1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /*
3 * Copyright (C) 2017 Joe Lawrence <[email protected]>
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 */
shadow_leak_ctor(void * obj,void * shadow_data,void * ctor_data)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
livepatch_fix1_dummy_alloc(void)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 + secs_to_jiffies(EXPIRE_PERIOD);
76
77 /*
78 * Patch: save the extra memory location into a SV_LEAK shadow
79 * variable. A patched dummy_free routine can later fetch this
80 * pointer to handle resource release.
81 */
82 leak = kzalloc(sizeof(*leak), GFP_KERNEL);
83 if (!leak)
84 goto err_leak;
85
86 shadow_leak = klp_shadow_alloc(d, SV_LEAK, sizeof(leak), GFP_KERNEL,
87 shadow_leak_ctor, &leak);
88 if (!shadow_leak) {
89 pr_err("%s: failed to allocate shadow variable for the leaking pointer: dummy @ %p, leak @ %p\n",
90 __func__, d, leak);
91 goto err_shadow;
92 }
93
94 pr_info("%s: dummy @ %p, expires @ %lx\n",
95 __func__, d, d->jiffies_expire);
96
97 return d;
98
99 err_shadow:
100 kfree(leak);
101 err_leak:
102 kfree(d);
103 return NULL;
104 }
105
livepatch_fix1_dummy_leak_dtor(void * obj,void * shadow_data)106 static void livepatch_fix1_dummy_leak_dtor(void *obj, void *shadow_data)
107 {
108 void *d = obj;
109 int **shadow_leak = shadow_data;
110
111 pr_info("%s: dummy @ %p, prevented leak @ %p\n",
112 __func__, d, *shadow_leak);
113 kfree(*shadow_leak);
114 }
115
livepatch_fix1_dummy_free(struct dummy * d)116 static void livepatch_fix1_dummy_free(struct dummy *d)
117 {
118 int **shadow_leak;
119
120 /*
121 * Patch: fetch the saved SV_LEAK shadow variable, detach and
122 * free it. Note: handle cases where this shadow variable does
123 * not exist (ie, dummy structures allocated before this livepatch
124 * was loaded.)
125 */
126 shadow_leak = klp_shadow_get(d, SV_LEAK);
127 if (shadow_leak)
128 klp_shadow_free(d, SV_LEAK, livepatch_fix1_dummy_leak_dtor);
129 else
130 pr_info("%s: dummy @ %p leaked!\n", __func__, d);
131
132 kfree(d);
133 }
134
135 static struct klp_func funcs[] = {
136 {
137 .old_name = "dummy_alloc",
138 .new_func = livepatch_fix1_dummy_alloc,
139 },
140 {
141 .old_name = "dummy_free",
142 .new_func = livepatch_fix1_dummy_free,
143 }, { }
144 };
145
146 static struct klp_object objs[] = {
147 {
148 .name = "livepatch_shadow_mod",
149 .funcs = funcs,
150 }, { }
151 };
152
153 static struct klp_patch patch = {
154 .mod = THIS_MODULE,
155 .objs = objs,
156 };
157
livepatch_shadow_fix1_init(void)158 static int livepatch_shadow_fix1_init(void)
159 {
160 return klp_enable_patch(&patch);
161 }
162
livepatch_shadow_fix1_exit(void)163 static void livepatch_shadow_fix1_exit(void)
164 {
165 /* Cleanup any existing SV_LEAK shadow variables */
166 klp_shadow_free_all(SV_LEAK, livepatch_fix1_dummy_leak_dtor);
167 }
168
169 module_init(livepatch_shadow_fix1_init);
170 module_exit(livepatch_shadow_fix1_exit);
171 MODULE_LICENSE("GPL");
172 MODULE_INFO(livepatch, "Y");
173