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
20 static DEFINE_STATIC_KEY_FALSE(stack_erasing_bypass);
21
stack_erasing_sysctl(struct ctl_table * table,int write,void __user * buffer,size_t * lenp,loff_t * ppos)22 int stack_erasing_sysctl(struct ctl_table *table, int write,
23 void __user *buffer, size_t *lenp, loff_t *ppos)
24 {
25 int ret = 0;
26 int state = !static_branch_unlikely(&stack_erasing_bypass);
27 int prev_state = state;
28
29 table->data = &state;
30 table->maxlen = sizeof(int);
31 ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
32 state = !!state;
33 if (ret || !write || state == prev_state)
34 return ret;
35
36 if (state)
37 static_branch_disable(&stack_erasing_bypass);
38 else
39 static_branch_enable(&stack_erasing_bypass);
40
41 pr_warn("stackleak: kernel stack erasing is %s\n",
42 state ? "enabled" : "disabled");
43 return ret;
44 }
45
46 #define skip_erasing() static_branch_unlikely(&stack_erasing_bypass)
47 #else
48 #define skip_erasing() false
49 #endif /* CONFIG_STACKLEAK_RUNTIME_DISABLE */
50
stackleak_erase(void)51 asmlinkage void notrace stackleak_erase(void)
52 {
53 /* It would be nice not to have 'kstack_ptr' and 'boundary' on stack */
54 unsigned long kstack_ptr = current->lowest_stack;
55 unsigned long boundary = (unsigned long)end_of_stack(current);
56 unsigned int poison_count = 0;
57 const unsigned int depth = STACKLEAK_SEARCH_DEPTH / sizeof(unsigned long);
58
59 if (skip_erasing())
60 return;
61
62 /* Check that 'lowest_stack' value is sane */
63 if (unlikely(kstack_ptr - boundary >= THREAD_SIZE))
64 kstack_ptr = boundary;
65
66 /* Search for the poison value in the kernel stack */
67 while (kstack_ptr > boundary && poison_count <= depth) {
68 if (*(unsigned long *)kstack_ptr == STACKLEAK_POISON)
69 poison_count++;
70 else
71 poison_count = 0;
72
73 kstack_ptr -= sizeof(unsigned long);
74 }
75
76 /*
77 * One 'long int' at the bottom of the thread stack is reserved and
78 * should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK=y).
79 */
80 if (kstack_ptr == boundary)
81 kstack_ptr += sizeof(unsigned long);
82
83 #ifdef CONFIG_STACKLEAK_METRICS
84 current->prev_lowest_stack = kstack_ptr;
85 #endif
86
87 /*
88 * Now write the poison value to the kernel stack. Start from
89 * 'kstack_ptr' and move up till the new 'boundary'. We assume that
90 * the stack pointer doesn't change when we write poison.
91 */
92 if (on_thread_stack())
93 boundary = current_stack_pointer;
94 else
95 boundary = current_top_of_stack();
96
97 while (kstack_ptr < boundary) {
98 *(unsigned long *)kstack_ptr = STACKLEAK_POISON;
99 kstack_ptr += sizeof(unsigned long);
100 }
101
102 /* Reset the 'lowest_stack' value for the next syscall */
103 current->lowest_stack = current_top_of_stack() - THREAD_SIZE/64;
104 }
105 NOKPROBE_SYMBOL(stackleak_erase);
106
stackleak_track_stack(void)107 void __used notrace stackleak_track_stack(void)
108 {
109 /*
110 * N.B. stackleak_erase() fills the kernel stack with the poison value,
111 * which has the register width. That code assumes that the value
112 * of 'lowest_stack' is aligned on the register width boundary.
113 *
114 * That is true for x86 and x86_64 because of the kernel stack
115 * alignment on these platforms (for details, see 'cc_stack_align' in
116 * arch/x86/Makefile). Take care of that when you port STACKLEAK to
117 * new platforms.
118 */
119 unsigned long sp = (unsigned long)&sp;
120
121 /*
122 * Having CONFIG_STACKLEAK_TRACK_MIN_SIZE larger than
123 * STACKLEAK_SEARCH_DEPTH makes the poison search in
124 * stackleak_erase() unreliable. Let's prevent that.
125 */
126 BUILD_BUG_ON(CONFIG_STACKLEAK_TRACK_MIN_SIZE > STACKLEAK_SEARCH_DEPTH);
127
128 if (sp < current->lowest_stack &&
129 sp >= (unsigned long)task_stack_page(current) +
130 sizeof(unsigned long)) {
131 current->lowest_stack = sp;
132 }
133 }
134 EXPORT_SYMBOL(stackleak_track_stack);
135