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 #ifndef __stackleak_poison
73 static __always_inline void __stackleak_poison 74 static __always_inline void __stackleak_poison(unsigned long erase_low,
74 75 unsigned long erase_high,
75 76 unsigned long poison)
76 { 77 {
77 while (erase_low < erase_high) { 78 while (erase_low < erase_high) {
78 *(unsigned long *)erase_low = 79 *(unsigned long *)erase_low = poison;
79 erase_low += sizeof(unsigned l 80 erase_low += sizeof(unsigned long);
80 } 81 }
81 } 82 }
82 #endif 83 #endif
83 84
84 static __always_inline void __stackleak_erase( 85 static __always_inline void __stackleak_erase(bool on_task_stack)
85 { 86 {
86 const unsigned long task_stack_low = s 87 const unsigned long task_stack_low = stackleak_task_low_bound(current);
87 const unsigned long task_stack_high = 88 const unsigned long task_stack_high = stackleak_task_high_bound(current);
88 unsigned long erase_low, erase_high; 89 unsigned long erase_low, erase_high;
89 90
90 erase_low = stackleak_find_top_of_pois 91 erase_low = stackleak_find_top_of_poison(task_stack_low,
91 92 current->lowest_stack);
92 93
93 #ifdef CONFIG_STACKLEAK_METRICS 94 #ifdef CONFIG_STACKLEAK_METRICS
94 current->prev_lowest_stack = erase_low 95 current->prev_lowest_stack = erase_low;
95 #endif 96 #endif
96 97
97 /* 98 /*
98 * Write poison to the task's stack be 99 * Write poison to the task's stack between 'erase_low' and
99 * 'erase_high'. 100 * 'erase_high'.
100 * 101 *
101 * If we're running on a different sta 102 * If we're running on a different stack (e.g. an entry trampoline
102 * stack) we can erase everything belo 103 * stack) we can erase everything below the pt_regs at the top of the
103 * task stack. 104 * task stack.
104 * 105 *
105 * If we're running on the task stack 106 * If we're running on the task stack itself, we must not clobber any
106 * stack used by this function and its 107 * stack used by this function and its caller. We assume that this
107 * function has a fixed-size stack fra 108 * function has a fixed-size stack frame, and the current stack pointer
108 * doesn't change while we write poiso 109 * doesn't change while we write poison.
109 */ 110 */
110 if (on_task_stack) 111 if (on_task_stack)
111 erase_high = current_stack_poi 112 erase_high = current_stack_pointer;
112 else 113 else
113 erase_high = task_stack_high; 114 erase_high = task_stack_high;
114 115
115 __stackleak_poison(erase_low, erase_hi 116 __stackleak_poison(erase_low, erase_high, STACKLEAK_POISON);
116 117
117 /* Reset the 'lowest_stack' value for 118 /* Reset the 'lowest_stack' value for the next syscall */
118 current->lowest_stack = task_stack_hig 119 current->lowest_stack = task_stack_high;
119 } 120 }
120 121
121 /* 122 /*
122 * Erase and poison the portion of the task st 123 * 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 124 * Can be called from the task stack or an entry stack when the task stack is
124 * no longer in use. 125 * no longer in use.
125 */ 126 */
126 asmlinkage void noinstr stackleak_erase(void) 127 asmlinkage void noinstr stackleak_erase(void)
127 { 128 {
128 if (skip_erasing()) 129 if (skip_erasing())
129 return; 130 return;
130 131
131 __stackleak_erase(on_thread_stack()); 132 __stackleak_erase(on_thread_stack());
132 } 133 }
133 134
134 /* 135 /*
135 * Erase and poison the portion of the task st 136 * Erase and poison the portion of the task stack used since the last erase.
136 * Can only be called from the task stack. 137 * Can only be called from the task stack.
137 */ 138 */
138 asmlinkage void noinstr stackleak_erase_on_tas 139 asmlinkage void noinstr stackleak_erase_on_task_stack(void)
139 { 140 {
140 if (skip_erasing()) 141 if (skip_erasing())
141 return; 142 return;
142 143
143 __stackleak_erase(true); 144 __stackleak_erase(true);
144 } 145 }
145 146
146 /* 147 /*
147 * Erase and poison the portion of the task st 148 * Erase and poison the portion of the task stack used since the last erase.
148 * Can only be called from a stack other than 149 * Can only be called from a stack other than the task stack.
149 */ 150 */
150 asmlinkage void noinstr stackleak_erase_off_ta 151 asmlinkage void noinstr stackleak_erase_off_task_stack(void)
151 { 152 {
152 if (skip_erasing()) 153 if (skip_erasing())
153 return; 154 return;
154 155
155 __stackleak_erase(false); 156 __stackleak_erase(false);
156 } 157 }
157 158
158 void __used __no_caller_saved_registers noinst 159 void __used __no_caller_saved_registers noinstr stackleak_track_stack(void)
159 { 160 {
160 unsigned long sp = current_stack_point 161 unsigned long sp = current_stack_pointer;
161 162
162 /* 163 /*
163 * Having CONFIG_STACKLEAK_TRACK_MIN_S 164 * Having CONFIG_STACKLEAK_TRACK_MIN_SIZE larger than
164 * STACKLEAK_SEARCH_DEPTH makes the po 165 * STACKLEAK_SEARCH_DEPTH makes the poison search in
165 * stackleak_erase() unreliable. Let's 166 * stackleak_erase() unreliable. Let's prevent that.
166 */ 167 */
167 BUILD_BUG_ON(CONFIG_STACKLEAK_TRACK_MI 168 BUILD_BUG_ON(CONFIG_STACKLEAK_TRACK_MIN_SIZE > STACKLEAK_SEARCH_DEPTH);
168 169
169 /* 'lowest_stack' should be aligned on 170 /* 'lowest_stack' should be aligned on the register width boundary */
170 sp = ALIGN(sp, sizeof(unsigned long)); 171 sp = ALIGN(sp, sizeof(unsigned long));
171 if (sp < current->lowest_stack && 172 if (sp < current->lowest_stack &&
172 sp >= stackleak_task_low_bound(cur 173 sp >= stackleak_task_low_bound(current)) {
173 current->lowest_stack = sp; 174 current->lowest_stack = sp;
174 } 175 }
175 } 176 }
176 EXPORT_SYMBOL(stackleak_track_stack); 177 EXPORT_SYMBOL(stackleak_track_stack);
177 178
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