1 /*
2 * Copyright (c) 2020 Apple Inc. All rights reserved.
3 *
4 * @APPLE_LICENSE_HEADER_START@
5 *
6 * This file contains Original Code and/or Modifications of Original Code
7 * as defined in and that are subject to the Apple Public Source License
8 * Version 2.0 (the 'License'). You may not use this file except in
9 * compliance with the License. Please obtain a copy of the License at
10 * http://www.opensource.apple.com/apsl/ and read it before using this
11 * file.
12 *
13 * The Original Code and all software distributed under the License are
14 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
15 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
16 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
17 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
18 * Please see the License for the specific language governing rights and
19 * limitations under the License.
20 *
21 * @APPLE_LICENSE_HEADER_END@
22 */
23
24 #include "system-version-compat-support.h"
25
26 #if SYSTEM_VERSION_COMPAT_ENABLED
27
28
29 #include <fcntl.h>
30 #include <stdbool.h>
31 #include <strings.h>
32 #include <sys/errno.h>
33 #include <sys/param.h>
34 #include <unistd.h>
35
36 #define PLAT_PREFIX_IOS "iOS"
37 #define PLAT_PREFIX_MACOS ""
38
39 #define COMPAT_SUFFIX_MACOS "Compat"
40 #define COMPAT_SUFFIX_IOS ""
41
42 #define SYSTEM_VERSION_PLIST_FILENAME "SystemVersion.plist"
43 #define SYSTEM_VERSION_PLIST_PATH ("/System/Library/CoreServices/" SYSTEM_VERSION_PLIST_FILENAME)
44
45 #define SYSTEM_VERSION_COMPAT_PLIST_FILENAME(platform_prefix, compat_suffix) (platform_prefix "SystemVersion" compat_suffix ".plist")
46
47 #define SYSTEM_VERSION_PLIST_FILENAMELEN strlen(SYSTEM_VERSION_PLIST_FILENAME)
48
49 #define SYSTEM_VERSION_COMPAT_PLIST_FILENAMELEN(platform_prefix, compat_suffix) strlen(SYSTEM_VERSION_COMPAT_PLIST_FILENAME(platform_prefix, compat_suffix))
50
51 #define SYSTEM_VERSION_PLIST_PATHLEN strlen(SYSTEM_VERSION_PLIST_PATH)
52
53 extern system_version_compat_mode_t system_version_compat_mode;
54
55 /*
56 * This routine determines whether the path specified matches the path of the SystemVersion plist file
57 * we are shimming accesses to. If the file name suffix matches, it's expected we'll call into the
58 * version_compat_open_shim() routine below which will do a full comparison on the expanded path.
59 *
60 * Parameters: orig_path The path suffix that was provided to the open{at} call.
61 *
62 * Returns: true if the path suffix matches the SystemVersion plist path we're shimming
63 * false otherwise
64 */
65 __attribute__((visibility("hidden")))
66 bool
_system_version_compat_check_path_suffix(const char * orig_path)67 _system_version_compat_check_path_suffix(const char *orig_path)
68 {
69 size_t path_str_len = strnlen(orig_path, MAXPATHLEN);
70 /*
71 * If the length of the filename we're opening is shorter than
72 * SYSTEM_VERSION_PLIST_FILENAME, bail.
73 */
74 if (path_str_len < SYSTEM_VERSION_PLIST_FILENAMELEN) {
75 return false;
76 }
77
78 /* If the path we're accessing doesn't end in SYSTEM_VERSION_PLIST_FILENAME, bail. */
79 if (strncmp(&orig_path[path_str_len - SYSTEM_VERSION_PLIST_FILENAMELEN], SYSTEM_VERSION_PLIST_FILENAME,
80 SYSTEM_VERSION_PLIST_FILENAMELEN) != 0) {
81 return false;
82 }
83
84 /* If modifying the path specified would exceed MAXPATHLEN, bail */
85 if (path_str_len == MAXPATHLEN) {
86 return false;
87 }
88
89 size_t compat_len = (system_version_compat_mode == SYSTEM_VERSION_COMPAT_MODE_IOS) ? SYSTEM_VERSION_COMPAT_PLIST_FILENAMELEN(PLAT_PREFIX_IOS, COMPAT_SUFFIX_IOS) : SYSTEM_VERSION_COMPAT_PLIST_FILENAMELEN(PLAT_PREFIX_MACOS, COMPAT_SUFFIX_MACOS);
90 if ((compat_len - SYSTEM_VERSION_PLIST_FILENAMELEN) > (MAXPATHLEN - path_str_len - 1)) {
91 return false;
92 }
93
94 /* Indicate that we should */
95 return true;
96 }
97
98 /*
99 * This routine determines whether we are trying to open the SystemVersion plist at SYSTEM_VERSION_PLIST_PATH.
100 * It's only used on targets that have the compatibility shim enabled (mainly older binaries).
101 *
102 * Note that this routine should * ABSOLUTELY NOT * be used as a general shim for accesses at all paths. We replace
103 * what the developer generally expected to be one system call with multiple additional system calls. We're ok
104 * with doing this here because we only do it for calls to open against files that match this very specific pattern
105 * (named SystemVersion.plist), but doing so for all calls to open could result in various issues. Specifically it's
106 * difficult to ensure the same cancellation semantics (around EINTR etc) that developers generally expect when replacing
107 * a single system call with multiple.
108 *
109 * This routine should return with the same semantics as the general open system calls that it is shimming - specifically
110 * it should leave errno and the return value matching what developers expect.
111 *
112 * It's expected that _version_compat_check_path_suffix() above was called prior to this call and returned true.
113 *
114 * We take the close, open and fcntl syscalls as parameters to make sure the variant we call matches the original call
115 * to open{at}.
116 *
117 * Parameters: opened_fd The file descriptor that was opened in the original open{at} call
118 * openat_fd The file descriptor passed to the original openat call (only used when use_openat is true)
119 * orig_path The original path suffix passed to open{at}
120 * oflag The original oflag passed to open{at}
121 * mode The original mode passed to open{at}
122 * close_syscall The appropriate syscall to use for closing file descriptors
123 * open_syscall The syscall that should be used for a new call to open.
124 * fctnl_syscall The appopriate syscall to use for fcntl.
125 *
126 * Returns: The original file descriptor if the open{at} access wasn't to SYSTEM_VERSION_PLIST_PATH
127 * A new file descriptor (with the original closed) if the expanded path matches SYSTEM_VERSION_PLIST_PATH
128 * The original file descriptor if the full path suffix does not match SYSTEM_VERSION_PLIST_PATH
129 * -1 (with errno set to EINTR) if the new open or fcntl calls received EINTR (with all new fds closed)
130 */
131 __attribute__((visibility("hidden")))
132 int
_system_version_compat_open_shim(int opened_fd,int openat_fd,const char * orig_path,int oflag,mode_t mode,int (* close_syscall)(int),int (* open_syscall)(const char *,int,mode_t),int (* openat_syscall)(int,const char *,int,mode_t),int (* fcntl_syscall)(int,int,long))133 _system_version_compat_open_shim(int opened_fd, int openat_fd, const char *orig_path, int oflag, mode_t mode,
134 int (*close_syscall)(int), int (*open_syscall)(const char *, int, mode_t),
135 int (*openat_syscall)(int, const char *, int, mode_t),
136 int (*fcntl_syscall)(int, int, long))
137 {
138 /* stash the errno from the original open{at} call */
139 int stashed_errno = errno;
140 char new_path[MAXPATHLEN];
141 size_t path_str_len = strnlen(orig_path, sizeof(new_path));
142
143 /* Resolve the full path of the file we've opened */
144 if (fcntl_syscall(opened_fd, F_GETPATH, new_path)) {
145 if (errno == EINTR) {
146 /* If we got EINTR, we close the file that was opened and return -1 & EINTR */
147 close_syscall(opened_fd);
148 errno = EINTR;
149 return -1;
150 } else {
151 /* otherwise we return the original file descriptor that was requested */
152 errno = stashed_errno;
153 return opened_fd;
154 }
155 }
156
157 /* Check to see whether the path matches SYSTEM_VERSION_PLIST_PATH */
158 size_t newpathlen = strnlen(new_path, MAXPATHLEN);
159 if (newpathlen != SYSTEM_VERSION_PLIST_PATHLEN) {
160 errno = stashed_errno;
161 return opened_fd;
162 }
163
164 if (strncmp(new_path, SYSTEM_VERSION_PLIST_PATH, SYSTEM_VERSION_PLIST_PATHLEN) != 0) {
165 errno = stashed_errno;
166 return opened_fd;
167 }
168
169 new_path[0] = '\0';
170
171 /*
172 * It looks like we're trying to access the SystemVersion plist. Let's try to open
173 * the compatibility plist and return that instead if it exists.
174 */
175 size_t prefix_str_len = path_str_len - SYSTEM_VERSION_PLIST_FILENAMELEN;
176 strlcpy(new_path, orig_path, (prefix_str_len + 1));
177 if (system_version_compat_mode == SYSTEM_VERSION_COMPAT_MODE_IOS) {
178 strlcat(new_path, SYSTEM_VERSION_COMPAT_PLIST_FILENAME(PLAT_PREFIX_IOS, COMPAT_SUFFIX_IOS), MAXPATHLEN);
179 } else {
180 strlcat(new_path, SYSTEM_VERSION_COMPAT_PLIST_FILENAME(PLAT_PREFIX_MACOS, COMPAT_SUFFIX_MACOS), MAXPATHLEN);
181 }
182
183 int new_fd = -1;
184 if (openat_syscall != NULL) {
185 new_fd = openat_syscall(openat_fd, new_path, oflag, mode);
186 } else {
187 new_fd = open_syscall(new_path, oflag, mode);
188 }
189 if ((new_fd == -1) && (errno == ENOENT)) {
190 /* The file doesn't exist, so return the original fd and errno. */
191 errno = stashed_errno;
192 return opened_fd;
193 }
194
195 /*
196 * Otherwise we close the first file we opened and populate errno
197 * with errno from the call to open{at}. (Note this covers the EINTR
198 * case and other failures).
199 */
200 stashed_errno = errno;
201 close_syscall(opened_fd);
202 errno = stashed_errno;
203 return new_fd;
204 }
205
206 #endif /* SYSTEM_VERSION_COMPAT_ENABLED */
207