1 /*
2 * memfd_create system call and file sealing support
3 *
4 * Code was originally included in shmem.c, and broken out to facilitate
5 * use by hugetlbfs as well as tmpfs.
6 *
7 * This file is released under the GPL.
8 */
9
10 #include <linux/fs.h>
11 #include <linux/vfs.h>
12 #include <linux/pagemap.h>
13 #include <linux/file.h>
14 #include <linux/mm.h>
15 #include <linux/sched/signal.h>
16 #include <linux/khugepaged.h>
17 #include <linux/syscalls.h>
18 #include <linux/hugetlb.h>
19 #include <linux/shmem_fs.h>
20 #include <linux/memfd.h>
21 #include <uapi/linux/memfd.h>
22
23 /*
24 * We need a tag: a new tag would expand every radix_tree_node by 8 bytes,
25 * so reuse a tag which we firmly believe is never set or cleared on tmpfs
26 * or hugetlbfs because they are memory only filesystems.
27 */
28 #define MEMFD_TAG_PINNED PAGECACHE_TAG_TOWRITE
29 #define LAST_SCAN 4 /* about 150ms max */
30
memfd_tag_pins(struct address_space * mapping)31 static void memfd_tag_pins(struct address_space *mapping)
32 {
33 struct radix_tree_iter iter;
34 void __rcu **slot;
35 pgoff_t start;
36 struct page *page;
37
38 lru_add_drain();
39 start = 0;
40 rcu_read_lock();
41
42 radix_tree_for_each_slot(slot, &mapping->i_pages, &iter, start) {
43 page = radix_tree_deref_slot(slot);
44 if (!page || radix_tree_exception(page)) {
45 if (radix_tree_deref_retry(page)) {
46 slot = radix_tree_iter_retry(&iter);
47 continue;
48 }
49 } else if (page_count(page) - page_mapcount(page) > 1) {
50 xa_lock_irq(&mapping->i_pages);
51 radix_tree_tag_set(&mapping->i_pages, iter.index,
52 MEMFD_TAG_PINNED);
53 xa_unlock_irq(&mapping->i_pages);
54 }
55
56 if (need_resched()) {
57 slot = radix_tree_iter_resume(slot, &iter);
58 cond_resched_rcu();
59 }
60 }
61 rcu_read_unlock();
62 }
63
64 /*
65 * Setting SEAL_WRITE requires us to verify there's no pending writer. However,
66 * via get_user_pages(), drivers might have some pending I/O without any active
67 * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages
68 * and see whether it has an elevated ref-count. If so, we tag them and wait for
69 * them to be dropped.
70 * The caller must guarantee that no new user will acquire writable references
71 * to those pages to avoid races.
72 */
memfd_wait_for_pins(struct address_space * mapping)73 static int memfd_wait_for_pins(struct address_space *mapping)
74 {
75 struct radix_tree_iter iter;
76 void __rcu **slot;
77 pgoff_t start;
78 struct page *page;
79 int error, scan;
80
81 memfd_tag_pins(mapping);
82
83 error = 0;
84 for (scan = 0; scan <= LAST_SCAN; scan++) {
85 if (!radix_tree_tagged(&mapping->i_pages, MEMFD_TAG_PINNED))
86 break;
87
88 if (!scan)
89 lru_add_drain_all();
90 else if (schedule_timeout_killable((HZ << scan) / 200))
91 scan = LAST_SCAN;
92
93 start = 0;
94 rcu_read_lock();
95 radix_tree_for_each_tagged(slot, &mapping->i_pages, &iter,
96 start, MEMFD_TAG_PINNED) {
97
98 page = radix_tree_deref_slot(slot);
99 if (radix_tree_exception(page)) {
100 if (radix_tree_deref_retry(page)) {
101 slot = radix_tree_iter_retry(&iter);
102 continue;
103 }
104
105 page = NULL;
106 }
107
108 if (page &&
109 page_count(page) - page_mapcount(page) != 1) {
110 if (scan < LAST_SCAN)
111 goto continue_resched;
112
113 /*
114 * On the last scan, we clean up all those tags
115 * we inserted; but make a note that we still
116 * found pages pinned.
117 */
118 error = -EBUSY;
119 }
120
121 xa_lock_irq(&mapping->i_pages);
122 radix_tree_tag_clear(&mapping->i_pages,
123 iter.index, MEMFD_TAG_PINNED);
124 xa_unlock_irq(&mapping->i_pages);
125 continue_resched:
126 if (need_resched()) {
127 slot = radix_tree_iter_resume(slot, &iter);
128 cond_resched_rcu();
129 }
130 }
131 rcu_read_unlock();
132 }
133
134 return error;
135 }
136
memfd_file_seals_ptr(struct file * file)137 static unsigned int *memfd_file_seals_ptr(struct file *file)
138 {
139 if (shmem_file(file))
140 return &SHMEM_I(file_inode(file))->seals;
141
142 #ifdef CONFIG_HUGETLBFS
143 if (is_file_hugepages(file))
144 return &HUGETLBFS_I(file_inode(file))->seals;
145 #endif
146
147 return NULL;
148 }
149
150 #define F_ALL_SEALS (F_SEAL_SEAL | \
151 F_SEAL_SHRINK | \
152 F_SEAL_GROW | \
153 F_SEAL_WRITE)
154
memfd_add_seals(struct file * file,unsigned int seals)155 static int memfd_add_seals(struct file *file, unsigned int seals)
156 {
157 struct inode *inode = file_inode(file);
158 unsigned int *file_seals;
159 int error;
160
161 /*
162 * SEALING
163 * Sealing allows multiple parties to share a tmpfs or hugetlbfs file
164 * but restrict access to a specific subset of file operations. Seals
165 * can only be added, but never removed. This way, mutually untrusted
166 * parties can share common memory regions with a well-defined policy.
167 * A malicious peer can thus never perform unwanted operations on a
168 * shared object.
169 *
170 * Seals are only supported on special tmpfs or hugetlbfs files and
171 * always affect the whole underlying inode. Once a seal is set, it
172 * may prevent some kinds of access to the file. Currently, the
173 * following seals are defined:
174 * SEAL_SEAL: Prevent further seals from being set on this file
175 * SEAL_SHRINK: Prevent the file from shrinking
176 * SEAL_GROW: Prevent the file from growing
177 * SEAL_WRITE: Prevent write access to the file
178 *
179 * As we don't require any trust relationship between two parties, we
180 * must prevent seals from being removed. Therefore, sealing a file
181 * only adds a given set of seals to the file, it never touches
182 * existing seals. Furthermore, the "setting seals"-operation can be
183 * sealed itself, which basically prevents any further seal from being
184 * added.
185 *
186 * Semantics of sealing are only defined on volatile files. Only
187 * anonymous tmpfs and hugetlbfs files support sealing. More
188 * importantly, seals are never written to disk. Therefore, there's
189 * no plan to support it on other file types.
190 */
191
192 if (!(file->f_mode & FMODE_WRITE))
193 return -EPERM;
194 if (seals & ~(unsigned int)F_ALL_SEALS)
195 return -EINVAL;
196
197 inode_lock(inode);
198
199 file_seals = memfd_file_seals_ptr(file);
200 if (!file_seals) {
201 error = -EINVAL;
202 goto unlock;
203 }
204
205 if (*file_seals & F_SEAL_SEAL) {
206 error = -EPERM;
207 goto unlock;
208 }
209
210 if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
211 error = mapping_deny_writable(file->f_mapping);
212 if (error)
213 goto unlock;
214
215 error = memfd_wait_for_pins(file->f_mapping);
216 if (error) {
217 mapping_allow_writable(file->f_mapping);
218 goto unlock;
219 }
220 }
221
222 *file_seals |= seals;
223 error = 0;
224
225 unlock:
226 inode_unlock(inode);
227 return error;
228 }
229
memfd_get_seals(struct file * file)230 static int memfd_get_seals(struct file *file)
231 {
232 unsigned int *seals = memfd_file_seals_ptr(file);
233
234 return seals ? *seals : -EINVAL;
235 }
236
memfd_fcntl(struct file * file,unsigned int cmd,unsigned long arg)237 long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
238 {
239 long error;
240
241 switch (cmd) {
242 case F_ADD_SEALS:
243 /* disallow upper 32bit */
244 if (arg > UINT_MAX)
245 return -EINVAL;
246
247 error = memfd_add_seals(file, arg);
248 break;
249 case F_GET_SEALS:
250 error = memfd_get_seals(file);
251 break;
252 default:
253 error = -EINVAL;
254 break;
255 }
256
257 return error;
258 }
259
260 #define MFD_NAME_PREFIX "memfd:"
261 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
262 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
263
264 #define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB)
265
SYSCALL_DEFINE2(memfd_create,const char __user *,uname,unsigned int,flags)266 SYSCALL_DEFINE2(memfd_create,
267 const char __user *, uname,
268 unsigned int, flags)
269 {
270 unsigned int *file_seals;
271 struct file *file;
272 int fd, error;
273 char *name;
274 long len;
275
276 if (!(flags & MFD_HUGETLB)) {
277 if (flags & ~(unsigned int)MFD_ALL_FLAGS)
278 return -EINVAL;
279 } else {
280 /* Allow huge page size encoding in flags. */
281 if (flags & ~(unsigned int)(MFD_ALL_FLAGS |
282 (MFD_HUGE_MASK << MFD_HUGE_SHIFT)))
283 return -EINVAL;
284 }
285
286 /* length includes terminating zero */
287 len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
288 if (len <= 0)
289 return -EFAULT;
290 if (len > MFD_NAME_MAX_LEN + 1)
291 return -EINVAL;
292
293 name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL);
294 if (!name)
295 return -ENOMEM;
296
297 strcpy(name, MFD_NAME_PREFIX);
298 if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) {
299 error = -EFAULT;
300 goto err_name;
301 }
302
303 /* terminating-zero may have changed after strnlen_user() returned */
304 if (name[len + MFD_NAME_PREFIX_LEN - 1]) {
305 error = -EFAULT;
306 goto err_name;
307 }
308
309 fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0);
310 if (fd < 0) {
311 error = fd;
312 goto err_name;
313 }
314
315 if (flags & MFD_HUGETLB) {
316 struct user_struct *user = NULL;
317
318 file = hugetlb_file_setup(name, 0, VM_NORESERVE, &user,
319 HUGETLB_ANONHUGE_INODE,
320 (flags >> MFD_HUGE_SHIFT) &
321 MFD_HUGE_MASK);
322 } else
323 file = shmem_file_setup(name, 0, VM_NORESERVE);
324 if (IS_ERR(file)) {
325 error = PTR_ERR(file);
326 goto err_fd;
327 }
328 file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
329 file->f_flags |= O_LARGEFILE;
330
331 if (flags & MFD_ALLOW_SEALING) {
332 file_seals = memfd_file_seals_ptr(file);
333 *file_seals &= ~F_SEAL_SEAL;
334 }
335
336 fd_install(fd, file);
337 kfree(name);
338 return fd;
339
340 err_fd:
341 put_unused_fd(fd);
342 err_name:
343 kfree(name);
344 return error;
345 }
346
