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 historically required it to be at a fixed offset |
9 | * from the percpu segment base. On x86_64, the offset is 40 bytes. |
10 | * |
11 | * The same segment is shared by percpu area and stack canary. On |
12 | * x86_64, percpu symbols are zero based and %gs (64-bit) points to the |
13 | * base of percpu area. The first occupant of the percpu area is always |
14 | * fixed_percpu_data which contains stack_canary at the appropriate |
15 | * offset. On x86_32, the stack canary is just a regular percpu |
16 | * variable. |
17 | * |
18 | * Putting percpu data in %fs on 32-bit is a minor optimization compared to |
19 | * using %gs. Since 32-bit userspace normally has %fs == 0, we are likely |
20 | * to load 0 into %fs on exit to usermode, whereas with percpu data in |
21 | * %gs, we are likely to load a non-null %gs on return to user mode. |
22 | * |
23 | * Once we are willing to require GCC 8.1 or better for 64-bit stackprotector |
24 | * support, we can remove some of this complexity. |
25 | */ |
26 | |
27 | #ifndef _ASM_STACKPROTECTOR_H |
28 | #define _ASM_STACKPROTECTOR_H 1 |
29 | |
30 | #ifdef CONFIG_STACKPROTECTOR |
31 | |
32 | #include <asm/tsc.h> |
33 | #include <asm/processor.h> |
34 | #include <asm/percpu.h> |
35 | #include <asm/desc.h> |
36 | |
37 | #include <linux/sched.h> |
38 | |
39 | /* |
40 | * Initialize the stackprotector canary value. |
41 | * |
42 | * NOTE: this must only be called from functions that never return |
43 | * and it must always be inlined. |
44 | * |
45 | * In addition, it should be called from a compilation unit for which |
46 | * stack protector is disabled. Alternatively, the caller should not end |
47 | * with a function call which gets tail-call optimized as that would |
48 | * lead to checking a modified canary value. |
49 | */ |
50 | static __always_inline void boot_init_stack_canary(void) |
51 | { |
52 | unsigned long canary = get_random_canary(); |
53 | |
54 | #ifdef CONFIG_X86_64 |
55 | BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40); |
56 | #endif |
57 | |
58 | current->stack_canary = canary; |
59 | #ifdef CONFIG_X86_64 |
60 | this_cpu_write(fixed_percpu_data.stack_canary, canary); |
61 | #else |
62 | this_cpu_write(__stack_chk_guard, canary); |
63 | #endif |
64 | } |
65 | |
66 | static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle) |
67 | { |
68 | #ifdef CONFIG_X86_64 |
69 | per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary; |
70 | #else |
71 | per_cpu(__stack_chk_guard, cpu) = idle->stack_canary; |
72 | #endif |
73 | } |
74 | |
75 | #else /* STACKPROTECTOR */ |
76 | |
77 | /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */ |
78 | |
79 | static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle) |
80 | { } |
81 | |
82 | #endif /* STACKPROTECTOR */ |
83 | #endif /* _ASM_STACKPROTECTOR_H */ |
84 | |