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 pr_info("%s: dummy @ %p, prevented leak @ %p\n",
113 __func__, d, *shadow_leak);
114 kfree(*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
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.