xref: /linux/arch/x86/include/asm/stackprotector.h (revision d39d0ed196aa1685bb24771e92f78633c66ac9cb)
1 /*
2  * GCC stack protector support.
3  *
4  * Stack protector works by putting predefined pattern at the start of
5  * the stack frame and verifying that it hasn't been overwritten when
6  * returning from the function.  The pattern is called stack canary
7  * and unfortunately gcc requires it to be at a fixed offset from %gs.
8  * On x86_64, the offset is 40 bytes and on x86_32 20 bytes.  x86_64
9  * and x86_32 use segment registers differently and thus handles this
10  * requirement differently.
11  *
12  * On x86_64, %gs is shared by percpu area and stack canary.  All
13  * percpu symbols are zero based and %gs points to the base of percpu
14  * area.  The first occupant of the percpu area is always
15  * irq_stack_union which contains stack_canary at offset 40.  Userland
16  * %gs is always saved and restored on kernel entry and exit using
17  * swapgs, so stack protector doesn't add any complexity there.
18  *
19  * On x86_32, it's slightly more complicated.  As in x86_64, %gs is
20  * used for userland TLS.  Unfortunately, some processors are much
21  * slower at loading segment registers with different value when
22  * entering and leaving the kernel, so the kernel uses %fs for percpu
23  * area and manages %gs lazily so that %gs is switched only when
24  * necessary, usually during task switch.
25  *
26  * As gcc requires the stack canary at %gs:20, %gs can't be managed
27  * lazily if stack protector is enabled, so the kernel saves and
28  * restores userland %gs on kernel entry and exit.  This behavior is
29  * controlled by CONFIG_X86_32_LAZY_GS and accessors are defined in
30  * system.h to hide the details.
31  */
32 
33 #ifndef _ASM_STACKPROTECTOR_H
34 #define _ASM_STACKPROTECTOR_H 1
35 
36 #ifdef CONFIG_CC_STACKPROTECTOR
37 
38 #include <asm/tsc.h>
39 #include <asm/processor.h>
40 #include <asm/percpu.h>
41 #include <asm/system.h>
42 #include <asm/desc.h>
43 #include <linux/random.h>
44 
45 /*
46  * 24 byte read-only segment initializer for stack canary.  Linker
47  * can't handle the address bit shifting.  Address will be set in
48  * head_32 for boot CPU and setup_per_cpu_areas() for others.
49  */
50 #define GDT_STACK_CANARY_INIT						\
51 	[GDT_ENTRY_STACK_CANARY] = GDT_ENTRY_INIT(0x4090, 0, 0x18),
52 
53 /*
54  * Initialize the stackprotector canary value.
55  *
56  * NOTE: this must only be called from functions that never return,
57  * and it must always be inlined.
58  */
59 static __always_inline void boot_init_stack_canary(void)
60 {
61 	u64 canary;
62 	u64 tsc;
63 
64 #ifdef CONFIG_X86_64
65 	BUILD_BUG_ON(offsetof(union irq_stack_union, stack_canary) != 40);
66 #endif
67 	/*
68 	 * We both use the random pool and the current TSC as a source
69 	 * of randomness. The TSC only matters for very early init,
70 	 * there it already has some randomness on most systems. Later
71 	 * on during the bootup the random pool has true entropy too.
72 	 */
73 	get_random_bytes(&canary, sizeof(canary));
74 	tsc = __native_read_tsc();
75 	canary += tsc + (tsc << 32UL);
76 
77 	current->stack_canary = canary;
78 #ifdef CONFIG_X86_64
79 	percpu_write(irq_stack_union.stack_canary, canary);
80 #else
81 	percpu_write(stack_canary.canary, canary);
82 #endif
83 }
84 
85 static inline void setup_stack_canary_segment(int cpu)
86 {
87 #ifdef CONFIG_X86_32
88 	unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
89 	struct desc_struct *gdt_table = get_cpu_gdt_table(cpu);
90 	struct desc_struct desc;
91 
92 	desc = gdt_table[GDT_ENTRY_STACK_CANARY];
93 	set_desc_base(&desc, canary);
94 	write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
95 #endif
96 }
97 
98 static inline void load_stack_canary_segment(void)
99 {
100 #ifdef CONFIG_X86_32
101 	asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
102 #endif
103 }
104 
105 #else	/* CC_STACKPROTECTOR */
106 
107 #define GDT_STACK_CANARY_INIT
108 
109 /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
110 
111 static inline void setup_stack_canary_segment(int cpu)
112 { }
113 
114 static inline void load_stack_canary_segment(void)
115 {
116 #ifdef CONFIG_X86_32
117 	asm volatile ("mov %0, %%gs" : : "r" (0));
118 #endif
119 }
120 
121 #endif	/* CC_STACKPROTECTOR */
122 #endif	/* _ASM_STACKPROTECTOR_H */
123