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  */
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 
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 
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