livepatch-shadow-fix1.c

  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
 42struct 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 */
 53static 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
 65static 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
 99err_shadow:
100	kfree(leak);
101err_leak:
102	kfree(d);
103	return NULL;
104}
105
106static 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
116static 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
135static 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
146static struct klp_object objs[] = {
147	{
148		.name = "livepatch_shadow_mod",
149		.funcs = funcs,
150	}, { }
151};
152
153static struct klp_patch patch = {
154	.mod = THIS_MODULE,
155	.objs = objs,
156};
157
158static int livepatch_shadow_fix1_init(void)
159{
160	return klp_enable_patch(&patch);
161}
162
163static 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
169module_init(livepatch_shadow_fix1_init);
170module_exit(livepatch_shadow_fix1_exit);
171MODULE_DESCRIPTION("Live patching demo for shadow variables");
172MODULE_LICENSE("GPL");
173MODULE_INFO(livepatch, "Y");