_STACK_CANARY];
  95        set_desc_base(&desc, canary);
  96        write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
  97#endif
  98}
  99
 100static inline void load_stack_canary_segment(void)
 101{
 102#ifdef CONFIG_X86_32
 103        asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
 104#endif
 105}
 106
 107#else   /* CC_STACKPROTECTOR */
 108
 109#define GDT_STACK_CANARY_INIT
 110
 111/* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
 112
 113static inline void setup_stack_canary_segment(int cpu)
 114{ }
 115
 116static inline void load_stack_canary_segment(void)
 117{
 118#ifdef CONFIG_X86_32
 119        asm volatile ("mov %0, %%gs" : : "r" (0));
 120#endif
 121}
 122
 123#endif  /* CC_STACKPROTECTOR */
 124#endif  /* _ASM_STACKPROTECTOR_H */
 125