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 approproate
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/random.h>
38 #include <linux/sched.h>
39
40 /*
41 * Initialize the stackprotector canary value.
42 *
43 * NOTE: this must only be called from functions that never return
44 * and it must always be inlined.
45 *
46 * In addition, it should be called from a compilation unit for which
47 * stack protector is disabled. Alternatively, the caller should not end
48 * with a function call which gets tail-call optimized as that would
49 * lead to checking a modified canary value.
50 */
boot_init_stack_canary(void)51 static __always_inline void boot_init_stack_canary(void)
52 {
53 u64 canary;
54 u64 tsc;
55
56 #ifdef CONFIG_X86_64
57 BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40);
58 #endif
59 /*
60 * We both use the random pool and the current TSC as a source
61 * of randomness. The TSC only matters for very early init,
62 * there it already has some randomness on most systems. Later
63 * on during the bootup the random pool has true entropy too.
64 */
65 get_random_bytes(&canary, sizeof(canary));
66 tsc = rdtsc();
67 canary += tsc + (tsc << 32UL);
68 canary &= CANARY_MASK;
69
70 current->stack_canary = canary;
71 #ifdef CONFIG_X86_64
72 this_cpu_write(fixed_percpu_data.stack_canary, canary);
73 #else
74 this_cpu_write(__stack_chk_guard, canary);
75 #endif
76 }
77
cpu_init_stack_canary(int cpu,struct task_struct * idle)78 static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
79 {
80 #ifdef CONFIG_X86_64
81 per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary;
82 #else
83 per_cpu(__stack_chk_guard, cpu) = idle->stack_canary;
84 #endif
85 }
86
87 #else /* STACKPROTECTOR */
88
89 /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
90
cpu_init_stack_canary(int cpu,struct task_struct * idle)91 static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
92 { }
93
94 #endif /* STACKPROTECTOR */
95 #endif /* _ASM_STACKPROTECTOR_H */
96