1 /*
2  * Copyright (C) 2017 Joe Lawrence <joe.lawrence@redhat.com>
3  *
4  * This program is free software; you can redistribute it and/or
5  * modify it under the terms of the GNU General Public License
6  * as published by the Free Software Foundation; either version 2
7  * of the License, or (at your option) any later version.
8  *
9  * This program is distributed in the hope that it will be useful,
10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
12  * GNU General Public License for more details.
13  *
14  * You should have received a copy of the GNU General Public License
15  * along with this program; if not, see <http://www.gnu.org/licenses/>.
16  */
17 
18 /*
19  * livepatch-shadow-fix1.c - Shadow variables, livepatch demo
20  *
21  * Purpose
22  * -------
23  *
24  * Fixes the memory leak introduced in livepatch-shadow-mod through the
25  * use of a shadow variable.  This fix demonstrates the "extending" of
26  * short-lived data structures by patching its allocation and release
27  * functions.
28  *
29  *
30  * Usage
31  * -----
32  *
33  * This module is not intended to be standalone.  See the "Usage"
34  * section of livepatch-shadow-mod.c.
35  */
36 
37 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
38 
39 #include <linux/module.h>
40 #include <linux/kernel.h>
41 #include <linux/livepatch.h>
42 #include <linux/slab.h>
43 
44 /* Shadow variable enums */
45 #define SV_LEAK		1
46 
47 /* Allocate new dummies every second */
48 #define ALLOC_PERIOD	1
49 /* Check for expired dummies after a few new ones have been allocated */
50 #define CLEANUP_PERIOD	(3 * ALLOC_PERIOD)
51 /* Dummies expire after a few cleanup instances */
52 #define EXPIRE_PERIOD	(4 * CLEANUP_PERIOD)
53 
54 struct dummy {
55 	struct list_head list;
56 	unsigned long jiffies_expire;
57 };
58 
59 struct dummy *livepatch_fix1_dummy_alloc(void)
60 {
61 	struct dummy *d;
62 	void *leak;
63 
64 	d = kzalloc(sizeof(*d), GFP_KERNEL);
65 	if (!d)
66 		return NULL;
67 
68 	d->jiffies_expire = jiffies +
69 		msecs_to_jiffies(1000 * EXPIRE_PERIOD);
70 
71 	/*
72 	 * Patch: save the extra memory location into a SV_LEAK shadow
73 	 * variable.  A patched dummy_free routine can later fetch this
74 	 * pointer to handle resource release.
75 	 */
76 	leak = kzalloc(sizeof(int), GFP_KERNEL);
77 	klp_shadow_alloc(d, SV_LEAK, &leak, sizeof(leak), GFP_KERNEL);
78 
79 	pr_info("%s: dummy @ %p, expires @ %lx\n",
80 		__func__, d, d->jiffies_expire);
81 
82 	return d;
83 }
84 
85 void livepatch_fix1_dummy_free(struct dummy *d)
86 {
87 	void **shadow_leak, *leak;
88 
89 	/*
90 	 * Patch: fetch the saved SV_LEAK shadow variable, detach and
91 	 * free it.  Note: handle cases where this shadow variable does
92 	 * not exist (ie, dummy structures allocated before this livepatch
93 	 * was loaded.)
94 	 */
95 	shadow_leak = klp_shadow_get(d, SV_LEAK);
96 	if (shadow_leak) {
97 		leak = *shadow_leak;
98 		klp_shadow_free(d, SV_LEAK);
99 		kfree(leak);
100 		pr_info("%s: dummy @ %p, prevented leak @ %p\n",
101 			 __func__, d, leak);
102 	} else {
103 		pr_info("%s: dummy @ %p leaked!\n", __func__, d);
104 	}
105 
106 	kfree(d);
107 }
108 
109 static struct klp_func funcs[] = {
110 	{
111 		.old_name = "dummy_alloc",
112 		.new_func = livepatch_fix1_dummy_alloc,
113 	},
114 	{
115 		.old_name = "dummy_free",
116 		.new_func = livepatch_fix1_dummy_free,
117 	}, { }
118 };
119 
120 static struct klp_object objs[] = {
121 	{
122 		.name = "livepatch_shadow_mod",
123 		.funcs = funcs,
124 	}, { }
125 };
126 
127 static struct klp_patch patch = {
128 	.mod = THIS_MODULE,
129 	.objs = objs,
130 };
131 
132 static int livepatch_shadow_fix1_init(void)
133 {
134 	int ret;
135 
136 	if (!klp_have_reliable_stack() && !patch.immediate) {
137 		/*
138 		 * WARNING: Be very careful when using 'patch.immediate' in
139 		 * your patches.  It's ok to use it for simple patches like
140 		 * this, but for more complex patches which change function
141 		 * semantics, locking semantics, or data structures, it may not
142 		 * be safe.  Use of this option will also prevent removal of
143 		 * the patch.
144 		 *
145 		 * See Documentation/livepatch/livepatch.txt for more details.
146 		 */
147 		patch.immediate = true;
148 		pr_notice("The consistency model isn't supported for your architecture.  Bypassing safety mechanisms and applying the patch immediately.\n");
149 	}
150 
151 	ret = klp_register_patch(&patch);
152 	if (ret)
153 		return ret;
154 	ret = klp_enable_patch(&patch);
155 	if (ret) {
156 		WARN_ON(klp_unregister_patch(&patch));
157 		return ret;
158 	}
159 	return 0;
160 }
161 
162 static void livepatch_shadow_fix1_exit(void)
163 {
164 	/* Cleanup any existing SV_LEAK shadow variables */
165 	klp_shadow_free_all(SV_LEAK);
166 
167 	WARN_ON(klp_unregister_patch(&patch));
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