1 /*
2  * Copyright (c) 2021 Omar Polo <op@omarpolo.com>
3  *
4  * Permission to use, copy, modify, and distribute this software for any
5  * purpose with or without fee is hereby granted, provided that the above
6  * copyright notice and this permission notice appear in all copies.
7  *
8  * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
9  * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
10  * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
11  * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
12  * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
13  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
14  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
15  */
16 
17 /*
18  * This an implementation of an OpenBSD' unveil(2) compatible API on
19  * top of Linux' landlock.
20  */
21 
22 #include <linux/landlock.h>
23 #include <linux/prctl.h>
24 
25 #include <sys/prctl.h>
26 #include <sys/stat.h>
27 #include <sys/syscall.h>
28 
29 #include <errno.h>
30 #include <fcntl.h>
31 #include <libgen.h>
32 #include <limits.h>
33 #include <string.h>
34 #include <unistd.h>
35 
36 #include "got_compat.h"
37 
38 /*
39  * What's the deal with landlock?  While distro with linux >= 5.13
40  * have the struct declarations, libc wrappers are missing.  The
41  * sample landlock code provided by the authors includes these "shims"
42  * in their example for the landlock API until libc provides them.
43  */
44 
45 #ifndef landlock_create_ruleset
46 static inline int
47 landlock_create_ruleset(const struct landlock_ruleset_attr *attr, size_t size,
48     __u32 flags)
49 {
50 	return syscall(__NR_landlock_create_ruleset, attr, size, flags);
51 }
52 #endif
53 
54 #ifndef landlock_add_rule
55 static inline int
56 landlock_add_rule(int ruleset_fd, enum landlock_rule_type type,
57     const void *attr, __u32 flags)
58 {
59 	return syscall(__NR_landlock_add_rule, ruleset_fd, type, attr, flags);
60 }
61 #endif
62 
63 #ifndef landlock_restrict_self
64 static inline int
65 landlock_restrict_self(int ruleset_fd, __u32 flags)
66 {
67 	return syscall(__NR_landlock_restrict_self, ruleset_fd, flags);
68 }
69 #endif
70 
71 static int landlock_fd = -1;
72 
73 static int
74 open_landlock(void)
75 {
76 	struct landlock_ruleset_attr rattr = {
77 		.handled_access_fs =	LANDLOCK_ACCESS_FS_EXECUTE	|
78 					LANDLOCK_ACCESS_FS_WRITE_FILE	|
79 					LANDLOCK_ACCESS_FS_READ_FILE	|
80 					LANDLOCK_ACCESS_FS_READ_DIR	|
81 					LANDLOCK_ACCESS_FS_REMOVE_DIR	|
82 					LANDLOCK_ACCESS_FS_REMOVE_FILE	|
83 					LANDLOCK_ACCESS_FS_MAKE_CHAR	|
84 					LANDLOCK_ACCESS_FS_MAKE_DIR	|
85 					LANDLOCK_ACCESS_FS_MAKE_REG	|
86 					LANDLOCK_ACCESS_FS_MAKE_SOCK	|
87 					LANDLOCK_ACCESS_FS_MAKE_FIFO	|
88 					LANDLOCK_ACCESS_FS_MAKE_BLOCK	|
89 					LANDLOCK_ACCESS_FS_MAKE_SYM,
90 	};
91 
92 	return landlock_create_ruleset(&rattr, sizeof(rattr), 0);
93 }
94 
95 static int
96 landlock_apply(void)
97 {
98 	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1)
99 		return -1;
100 
101 	if (landlock_restrict_self(landlock_fd, 0))
102 		return -1;
103 
104 	close(landlock_fd);
105 	landlock_fd = -1;
106 	return 0;
107 }
108 
109 static int
110 parse_permissions(const char *permission)
111 {
112 	int perm = 0;
113 
114 	for (; *permission; ++permission) {
115 		switch (*permission) {
116 		case 'r':
117 			perm |= LANDLOCK_ACCESS_FS_READ_FILE;
118 			perm |= LANDLOCK_ACCESS_FS_READ_DIR;
119 			break;
120 		case 'w':
121 			perm |= LANDLOCK_ACCESS_FS_WRITE_FILE;
122 			break;
123 		case 'x':
124 			perm |= LANDLOCK_ACCESS_FS_EXECUTE;
125 			break;
126 		case 'c':
127 			perm |= LANDLOCK_ACCESS_FS_REMOVE_DIR;
128 			perm |= LANDLOCK_ACCESS_FS_REMOVE_FILE;
129 			perm |= LANDLOCK_ACCESS_FS_MAKE_CHAR;
130 			perm |= LANDLOCK_ACCESS_FS_MAKE_DIR;
131 			perm |= LANDLOCK_ACCESS_FS_MAKE_REG;
132 			perm |= LANDLOCK_ACCESS_FS_MAKE_SOCK;
133 			perm |= LANDLOCK_ACCESS_FS_MAKE_FIFO;
134 			perm |= LANDLOCK_ACCESS_FS_MAKE_BLOCK;
135 			perm |= LANDLOCK_ACCESS_FS_MAKE_SYM;
136 			break;
137 		default:
138 			return -1;
139 		}
140 	}
141 
142 	return perm;
143 }
144 
145 static int
146 landlock_unveil_path(const char *path, int permissions)
147 {
148 	struct landlock_path_beneath_attr pb;
149 	struct stat sb;
150 	int fd, err, saved_errno;
151 	char fpath[PATH_MAX];
152 
153 	pb.allowed_access = permissions;
154 	if ((pb.parent_fd = open(path, O_PATH)) == -1)
155 		return -1;
156 
157 	if (fstat(pb.parent_fd, &sb) == -1)
158 		return -1;
159 
160 	if (!S_ISDIR(sb.st_mode)) {
161 		close(pb.parent_fd);
162 
163 		if (strlcpy(fpath, path, sizeof(fpath)) >= sizeof(fpath)) {
164 			errno = ENAMETOOLONG;
165 			return -1;
166 		}
167 
168 		permissions |= LANDLOCK_ACCESS_FS_READ_FILE;
169 		permissions |= LANDLOCK_ACCESS_FS_READ_DIR;
170 		return landlock_unveil_path(dirname(fpath), permissions);
171 	}
172 
173 	err = landlock_add_rule(landlock_fd, LANDLOCK_RULE_PATH_BENEATH,
174 	    &pb, 0);
175 	saved_errno = errno;
176 	close(pb.parent_fd);
177 	errno = saved_errno;
178 	return err ? -1 : 0;
179 }
180 
181 int
182 landlock_unveil(const char *path, const char *permissions)
183 {
184 	int perms;
185 
186 	if (landlock_fd == -1) {
187 		if ((landlock_fd = open_landlock()) == -1)
188 			return -1;
189 
190 		/* XXX: use rpath on the current executable */
191 		if (landlock_unveil("/lib64", "rx") == -1)
192 			return -1;
193 	}
194 
195 	if (path == NULL && permissions == NULL)
196 		return landlock_apply();
197 
198 	if (path == NULL ||
199 	    permissions == NULL ||
200 	    (perms = parse_permissions(permissions)) == -1) {
201 		errno = EINVAL;
202 		return -1;
203 	}
204 
205 	return landlock_unveil_path(path, perms);
206 }
207 
208 int
209 landlock_no_fs(void)
210 {
211 	if ((landlock_fd = open_landlock()) == -1)
212 		return -1;
213 
214 	return landlock_apply();
215 }