Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/dtor/input
[linux-btrfs-devel.git] / arch / x86 / include / asm / stackprotector.h
blob157517763565668d050a628f98630199764d825c
1 /*
2 * GCC stack protector support.
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.
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.
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.
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.
33 #ifndef _ASM_STACKPROTECTOR_H
34 #define _ASM_STACKPROTECTOR_H 1
36 #ifdef CONFIG_CC_STACKPROTECTOR
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>
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.
50 #define GDT_STACK_CANARY_INIT \
51 [GDT_ENTRY_STACK_CANARY] = GDT_ENTRY_INIT(0x4090, 0, 0x18),
54 * Initialize the stackprotector canary value.
56 * NOTE: this must only be called from functions that never return,
57 * and it must always be inlined.
59 static __always_inline void boot_init_stack_canary(void)
61 u64 canary;
62 u64 tsc;
64 #ifdef CONFIG_X86_64
65 BUILD_BUG_ON(offsetof(union irq_stack_union, stack_canary) != 40);
66 #endif
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.
73 get_random_bytes(&canary, sizeof(canary));
74 tsc = __native_read_tsc();
75 canary += tsc + (tsc << 32UL);
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
85 static inline void setup_stack_canary_segment(int cpu)
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;
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
98 static inline void load_stack_canary_segment(void)
100 #ifdef CONFIG_X86_32
101 asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
102 #endif
105 #else /* CC_STACKPROTECTOR */
107 #define GDT_STACK_CANARY_INIT
109 /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
111 static inline void setup_stack_canary_segment(int cpu)
114 static inline void load_stack_canary_segment(void)
116 #ifdef CONFIG_X86_32
117 asm volatile ("mov %0, %%gs" : : "r" (0));
118 #endif
121 #endif /* CC_STACKPROTECTOR */
122 #endif /* _ASM_STACKPROTECTOR_H */