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git.proxmox.com Git - mirror_ubuntu-focal-kernel.git/blob - arch/x86/include/asm/stackprotector.h
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
39 #include <asm/processor.h>
40 #include <asm/percpu.h>
43 #include <linux/random.h>
44 #include <linux/sched.h>
47 * 24 byte read-only segment initializer for stack canary. Linker
48 * can't handle the address bit shifting. Address will be set in
49 * head_32 for boot CPU and setup_per_cpu_areas() for others.
51 #define GDT_STACK_CANARY_INIT \
52 [GDT_ENTRY_STACK_CANARY] = GDT_ENTRY_INIT(0x4090, 0, 0x18),
55 * Initialize the stackprotector canary value.
57 * NOTE: this must only be called from functions that never return,
58 * and it must always be inlined.
60 static __always_inline
void boot_init_stack_canary(void)
66 BUILD_BUG_ON(offsetof(union irq_stack_union
, stack_canary
) != 40);
69 * We both use the random pool and the current TSC as a source
70 * of randomness. The TSC only matters for very early init,
71 * there it already has some randomness on most systems. Later
72 * on during the bootup the random pool has true entropy too.
74 get_random_bytes(&canary
, sizeof(canary
));
76 canary
+= tsc
+ (tsc
<< 32UL);
78 current
->stack_canary
= canary
;
80 this_cpu_write(irq_stack_union
.stack_canary
, canary
);
82 this_cpu_write(stack_canary
.canary
, canary
);
86 static inline void setup_stack_canary_segment(int cpu
)
89 unsigned long canary
= (unsigned long)&per_cpu(stack_canary
, cpu
);
90 struct desc_struct
*gdt_table
= get_cpu_gdt_table(cpu
);
91 struct desc_struct desc
;
93 desc
= gdt_table
[GDT_ENTRY_STACK_CANARY
];
94 set_desc_base(&desc
, canary
);
95 write_gdt_entry(gdt_table
, GDT_ENTRY_STACK_CANARY
, &desc
, DESCTYPE_S
);
99 static inline void load_stack_canary_segment(void)
102 asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY
) : "memory");
106 #else /* CC_STACKPROTECTOR */
108 #define GDT_STACK_CANARY_INIT
110 /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
112 static inline void setup_stack_canary_segment(int cpu
)
115 static inline void load_stack_canary_segment(void)
118 asm volatile ("mov %0, %%gs" : : "r" (0));
122 #endif /* CC_STACKPROTECTOR */
123 #endif /* _ASM_STACKPROTECTOR_H */