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 */
boot_init_stack_canary(void)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
cpu_init_stack_canary(int cpu,struct task_struct * idle)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
cpu_init_stack_canary(int cpu,struct task_struct * idle)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