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 <alex.popov@linux.com>
8 *
9 * STACKLEAK reduces the information which kernel stack leak bugs can
10 * reveal and blocks some uninitialized stack variable attacks.
11 */
12
13 #include <linux/stackleak.h>
14 #include <linux/kprobes.h>
15
16 #ifdef CONFIG_STACKLEAK_RUNTIME_DISABLE
17 #include <linux/jump_label.h>
18 #include <linux/sysctl.h>
19 #include <linux/init.h>
20
21 static DEFINE_STATIC_KEY_FALSE(stack_erasing_bypass);
22
23 #ifdef CONFIG_SYSCTL
stack_erasing_sysctl(struct ctl_table * table,int write,void __user * buffer,size_t * lenp,loff_t * ppos)24 static int stack_erasing_sysctl(struct ctl_table *table, int write,
25 void __user *buffer, size_t *lenp, loff_t *ppos)
26 {
27 int ret = 0;
28 int state = !static_branch_unlikely(&stack_erasing_bypass);
29 int prev_state = state;
30
31 table->data = &state;
32 table->maxlen = sizeof(int);
33 ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
34 state = !!state;
35 if (ret || !write || state == prev_state)
36 return ret;
37
38 if (state)
39 static_branch_disable(&stack_erasing_bypass);
40 else
41 static_branch_enable(&stack_erasing_bypass);
42
43 pr_warn("stackleak: kernel stack erasing is %s\n",
44 state ? "enabled" : "disabled");
45 return ret;
46 }
47 static struct ctl_table stackleak_sysctls[] = {
48 {
49 .procname = "stack_erasing",
50 .data = NULL,
51 .maxlen = sizeof(int),
52 .mode = 0600,
53 .proc_handler = stack_erasing_sysctl,
54 .extra1 = SYSCTL_ZERO,
55 .extra2 = SYSCTL_ONE,
56 },
57 {}
58 };
59
stackleak_sysctls_init(void)60 static int __init stackleak_sysctls_init(void)
61 {
62 register_sysctl_init("kernel", stackleak_sysctls);
63 return 0;
64 }
65 late_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_STACKLEAK_RUNTIME_DISABLE */
72
73 #ifndef __stackleak_poison
__stackleak_poison(unsigned long erase_low,unsigned long erase_high,unsigned long poison)74 static __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
__stackleak_erase(bool on_task_stack)85 static __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_STACKLEAK_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, STACKLEAK_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 */
stackleak_erase(void)127 asmlinkage 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 */
stackleak_erase_on_task_stack(void)139 asmlinkage 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 */
stackleak_erase_off_task_stack(void)151 asmlinkage void noinstr stackleak_erase_off_task_stack(void)
152 {
153 if (skip_erasing())
154 return;
155
156 __stackleak_erase(false);
157 }
158
stackleak_track_stack(void)159 void __used __no_caller_saved_registers noinstr stackleak_track_stack(void)
160 {
161 unsigned long sp = current_stack_pointer;
162
163 /*
164 * Having CONFIG_STACKLEAK_TRACK_MIN_SIZE larger than
165 * STACKLEAK_SEARCH_DEPTH makes the poison search in
166 * stackleak_erase() unreliable. Let's prevent that.
167 */
168 BUILD_BUG_ON(CONFIG_STACKLEAK_TRACK_MIN_SIZE > STACKLEAK_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 }
177 EXPORT_SYMBOL(stackleak_track_stack);
178
