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 * fixed_percpu_data 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_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 * In addition, it should be called from a compilation unit for which
62 * stack protector is disabled. Alternatively, the caller should not end
63 * with a function call which gets tail-call optimized as that would
64 * lead to checking a modified canary value.
65 */
66 static __always_inline void boot_init_stack_canary(void)
67 {
68 u64 canary;
69 u64 tsc;
70
71 #ifdef CONFIG_X86_64
72 BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40);
73 #endif
74 /*
75 * We both use the random pool and the current TSC as a source
76 * of randomness. The TSC only matters for very early init,
77 * there it already has some randomness on most systems. Later
78 * on during the bootup the random pool has true entropy too.
79 */
80 get_random_bytes(&canary, sizeof(canary));
81 tsc = rdtsc();
82 canary += tsc + (tsc << 32UL);
83 canary &= CANARY_MASK;
84
85 current->stack_canary = canary;
86 #ifdef CONFIG_X86_64
87 this_cpu_write(fixed_percpu_data.stack_canary, canary);
88 #else
89 this_cpu_write(stack_canary.canary, canary);
90 #endif
91 }
92
93 static inline void setup_stack_canary_segment(int cpu)
94 {
95 #ifdef CONFIG_X86_32
96 unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
97 struct desc_struct *gdt_table = get_cpu_gdt_rw(cpu);
98 struct desc_struct desc;
99
100 desc = gdt_table[GDT_ENTRY_STACK_CANARY];
101 set_desc_base(&desc, canary);
102 write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
103 #endif
104 }
105
106 static inline void load_stack_canary_segment(void)
107 {
108 #ifdef CONFIG_X86_32
109 asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
110 #endif
111 }
112
113 #else /* STACKPROTECTOR */
114
115 #define GDT_STACK_CANARY_INIT
116
117 /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
118
119 static inline void setup_stack_canary_segment(int cpu)
120 { }
121
122 static inline void load_stack_canary_segment(void)
123 {
124 #ifdef CONFIG_X86_32
125 asm volatile ("mov %0, %%gs" : : "r" (0));
126 #endif
127 }
128
129 #endif /* STACKPROTECTOR */
130 #endif /* _ASM_STACKPROTECTOR_H */