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 #include <linux/random.h>
43
44 /*
45 * 24 byte read-only segment initializer for stack canary. Linker
46 * can't handle the address bit shifting. Address will be set in
47 * head_32 for boot CPU and setup_per_cpu_areas() for others.
48 */
49 #define GDT_STACK_CANARY_INIT \
50 [GDT_ENTRY_STACK_CANARY] = GDT_ENTRY_INIT(0x4090, 0, 0x18),
51
52 /*
53 * Initialize the stackprotector canary value.
54 *
55 * NOTE: this must only be called from functions that never return,
56 * and it must always be inlined.
57 */
boot_init_stack_canary(void)58 static __always_inline void boot_init_stack_canary(void)
59 {
60 u64 canary;
61 u64 tsc;
62
63 #ifdef CONFIG_X86_64
64 BUILD_BUG_ON(offsetof(union irq_stack_union, stack_canary) != 40);
65 #endif
66 /*
67 * We both use the random pool and the current TSC as a source
68 * of randomness. The TSC only matters for very early init,
69 * there it already has some randomness on most systems. Later
70 * on during the bootup the random pool has true entropy too.
71 */
72 get_random_bytes(&canary, sizeof(canary));
73 tsc = __native_read_tsc();
74 canary += tsc + (tsc << 32UL);
75
76 current->stack_canary = canary;
77 #ifdef CONFIG_X86_64
78 percpu_write(irq_stack_union.stack_canary, canary);
79 #else
80 percpu_write(stack_canary.canary, canary);
81 #endif
82 }
83
setup_stack_canary_segment(int cpu)84 static inline void setup_stack_canary_segment(int cpu)
85 {
86 #ifdef CONFIG_X86_32
87 unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
88 struct desc_struct *gdt_table = get_cpu_gdt_table(cpu);
89 struct desc_struct desc;
90
91 desc = gdt_table[GDT_ENTRY_STACK_CANARY];
92 set_desc_base(&desc, canary);
93 write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
94 #endif
95 }
96
load_stack_canary_segment(void)97 static inline void load_stack_canary_segment(void)
98 {
99 #ifdef CONFIG_X86_32
100 asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
101 #endif
102 }
103
104 #else /* CC_STACKPROTECTOR */
105
106 #define GDT_STACK_CANARY_INIT
107
108 /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
109
setup_stack_canary_segment(int cpu)110 static inline void setup_stack_canary_segment(int cpu)
111 { }
112
load_stack_canary_segment(void)113 static inline void load_stack_canary_segment(void)
114 {
115 #ifdef CONFIG_X86_32
116 asm volatile ("mov %0, %%gs" : : "r" (0));
117 #endif
118 }
119
120 #endif /* CC_STACKPROTECTOR */
121 #endif /* _ASM_STACKPROTECTOR_H */
122