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 	void **shadow_leak = shadow_data;
56 	void *leak = ctor_data;
57 
58 	*shadow_leak = leak;
59 	return 0;
60 }
61 
62 static struct dummy *livepatch_fix1_dummy_alloc(void)
63 {
64 	struct dummy *d;
65 	void *leak;
66 
67 	d = kzalloc(sizeof(*d), GFP_KERNEL);
68 	if (!d)
69 		return NULL;
70 
71 	d->jiffies_expire = jiffies +
72 		msecs_to_jiffies(1000 * EXPIRE_PERIOD);
73 
74 	/*
75 	 * Patch: save the extra memory location into a SV_LEAK shadow
76 	 * variable.  A patched dummy_free routine can later fetch this
77 	 * pointer to handle resource release.
78 	 */
79 	leak = kzalloc(sizeof(int), GFP_KERNEL);
80 	if (!leak) {
81 		kfree(d);
82 		return NULL;
83 	}
84 
85 	klp_shadow_alloc(d, SV_LEAK, sizeof(leak), GFP_KERNEL,
86 			 shadow_leak_ctor, leak);
87 
88 	pr_info("%s: dummy @ %p, expires @ %lx\n",
89 		__func__, d, d->jiffies_expire);
90 
91 	return d;
92 }
93 
94 static void livepatch_fix1_dummy_leak_dtor(void *obj, void *shadow_data)
95 {
96 	void *d = obj;
97 	void **shadow_leak = shadow_data;
98 
99 	kfree(*shadow_leak);
100 	pr_info("%s: dummy @ %p, prevented leak @ %p\n",
101 			 __func__, d, *shadow_leak);
102 }
103 
104 static void livepatch_fix1_dummy_free(struct dummy *d)
105 {
106 	void **shadow_leak;
107 
108 	/*
109 	 * Patch: fetch the saved SV_LEAK shadow variable, detach and
110 	 * free it.  Note: handle cases where this shadow variable does
111 	 * not exist (ie, dummy structures allocated before this livepatch
112 	 * was loaded.)
113 	 */
114 	shadow_leak = klp_shadow_get(d, SV_LEAK);
115 	if (shadow_leak)
116 		klp_shadow_free(d, SV_LEAK, livepatch_fix1_dummy_leak_dtor);
117 	else
118 		pr_info("%s: dummy @ %p leaked!\n", __func__, d);
119 
120 	kfree(d);
121 }
122 
123 static struct klp_func funcs[] = {
124 	{
125 		.old_name = "dummy_alloc",
126 		.new_func = livepatch_fix1_dummy_alloc,
127 	},
128 	{
129 		.old_name = "dummy_free",
130 		.new_func = livepatch_fix1_dummy_free,
131 	}, { }
132 };
133 
134 static struct klp_object objs[] = {
135 	{
136 		.name = "livepatch_shadow_mod",
137 		.funcs = funcs,
138 	}, { }
139 };
140 
141 static struct klp_patch patch = {
142 	.mod = THIS_MODULE,
143 	.objs = objs,
144 };
145 
146 static int livepatch_shadow_fix1_init(void)
147 {
148 	return klp_enable_patch(&patch);
149 }
150 
151 static void livepatch_shadow_fix1_exit(void)
152 {
153 	/* Cleanup any existing SV_LEAK shadow variables */
154 	klp_shadow_free_all(SV_LEAK, livepatch_fix1_dummy_leak_dtor);
155 }
156 
157 module_init(livepatch_shadow_fix1_init);
158 module_exit(livepatch_shadow_fix1_exit);
159 MODULE_LICENSE("GPL");
160 MODULE_INFO(livepatch, "Y");
161