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