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/desc.h>
42#include <linux/random.h>
43
44/*
45 * 24 byte read-only segment initializer for stack canary.  Linker
46 * can't handle the address bit shifting.  Address will be set in
47 * head_32 for boot CPU and setup_per_cpu_areas() for others.
48 */
49#define GDT_STACK_CANARY_INIT						\
50	[GDT_ENTRY_STACK_CANARY] = GDT_ENTRY_INIT(0x4090, 0, 0x18),
51
52/*
53 * Initialize the stackprotector canary value.
54 *
55 * NOTE: this must only be called from functions that never return,
56 * and it must always be inlined.
57 */
58static __always_inline void boot_init_stack_canary(void)
59{
60	u64 canary;
61	u64 tsc;
62
63#ifdef CONFIG_X86_64
64	BUILD_BUG_ON(offsetof(union irq_stack_union, stack_canary) != 40);
65#endif
66	/*
67	 * We both use the random pool and the current TSC as a source
68	 * of randomness. The TSC only matters for very early init,
69	 * there it already has some randomness on most systems. Later
70	 * on during the bootup the random pool has true entropy too.
71	 */
72	get_random_bytes(&canary, sizeof(canary));
73	tsc = __native_read_tsc();
74	canary += tsc + (tsc << 32UL);
75
76	current->stack_canary = canary;
77#ifdef CONFIG_X86_64
78	this_cpu_write(irq_stack_union.stack_canary, canary);
79#else
80	this_cpu_write(stack_canary.canary, canary);
81#endif
82}
83
84static inline void setup_stack_canary_segment(int cpu)
85{
86#ifdef CONFIG_X86_32
87	unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
88	struct desc_struct *gdt_table = get_cpu_gdt_table(cpu);
89	struct desc_struct desc;
90
91	desc = gdt_table[GDT_ENTRY_STACK_CANARY];
92	set_desc_base(&desc, canary);
93	write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
94#endif
95}
96
97static inline void load_stack_canary_segment(void)
98{
99#ifdef CONFIG_X86_32
100	asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
101#endif
102}
103
104#else	/* CC_STACKPROTECTOR */
105
106#define GDT_STACK_CANARY_INIT
107
108/* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
109
110static inline void setup_stack_canary_segment(int cpu)
111{ }
112
113static inline void load_stack_canary_segment(void)
114{
115#ifdef CONFIG_X86_32
116	asm volatile ("mov %0, %%gs" : : "r" (0));
117#endif
118}
119
120#endif	/* CC_STACKPROTECTOR */
121#endif	/* _ASM_STACKPROTECTOR_H */
122