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
21static DEFINE_STATIC_KEY_FALSE(stack_erasing_bypass);
22
23#ifdef CONFIG_SYSCTL
24static 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}
47static 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
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_STACKLEAK_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_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 */
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 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}
177EXPORT_SYMBOL(stackleak_track_stack);
178