1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /*
3  * Copyright (C) 2017 Joe Lawrence <joe.lawrence@redhat.com>
4  */
5 
6 /*
7  * livepatch-shadow-fix1.c - Shadow variables, livepatch demo
8  *
9  * Purpose
10  * -------
11  *
12  * Fixes the memory leak introduced in livepatch-shadow-mod through the
13  * use of a shadow variable.  This fix demonstrates the "extending" of
14  * short-lived data structures by patching its allocation and release
15  * functions.
16  *
17  *
18  * Usage
19  * -----
20  *
21  * This module is not intended to be standalone.  See the "Usage"
22  * section of livepatch-shadow-mod.c.
23  */
24 
25 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
26 
27 #include <linux/module.h>
28 #include <linux/kernel.h>
29 #include <linux/livepatch.h>
30 #include <linux/slab.h>
31 
32 /* Shadow variable enums */
33 #define SV_LEAK		1
34 
35 /* Allocate new dummies every second */
36 #define ALLOC_PERIOD	1
37 /* Check for expired dummies after a few new ones have been allocated */
38 #define CLEANUP_PERIOD	(3 * ALLOC_PERIOD)
39 /* Dummies expire after a few cleanup instances */
40 #define EXPIRE_PERIOD	(4 * CLEANUP_PERIOD)
41 
42 struct dummy {
43 	struct list_head list;
44 	unsigned long jiffies_expire;
45 };
46 
47 /*
48  * The constructor makes more sense together with klp_shadow_get_or_alloc().
49  * In this example, it would be safe to assign the pointer also to the shadow
50  * variable returned by klp_shadow_alloc().  But we wanted to show the more
51  * complicated use of the API.
52  */
53 static int shadow_leak_ctor(void *obj, void *shadow_data, void *ctor_data)
54 {
55 	int **shadow_leak = shadow_data;
56 	int **leak = ctor_data;
57 
58 	if (!ctor_data)
59 		return -EINVAL;
60 
61 	*shadow_leak = *leak;
62 	return 0;
63 }
64 
65 static struct dummy *livepatch_fix1_dummy_alloc(void)
66 {
67 	struct dummy *d;
68 	int *leak;
69 	int **shadow_leak;
70 
71 	d = kzalloc(sizeof(*d), GFP_KERNEL);
72 	if (!d)
73 		return NULL;
74 
75 	d->jiffies_expire = jiffies +
76 		msecs_to_jiffies(1000 * EXPIRE_PERIOD);
77 
78 	/*
79 	 * Patch: save the extra memory location into a SV_LEAK shadow
80 	 * variable.  A patched dummy_free routine can later fetch this
81 	 * pointer to handle resource release.
82 	 */
83 	leak = kzalloc(sizeof(*leak), GFP_KERNEL);
84 	if (!leak)
85 		goto err_leak;
86 
87 	shadow_leak = klp_shadow_alloc(d, SV_LEAK, sizeof(leak), GFP_KERNEL,
88 				       shadow_leak_ctor, &leak);
89 	if (!shadow_leak) {
90 		pr_err("%s: failed to allocate shadow variable for the leaking pointer: dummy @ %p, leak @ %p\n",
91 		       __func__, d, leak);
92 		goto err_shadow;
93 	}
94 
95 	pr_info("%s: dummy @ %p, expires @ %lx\n",
96 		__func__, d, d->jiffies_expire);
97 
98 	return d;
99 
100 err_shadow:
101 	kfree(leak);
102 err_leak:
103 	kfree(d);
104 	return NULL;
105 }
106 
107 static void livepatch_fix1_dummy_leak_dtor(void *obj, void *shadow_data)
108 {
109 	void *d = obj;
110 	int **shadow_leak = shadow_data;
111 
112 	kfree(*shadow_leak);
113 	pr_info("%s: dummy @ %p, prevented leak @ %p\n",
114 			 __func__, d, *shadow_leak);
115 }
116 
117 static void livepatch_fix1_dummy_free(struct dummy *d)
118 {
119 	int **shadow_leak;
120 
121 	/*
122 	 * Patch: fetch the saved SV_LEAK shadow variable, detach and
123 	 * free it.  Note: handle cases where this shadow variable does
124 	 * not exist (ie, dummy structures allocated before this livepatch
125 	 * was loaded.)
126 	 */
127 	shadow_leak = klp_shadow_get(d, SV_LEAK);
128 	if (shadow_leak)
129 		klp_shadow_free(d, SV_LEAK, livepatch_fix1_dummy_leak_dtor);
130 	else
131 		pr_info("%s: dummy @ %p leaked!\n", __func__, d);
132 
133 	kfree(d);
134 }
135 
136 static struct klp_func funcs[] = {
137 	{
138 		.old_name = "dummy_alloc",
139 		.new_func = livepatch_fix1_dummy_alloc,
140 	},
141 	{
142 		.old_name = "dummy_free",
143 		.new_func = livepatch_fix1_dummy_free,
144 	}, { }
145 };
146 
147 static struct klp_object objs[] = {
148 	{
149 		.name = "livepatch_shadow_mod",
150 		.funcs = funcs,
151 	}, { }
152 };
153 
154 static struct klp_patch patch = {
155 	.mod = THIS_MODULE,
156 	.objs = objs,
157 };
158 
159 static int livepatch_shadow_fix1_init(void)
160 {
161 	return klp_enable_patch(&patch);
162 }
163 
164 static void livepatch_shadow_fix1_exit(void)
165 {
166 	/* Cleanup any existing SV_LEAK shadow variables */
167 	klp_shadow_free_all(SV_LEAK, livepatch_fix1_dummy_leak_dtor);
168 }
169 
170 module_init(livepatch_shadow_fix1_init);
171 module_exit(livepatch_shadow_fix1_exit);
172 MODULE_LICENSE("GPL");
173 MODULE_INFO(livepatch, "Y");
174