xref: /openbmc/linux/kernel/kcmp.c (revision 09c434b8)
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  */
37 static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
38 
39 static 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  */
50 static int kcmp_ptr(void *v1, void *v2, enum kcmp_type type)
51 {
52 	long t1, t2;
53 
54 	t1 = kptr_obfuscate((long)v1, type);
55 	t2 = kptr_obfuscate((long)v2, type);
56 
57 	return (t1 < t2) | ((t1 > t2) << 1);
58 }
59 
60 /* The caller must have pinned the task */
61 static struct file *
62 get_file_raw_ptr(struct task_struct *task, unsigned int idx)
63 {
64 	struct file *file = NULL;
65 
66 	task_lock(task);
67 	rcu_read_lock();
68 
69 	if (task->files)
70 		file = fcheck_files(task->files, idx);
71 
72 	rcu_read_unlock();
73 	task_unlock(task);
74 
75 	return file;
76 }
77 
78 static void kcmp_unlock(struct mutex *m1, struct mutex *m2)
79 {
80 	if (likely(m2 != m1))
81 		mutex_unlock(m2);
82 	mutex_unlock(m1);
83 }
84 
85 static int kcmp_lock(struct mutex *m1, struct mutex *m2)
86 {
87 	int err;
88 
89 	if (m2 > m1)
90 		swap(m1, m2);
91 
92 	err = mutex_lock_killable(m1);
93 	if (!err && likely(m1 != m2)) {
94 		err = mutex_lock_killable_nested(m2, SINGLE_DEPTH_NESTING);
95 		if (err)
96 			mutex_unlock(m1);
97 	}
98 
99 	return err;
100 }
101 
102 #ifdef CONFIG_EPOLL
103 static int kcmp_epoll_target(struct task_struct *task1,
104 			     struct task_struct *task2,
105 			     unsigned long idx1,
106 			     struct kcmp_epoll_slot __user *uslot)
107 {
108 	struct file *filp, *filp_epoll, *filp_tgt;
109 	struct kcmp_epoll_slot slot;
110 	struct files_struct *files;
111 
112 	if (copy_from_user(&slot, uslot, sizeof(slot)))
113 		return -EFAULT;
114 
115 	filp = get_file_raw_ptr(task1, idx1);
116 	if (!filp)
117 		return -EBADF;
118 
119 	files = get_files_struct(task2);
120 	if (!files)
121 		return -EBADF;
122 
123 	spin_lock(&files->file_lock);
124 	filp_epoll = fcheck_files(files, slot.efd);
125 	if (filp_epoll)
126 		get_file(filp_epoll);
127 	else
128 		filp_tgt = ERR_PTR(-EBADF);
129 	spin_unlock(&files->file_lock);
130 	put_files_struct(files);
131 
132 	if (filp_epoll) {
133 		filp_tgt = get_epoll_tfile_raw_ptr(filp_epoll, slot.tfd, slot.toff);
134 		fput(filp_epoll);
135 	}
136 
137 	if (IS_ERR(filp_tgt))
138 		return PTR_ERR(filp_tgt);
139 
140 	return kcmp_ptr(filp, filp_tgt, KCMP_FILE);
141 }
142 #else
143 static int kcmp_epoll_target(struct task_struct *task1,
144 			     struct task_struct *task2,
145 			     unsigned long idx1,
146 			     struct kcmp_epoll_slot __user *uslot)
147 {
148 	return -EOPNOTSUPP;
149 }
150 #endif
151 
152 SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
153 		unsigned long, idx1, unsigned long, idx2)
154 {
155 	struct task_struct *task1, *task2;
156 	int ret;
157 
158 	rcu_read_lock();
159 
160 	/*
161 	 * Tasks are looked up in caller's PID namespace only.
162 	 */
163 	task1 = find_task_by_vpid(pid1);
164 	task2 = find_task_by_vpid(pid2);
165 	if (!task1 || !task2)
166 		goto err_no_task;
167 
168 	get_task_struct(task1);
169 	get_task_struct(task2);
170 
171 	rcu_read_unlock();
172 
173 	/*
174 	 * One should have enough rights to inspect task details.
175 	 */
176 	ret = kcmp_lock(&task1->signal->cred_guard_mutex,
177 			&task2->signal->cred_guard_mutex);
178 	if (ret)
179 		goto err;
180 	if (!ptrace_may_access(task1, PTRACE_MODE_READ_REALCREDS) ||
181 	    !ptrace_may_access(task2, PTRACE_MODE_READ_REALCREDS)) {
182 		ret = -EPERM;
183 		goto err_unlock;
184 	}
185 
186 	switch (type) {
187 	case KCMP_FILE: {
188 		struct file *filp1, *filp2;
189 
190 		filp1 = get_file_raw_ptr(task1, idx1);
191 		filp2 = get_file_raw_ptr(task2, idx2);
192 
193 		if (filp1 && filp2)
194 			ret = kcmp_ptr(filp1, filp2, KCMP_FILE);
195 		else
196 			ret = -EBADF;
197 		break;
198 	}
199 	case KCMP_VM:
200 		ret = kcmp_ptr(task1->mm, task2->mm, KCMP_VM);
201 		break;
202 	case KCMP_FILES:
203 		ret = kcmp_ptr(task1->files, task2->files, KCMP_FILES);
204 		break;
205 	case KCMP_FS:
206 		ret = kcmp_ptr(task1->fs, task2->fs, KCMP_FS);
207 		break;
208 	case KCMP_SIGHAND:
209 		ret = kcmp_ptr(task1->sighand, task2->sighand, KCMP_SIGHAND);
210 		break;
211 	case KCMP_IO:
212 		ret = kcmp_ptr(task1->io_context, task2->io_context, KCMP_IO);
213 		break;
214 	case KCMP_SYSVSEM:
215 #ifdef CONFIG_SYSVIPC
216 		ret = kcmp_ptr(task1->sysvsem.undo_list,
217 			       task2->sysvsem.undo_list,
218 			       KCMP_SYSVSEM);
219 #else
220 		ret = -EOPNOTSUPP;
221 #endif
222 		break;
223 	case KCMP_EPOLL_TFD:
224 		ret = kcmp_epoll_target(task1, task2, idx1, (void *)idx2);
225 		break;
226 	default:
227 		ret = -EINVAL;
228 		break;
229 	}
230 
231 err_unlock:
232 	kcmp_unlock(&task1->signal->cred_guard_mutex,
233 		    &task2->signal->cred_guard_mutex);
234 err:
235 	put_task_struct(task1);
236 	put_task_struct(task2);
237 
238 	return ret;
239 
240 err_no_task:
241 	rcu_read_unlock();
242 	return -ESRCH;
243 }
244 
245 static __init int kcmp_cookies_init(void)
246 {
247 	int i;
248 
249 	get_random_bytes(cookies, sizeof(cookies));
250 
251 	for (i = 0; i < KCMP_TYPES; i++)
252 		cookies[i][1] |= (~(~0UL >>  1) | 1);
253 
254 	return 0;
255 }
256 arch_initcall(kcmp_cookies_init);
257