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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 */
50static __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
66static 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
79static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
80{ }
81
82#endif /* STACKPROTECTOR */
83#endif /* _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 * 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 */
66static __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
93static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
94{
95#ifdef CONFIG_X86_64
96 per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary;
97#else
98 per_cpu(stack_canary.canary, cpu) = idle->stack_canary;
99#endif
100}
101
102static inline void setup_stack_canary_segment(int cpu)
103{
104#ifdef CONFIG_X86_32
105 unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
106 struct desc_struct *gdt_table = get_cpu_gdt_rw(cpu);
107 struct desc_struct desc;
108
109 desc = gdt_table[GDT_ENTRY_STACK_CANARY];
110 set_desc_base(&desc, canary);
111 write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
112#endif
113}
114
115static inline void load_stack_canary_segment(void)
116{
117#ifdef CONFIG_X86_32
118 asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
119#endif
120}
121
122#else /* STACKPROTECTOR */
123
124#define GDT_STACK_CANARY_INIT
125
126/* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
127
128static inline void setup_stack_canary_segment(int cpu)
129{ }
130
131static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
132{ }
133
134static inline void load_stack_canary_segment(void)
135{
136#ifdef CONFIG_X86_32
137 asm volatile ("mov %0, %%gs" : : "r" (0));
138#endif
139}
140
141#endif /* STACKPROTECTOR */
142#endif /* _ASM_STACKPROTECTOR_H */