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/system.h>
  42 #include <asm/desc.h>
  43 #include <linux/random.h>
  44
  45 /*
  46  * 24 byte read-only segment initializer for stack canary.  Linker
  47  * can't handle the address bit shifting.  Address will be set in
  48  * head_32 for boot CPU and setup_per_cpu_areas() for others.
  49  */
  50 #define GDT_STACK_CANARY_INIT                                           \
  51         [GDT_ENTRY_STACK_CANARY] = { { { 0x00000018, 0x00409000 } } },
  52
  53 /*
  54  * Initialize the stackprotector canary value.
  55  *
  56  * NOTE: this must only be called from functions that never return,
  57  * and it must always be inlined.
  58  */
  59 static __always_inline void boot_init_stack_canary(void)
  60 {
  61         u64 canary;
  62         u64 tsc;
  63
  64 #ifdef CONFIG_X86_64
  65         BUILD_BUG_ON(offsetof(union irq_stack_union, stack_canary) != 40);
  66 #endif
  67         /*
  68          * We both use the random pool and the current TSC as a source
  69          * of randomness. The TSC only matters for very early init,
  70          * there it already has some randomness on most systems. Later
  71          * on during the bootup the random pool has true entropy too.
  72          */
  73         get_random_bytes(&canary, sizeof(canary));
  74         tsc = __native_read_tsc();
  75         canary += tsc + (tsc << 32UL);
  76
  77         current->stack_canary = canary;
  78 #ifdef CONFIG_X86_64
  79         percpu_write(irq_stack_union.stack_canary, canary);
  80 #else
  81         percpu_write(stack_canary.canary, canary);
  82 #endif
  83 }
  84
  85 static inline void setup_stack_canary_segment(int cpu)
  86 {
  87 #ifdef CONFIG_X86_32
  88         unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
  89         struct desc_struct *gdt_table = get_cpu_gdt_table(cpu);
  90         struct desc_struct desc;
  91
  92         desc = gdt_table[GDT_ENTRY_STACK_CANARY];
  93         desc.base0 = canary & 0xffff;
  94         desc.base1 = (canary >> 16) & 0xff;
  95         desc.base2 = (canary >> 24) & 0xff;
  96         write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
  97 #endif
  98 }
  99
 100 static 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
 113 static inline void setup_stack_canary_segment(int cpu)
 114 { }
 115
 116 static 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 */