1// SPDX-License-Identifier: GPL-2.0
2#include <linux/kernel.h>
3#include <linux/syscalls.h>
4#include <linux/fdtable.h>
5#include <linux/string.h>
6#include <linux/random.h>
7#include <linux/module.h>
8#include <linux/ptrace.h>
9#include <linux/init.h>
10#include <linux/errno.h>
11#include <linux/cache.h>
12#include <linux/bug.h>
13#include <linux/err.h>
14#include <linux/kcmp.h>
15#include <linux/capability.h>
16#include <linux/list.h>
17#include <linux/eventpoll.h>
18#include <linux/file.h>
19
20#include <asm/unistd.h>
21
22/*
23 * We don't expose the real in-memory order of objects for security reasons.
24 * But still the comparison results should be suitable for sorting. So we
25 * obfuscate kernel pointers values and compare the production instead.
26 *
27 * The obfuscation is done in two steps. First we xor the kernel pointer with
28 * a random value, which puts pointer into a new position in a reordered space.
29 * Secondly we multiply the xor production with a large odd random number to
30 * permute its bits even more (the odd multiplier guarantees that the product
31 * is unique ever after the high bits are truncated, since any odd number is
32 * relative prime to 2^n).
33 *
34 * Note also that the obfuscation itself is invisible to userspace and if needed
35 * it can be changed to an alternate scheme.
36 */
37static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
38
39static long kptr_obfuscate(long v, int type)
40{
41 return (v ^ cookies[type][0]) * cookies[type][1];
42}
43
44/*
45 * 0 - equal, i.e. v1 = v2
46 * 1 - less than, i.e. v1 < v2
47 * 2 - greater than, i.e. v1 > v2
48 * 3 - not equal but ordering unavailable (reserved for future)
49 */
50static int kcmp_ptr(void *v1, void *v2, enum kcmp_type type)
51{
52 long t1, t2;
53
54 t1 = kptr_obfuscate(v: (long)v1, type);
55 t2 = kptr_obfuscate(v: (long)v2, type);
56
57 return (t1 < t2) | ((t1 > t2) << 1);
58}
59
60/* The caller must have pinned the task */
61static struct file *
62get_file_raw_ptr(struct task_struct *task, unsigned int idx)
63{
64 struct file *file;
65
66 file = fget_task(task, fd: idx);
67 if (file)
68 fput(file);
69
70 return file;
71}
72
73static void kcmp_unlock(struct rw_semaphore *l1, struct rw_semaphore *l2)
74{
75 if (likely(l2 != l1))
76 up_read(sem: l2);
77 up_read(sem: l1);
78}
79
80static int kcmp_lock(struct rw_semaphore *l1, struct rw_semaphore *l2)
81{
82 int err;
83
84 if (l2 > l1)
85 swap(l1, l2);
86
87 err = down_read_killable(sem: l1);
88 if (!err && likely(l1 != l2)) {
89 err = down_read_killable_nested(sem: l2, SINGLE_DEPTH_NESTING);
90 if (err)
91 up_read(sem: l1);
92 }
93
94 return err;
95}
96
97#ifdef CONFIG_EPOLL
98static int kcmp_epoll_target(struct task_struct *task1,
99 struct task_struct *task2,
100 unsigned long idx1,
101 struct kcmp_epoll_slot __user *uslot)
102{
103 struct file *filp, *filp_epoll, *filp_tgt;
104 struct kcmp_epoll_slot slot;
105
106 if (copy_from_user(to: &slot, from: uslot, n: sizeof(slot)))
107 return -EFAULT;
108
109 filp = get_file_raw_ptr(task: task1, idx: idx1);
110 if (!filp)
111 return -EBADF;
112
113 filp_epoll = fget_task(task: task2, fd: slot.efd);
114 if (!filp_epoll)
115 return -EBADF;
116
117 filp_tgt = get_epoll_tfile_raw_ptr(file: filp_epoll, tfd: slot.tfd, toff: slot.toff);
118 fput(filp_epoll);
119
120 if (IS_ERR(ptr: filp_tgt))
121 return PTR_ERR(ptr: filp_tgt);
122
123 return kcmp_ptr(v1: filp, v2: filp_tgt, type: KCMP_FILE);
124}
125#else
126static int kcmp_epoll_target(struct task_struct *task1,
127 struct task_struct *task2,
128 unsigned long idx1,
129 struct kcmp_epoll_slot __user *uslot)
130{
131 return -EOPNOTSUPP;
132}
133#endif
134
135SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
136 unsigned long, idx1, unsigned long, idx2)
137{
138 struct task_struct *task1, *task2;
139 int ret;
140
141 rcu_read_lock();
142
143 /*
144 * Tasks are looked up in caller's PID namespace only.
145 */
146 task1 = find_task_by_vpid(nr: pid1);
147 task2 = find_task_by_vpid(nr: pid2);
148 if (unlikely(!task1 || !task2))
149 goto err_no_task;
150
151 get_task_struct(t: task1);
152 get_task_struct(t: task2);
153
154 rcu_read_unlock();
155
156 /*
157 * One should have enough rights to inspect task details.
158 */
159 ret = kcmp_lock(l1: &task1->signal->exec_update_lock,
160 l2: &task2->signal->exec_update_lock);
161 if (ret)
162 goto err;
163 if (!ptrace_may_access(task: task1, PTRACE_MODE_READ_REALCREDS) ||
164 !ptrace_may_access(task: task2, PTRACE_MODE_READ_REALCREDS)) {
165 ret = -EPERM;
166 goto err_unlock;
167 }
168
169 switch (type) {
170 case KCMP_FILE: {
171 struct file *filp1, *filp2;
172
173 filp1 = get_file_raw_ptr(task: task1, idx: idx1);
174 filp2 = get_file_raw_ptr(task: task2, idx: idx2);
175
176 if (filp1 && filp2)
177 ret = kcmp_ptr(v1: filp1, v2: filp2, type: KCMP_FILE);
178 else
179 ret = -EBADF;
180 break;
181 }
182 case KCMP_VM:
183 ret = kcmp_ptr(v1: task1->mm, v2: task2->mm, type: KCMP_VM);
184 break;
185 case KCMP_FILES:
186 ret = kcmp_ptr(v1: task1->files, v2: task2->files, type: KCMP_FILES);
187 break;
188 case KCMP_FS:
189 ret = kcmp_ptr(v1: task1->fs, v2: task2->fs, type: KCMP_FS);
190 break;
191 case KCMP_SIGHAND:
192 ret = kcmp_ptr(v1: task1->sighand, v2: task2->sighand, type: KCMP_SIGHAND);
193 break;
194 case KCMP_IO:
195 ret = kcmp_ptr(v1: task1->io_context, v2: task2->io_context, type: KCMP_IO);
196 break;
197 case KCMP_SYSVSEM:
198#ifdef CONFIG_SYSVIPC
199 ret = kcmp_ptr(v1: task1->sysvsem.undo_list,
200 v2: task2->sysvsem.undo_list,
201 type: KCMP_SYSVSEM);
202#else
203 ret = -EOPNOTSUPP;
204#endif
205 break;
206 case KCMP_EPOLL_TFD:
207 ret = kcmp_epoll_target(task1, task2, idx1, uslot: (void *)idx2);
208 break;
209 default:
210 ret = -EINVAL;
211 break;
212 }
213
214err_unlock:
215 kcmp_unlock(l1: &task1->signal->exec_update_lock,
216 l2: &task2->signal->exec_update_lock);
217err:
218 put_task_struct(t: task1);
219 put_task_struct(t: task2);
220
221 return ret;
222
223err_no_task:
224 rcu_read_unlock();
225 return -ESRCH;
226}
227
228static __init int kcmp_cookies_init(void)
229{
230 int i;
231
232 get_random_bytes(buf: cookies, len: sizeof(cookies));
233
234 for (i = 0; i < KCMP_TYPES; i++)
235 cookies[i][1] |= (~(~0UL >> 1) | 1);
236
237 return 0;
238}
239arch_initcall(kcmp_cookies_init);
240

source code of linux/kernel/kcmp.c