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 static __always_inline void boot_init_stack_canary(void)
62 {
63 	u64 canary;
64 	u64 tsc;
65 
66 #ifdef CONFIG_X86_64
67 	BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40);
68 #endif
69 	/*
70 	 * We both use the random pool and the current TSC as a source
71 	 * of randomness. The TSC only matters for very early init,
72 	 * there it already has some randomness on most systems. Later
73 	 * on during the bootup the random pool has true entropy too.
74 	 */
75 	get_random_bytes(&canary, sizeof(canary));
76 	tsc = rdtsc();
77 	canary += tsc + (tsc << 32UL);
78 	canary &= CANARY_MASK;
79 
80 	current->stack_canary = canary;
81 #ifdef CONFIG_X86_64
82 	this_cpu_write(fixed_percpu_data.stack_canary, canary);
83 #else
84 	this_cpu_write(stack_canary.canary, canary);
85 #endif
86 }
87 
88 static inline void setup_stack_canary_segment(int cpu)
89 {
90 #ifdef CONFIG_X86_32
91 	unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
92 	struct desc_struct *gdt_table = get_cpu_gdt_rw(cpu);
93 	struct desc_struct desc;
94 
95 	desc = gdt_table[GDT_ENTRY_STACK_CANARY];
96 	set_desc_base(&desc, canary);
97 	write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
98 #endif
99 }
100 
101 static inline void load_stack_canary_segment(void)
102 {
103 #ifdef CONFIG_X86_32
104 	asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
105 #endif
106 }
107 
108 #else	/* STACKPROTECTOR */
109 
110 #define GDT_STACK_CANARY_INIT
111 
112 /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
113 
114 static inline void setup_stack_canary_segment(int cpu)
115 { }
116 
117 static inline void load_stack_canary_segment(void)
118 {
119 #ifdef CONFIG_X86_32
120 	asm volatile ("mov %0, %%gs" : : "r" (0));
121 #endif
122 }
123 
124 #endif	/* STACKPROTECTOR */
125 #endif	/* _ASM_STACKPROTECTOR_H */
126