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1// SPDX-License-Identifier: GPL-2.0
2/*
3 * This code fills the used part of the kernel stack with a poison value
4 * before returning to userspace. It's part of the STACKLEAK feature
5 * ported from grsecurity/PaX.
6 *
7 * Author: Alexander Popov <[email protected]>
8 *
9 * KSTACK_ERASE reduces the information which kernel stack leak bugs can
10 * reveal and blocks some uninitialized stack variable attacks.
11 */
12
13#include <linux/kstack_erase.h>
14#include <linux/kprobes.h>
15
16#ifdef CONFIG_KSTACK_ERASE_RUNTIME_DISABLE
17#include <linux/jump_label.h>
18#include <linux/string_choices.h>
19#include <linux/sysctl.h>
20#include <linux/init.h>
21
22static DEFINE_STATIC_KEY_FALSE(stack_erasing_bypass);
23
24#ifdef CONFIG_SYSCTL
25static int stack_erasing_sysctl(const struct ctl_table *table, int write,
26 void *buffer, size_t *lenp, loff_t *ppos)
27{
28 int ret = 0;
29 int state = !static_branch_unlikely(&stack_erasing_bypass);
30 int prev_state = state;
31 struct ctl_table table_copy = *table;
32
33 table_copy.data = &state;
34 ret = proc_dointvec_minmax(&table_copy, write, buffer, lenp, ppos);
35 state = !!state;
36 if (ret || !write || state == prev_state)
37 return ret;
38
39 if (state)
40 static_branch_disable(&stack_erasing_bypass);
41 else
42 static_branch_enable(&stack_erasing_bypass);
43
44 pr_warn("stackleak: kernel stack erasing is %s\n",
45 str_enabled_disabled(state));
46 return ret;
47}
48static const struct ctl_table stackleak_sysctls[] = {
49 {
50 .procname = "stack_erasing",
51 .data = NULL,
52 .maxlen = sizeof(int),
53 .mode = 0600,
54 .proc_handler = stack_erasing_sysctl,
55 .extra1 = SYSCTL_ZERO,
56 .extra2 = SYSCTL_ONE,
57 },
58};
59
60static int __init stackleak_sysctls_init(void)
61{
62 register_sysctl_init("kernel", stackleak_sysctls);
63 return 0;
64}
65late_initcall(stackleak_sysctls_init);
66#endif /* CONFIG_SYSCTL */
67
68#define skip_erasing() static_branch_unlikely(&stack_erasing_bypass)
69#else
70#define skip_erasing() false
71#endif /* CONFIG_KSTACK_ERASE_RUNTIME_DISABLE */
72
73#ifndef __stackleak_poison
74static __always_inline void __stackleak_poison(unsigned long erase_low,
75 unsigned long erase_high,
76 unsigned long poison)
77{
78 while (erase_low < erase_high) {
79 *(unsigned long *)erase_low = poison;
80 erase_low += sizeof(unsigned long);
81 }
82}
83#endif
84
85static __always_inline void __stackleak_erase(bool on_task_stack)
86{
87 const unsigned long task_stack_low = stackleak_task_low_bound(current);
88 const unsigned long task_stack_high = stackleak_task_high_bound(current);
89 unsigned long erase_low, erase_high;
90
91 erase_low = stackleak_find_top_of_poison(task_stack_low,
92 current->lowest_stack);
93
94#ifdef CONFIG_KSTACK_ERASE_METRICS
95 current->prev_lowest_stack = erase_low;
96#endif
97
98 /*
99 * Write poison to the task's stack between 'erase_low' and
100 * 'erase_high'.
101 *
102 * If we're running on a different stack (e.g. an entry trampoline
103 * stack) we can erase everything below the pt_regs at the top of the
104 * task stack.
105 *
106 * If we're running on the task stack itself, we must not clobber any
107 * stack used by this function and its caller. We assume that this
108 * function has a fixed-size stack frame, and the current stack pointer
109 * doesn't change while we write poison.
110 */
111 if (on_task_stack)
112 erase_high = current_stack_pointer;
113 else
114 erase_high = task_stack_high;
115
116 __stackleak_poison(erase_low, erase_high, KSTACK_ERASE_POISON);
117
118 /* Reset the 'lowest_stack' value for the next syscall */
119 current->lowest_stack = task_stack_high;
120}
121
122/*
123 * Erase and poison the portion of the task stack used since the last erase.
124 * Can be called from the task stack or an entry stack when the task stack is
125 * no longer in use.
126 */
127asmlinkage void noinstr stackleak_erase(void)
128{
129 if (skip_erasing())
130 return;
131
132 __stackleak_erase(on_thread_stack());
133}
134
135/*
136 * Erase and poison the portion of the task stack used since the last erase.
137 * Can only be called from the task stack.
138 */
139asmlinkage void noinstr stackleak_erase_on_task_stack(void)
140{
141 if (skip_erasing())
142 return;
143
144 __stackleak_erase(true);
145}
146
147/*
148 * Erase and poison the portion of the task stack used since the last erase.
149 * Can only be called from a stack other than the task stack.
150 */
151asmlinkage void noinstr stackleak_erase_off_task_stack(void)
152{
153 if (skip_erasing())
154 return;
155
156 __stackleak_erase(false);
157}
158
159void __used __no_caller_saved_registers noinstr __sanitizer_cov_stack_depth(void)
160{
161 unsigned long sp = current_stack_pointer;
162
163 /*
164 * Having CONFIG_KSTACK_ERASE_TRACK_MIN_SIZE larger than
165 * KSTACK_ERASE_SEARCH_DEPTH makes the poison search in
166 * stackleak_erase() unreliable. Let's prevent that.
167 */
168 BUILD_BUG_ON(CONFIG_KSTACK_ERASE_TRACK_MIN_SIZE > KSTACK_ERASE_SEARCH_DEPTH);
169
170 /* 'lowest_stack' should be aligned on the register width boundary */
171 sp = ALIGN(sp, sizeof(unsigned long));
172 if (sp < current->lowest_stack &&
173 sp >= stackleak_task_low_bound(current)) {
174 current->lowest_stack = sp;
175 }
176}
177EXPORT_SYMBOL(__sanitizer_cov_stack_depth);