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 historically required it to be at a fixed offset
 9 * from the percpu segment base.  On x86_64, the offset is 40 bytes.
 
 
10 *
11 * The same segment is shared by percpu area and stack canary.  On
12 * x86_64, percpu symbols are zero based and %gs (64-bit) points to the
13 * base of percpu area.  The first occupant of the percpu area is always
14 * fixed_percpu_data which contains stack_canary at the appropriate
15 * offset.  On x86_32, the stack canary is just a regular percpu
16 * variable.
17 *
18 * Putting percpu data in %fs on 32-bit is a minor optimization compared to
19 * using %gs.  Since 32-bit userspace normally has %fs == 0, we are likely
20 * to load 0 into %fs on exit to usermode, whereas with percpu data in
21 * %gs, we are likely to load a non-null %gs on return to user mode.
 
 
22 *
23 * Once we are willing to require GCC 8.1 or better for 64-bit stackprotector
24 * support, we can remove some of this complexity.
 
 
 
25 */
26
27#ifndef _ASM_STACKPROTECTOR_H
28#define _ASM_STACKPROTECTOR_H 1
29
30#ifdef CONFIG_STACKPROTECTOR
31
32#include <asm/tsc.h>
33#include <asm/processor.h>
34#include <asm/percpu.h>
35#include <asm/desc.h>
36
 
37#include <linux/sched.h>
38
39/*
 
 
 
 
 
 
 
 
40 * Initialize the stackprotector canary value.
41 *
42 * NOTE: this must only be called from functions that never return
43 * and it must always be inlined.
44 *
45 * In addition, it should be called from a compilation unit for which
46 * stack protector is disabled. Alternatively, the caller should not end
47 * with a function call which gets tail-call optimized as that would
48 * lead to checking a modified canary value.
49 */
50static __always_inline void boot_init_stack_canary(void)
51{
52	unsigned long canary = get_random_canary();
 
53
54#ifdef CONFIG_X86_64
55	BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40);
56#endif
 
 
 
 
 
 
 
 
 
 
57
58	current->stack_canary = canary;
59#ifdef CONFIG_X86_64
60	this_cpu_write(fixed_percpu_data.stack_canary, canary);
61#else
62	this_cpu_write(__stack_chk_guard, canary);
63#endif
64}
65
66static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
67{
68#ifdef CONFIG_X86_64
69	per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary;
70#else
71	per_cpu(__stack_chk_guard, cpu) = idle->stack_canary;
 
 
 
 
72#endif
73}
74
75#else	/* STACKPROTECTOR */
 
 
 
 
 
 
 
 
 
76
77/* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
78
79static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
80{ }
81
82#endif	/* STACKPROTECTOR */
 
 
 
 
 
 
 
83#endif	/* _ASM_STACKPROTECTOR_H */

  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 * irq_stack_union 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_CC_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 */
 61static __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(union irq_stack_union, 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(irq_stack_union.stack_canary, canary);
 83#else
 84	this_cpu_write(stack_canary.canary, canary);
 85#endif
 86}
 87
 88static 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
101static 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	/* CC_STACKPROTECTOR */
109
110#define GDT_STACK_CANARY_INIT
111
112/* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
113
114static inline void setup_stack_canary_segment(int cpu)
115{ }
116
117static 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	/* CC_STACKPROTECTOR */
125#endif	/* _ASM_STACKPROTECTOR_H */