Root/kernel/kcmp.c

1#include <linux/kernel.h>
2#include <linux/syscalls.h>
3#include <linux/fdtable.h>
4#include <linux/string.h>
5#include <linux/random.h>
6#include <linux/module.h>
7#include <linux/ptrace.h>
8#include <linux/init.h>
9#include <linux/errno.h>
10#include <linux/cache.h>
11#include <linux/bug.h>
12#include <linux/err.h>
13#include <linux/kcmp.h>
14
15#include <asm/unistd.h>
16
17/*
18 * We don't expose the real in-memory order of objects for security reasons.
19 * But still the comparison results should be suitable for sorting. So we
20 * obfuscate kernel pointers values and compare the production instead.
21 *
22 * The obfuscation is done in two steps. First we xor the kernel pointer with
23 * a random value, which puts pointer into a new position in a reordered space.
24 * Secondly we multiply the xor production with a large odd random number to
25 * permute its bits even more (the odd multiplier guarantees that the product
26 * is unique ever after the high bits are truncated, since any odd number is
27 * relative prime to 2^n).
28 *
29 * Note also that the obfuscation itself is invisible to userspace and if needed
30 * it can be changed to an alternate scheme.
31 */
32static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
33
34static long kptr_obfuscate(long v, int type)
35{
36    return (v ^ cookies[type][0]) * cookies[type][1];
37}
38
39/*
40 * 0 - equal, i.e. v1 = v2
41 * 1 - less than, i.e. v1 < v2
42 * 2 - greater than, i.e. v1 > v2
43 * 3 - not equal but ordering unavailable (reserved for future)
44 */
45static int kcmp_ptr(void *v1, void *v2, enum kcmp_type type)
46{
47    long ret;
48
49    ret = kptr_obfuscate((long)v1, type) - kptr_obfuscate((long)v2, type);
50
51    return (ret < 0) | ((ret > 0) << 1);
52}
53
54/* The caller must have pinned the task */
55static struct file *
56get_file_raw_ptr(struct task_struct *task, unsigned int idx)
57{
58    struct file *file = NULL;
59
60    task_lock(task);
61    rcu_read_lock();
62
63    if (task->files)
64        file = fcheck_files(task->files, idx);
65
66    rcu_read_unlock();
67    task_unlock(task);
68
69    return file;
70}
71
72static void kcmp_unlock(struct mutex *m1, struct mutex *m2)
73{
74    if (likely(m2 != m1))
75        mutex_unlock(m2);
76    mutex_unlock(m1);
77}
78
79static int kcmp_lock(struct mutex *m1, struct mutex *m2)
80{
81    int err;
82
83    if (m2 > m1)
84        swap(m1, m2);
85
86    err = mutex_lock_killable(m1);
87    if (!err && likely(m1 != m2)) {
88        err = mutex_lock_killable_nested(m2, SINGLE_DEPTH_NESTING);
89        if (err)
90            mutex_unlock(m1);
91    }
92
93    return err;
94}
95
96SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
97        unsigned long, idx1, unsigned long, idx2)
98{
99    struct task_struct *task1, *task2;
100    int ret;
101
102    rcu_read_lock();
103
104    /*
105     * Tasks are looked up in caller's PID namespace only.
106     */
107    task1 = find_task_by_vpid(pid1);
108    task2 = find_task_by_vpid(pid2);
109    if (!task1 || !task2)
110        goto err_no_task;
111
112    get_task_struct(task1);
113    get_task_struct(task2);
114
115    rcu_read_unlock();
116
117    /*
118     * One should have enough rights to inspect task details.
119     */
120    ret = kcmp_lock(&task1->signal->cred_guard_mutex,
121            &task2->signal->cred_guard_mutex);
122    if (ret)
123        goto err;
124    if (!ptrace_may_access(task1, PTRACE_MODE_READ) ||
125        !ptrace_may_access(task2, PTRACE_MODE_READ)) {
126        ret = -EPERM;
127        goto err_unlock;
128    }
129
130    switch (type) {
131    case KCMP_FILE: {
132        struct file *filp1, *filp2;
133
134        filp1 = get_file_raw_ptr(task1, idx1);
135        filp2 = get_file_raw_ptr(task2, idx2);
136
137        if (filp1 && filp2)
138            ret = kcmp_ptr(filp1, filp2, KCMP_FILE);
139        else
140            ret = -EBADF;
141        break;
142    }
143    case KCMP_VM:
144        ret = kcmp_ptr(task1->mm, task2->mm, KCMP_VM);
145        break;
146    case KCMP_FILES:
147        ret = kcmp_ptr(task1->files, task2->files, KCMP_FILES);
148        break;
149    case KCMP_FS:
150        ret = kcmp_ptr(task1->fs, task2->fs, KCMP_FS);
151        break;
152    case KCMP_SIGHAND:
153        ret = kcmp_ptr(task1->sighand, task2->sighand, KCMP_SIGHAND);
154        break;
155    case KCMP_IO:
156        ret = kcmp_ptr(task1->io_context, task2->io_context, KCMP_IO);
157        break;
158    case KCMP_SYSVSEM:
159#ifdef CONFIG_SYSVIPC
160        ret = kcmp_ptr(task1->sysvsem.undo_list,
161                   task2->sysvsem.undo_list,
162                   KCMP_SYSVSEM);
163#else
164        ret = -EOPNOTSUPP;
165#endif
166        break;
167    default:
168        ret = -EINVAL;
169        break;
170    }
171
172err_unlock:
173    kcmp_unlock(&task1->signal->cred_guard_mutex,
174            &task2->signal->cred_guard_mutex);
175err:
176    put_task_struct(task1);
177    put_task_struct(task2);
178
179    return ret;
180
181err_no_task:
182    rcu_read_unlock();
183    return -ESRCH;
184}
185
186static __init int kcmp_cookies_init(void)
187{
188    int i;
189
190    get_random_bytes(cookies, sizeof(cookies));
191
192    for (i = 0; i < KCMP_TYPES; i++)
193        cookies[i][1] |= (~(~0UL >> 1) | 1);
194
195    return 0;
196}
197arch_initcall(kcmp_cookies_init);
198

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