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