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 <linux/pid_namespace.h>
22 #include <uapi/linux/memfd.h>
23
24 /*
25 * We need a tag: a new tag would expand every xa_node by 8 bytes,
26 * so reuse a tag which we firmly believe is never set or cleared on tmpfs
27 * or hugetlbfs because they are memory only filesystems.
28 */
29 #define MEMFD_TAG_PINNED PAGECACHE_TAG_TOWRITE
30 #define LAST_SCAN 4 /* about 150ms max */
31
memfd_folio_has_extra_refs(struct folio * folio)32 static bool memfd_folio_has_extra_refs(struct folio *folio)
33 {
34 return folio_ref_count(folio) - folio_mapcount(folio) !=
35 folio_nr_pages(folio);
36 }
37
memfd_tag_pins(struct xa_state * xas)38 static void memfd_tag_pins(struct xa_state *xas)
39 {
40 struct folio *folio;
41 int latency = 0;
42
43 lru_add_drain();
44
45 xas_lock_irq(xas);
46 xas_for_each(xas, folio, ULONG_MAX) {
47 if (!xa_is_value(folio) && memfd_folio_has_extra_refs(folio))
48 xas_set_mark(xas, MEMFD_TAG_PINNED);
49
50 if (++latency < XA_CHECK_SCHED)
51 continue;
52 latency = 0;
53
54 xas_pause(xas);
55 xas_unlock_irq(xas);
56 cond_resched();
57 xas_lock_irq(xas);
58 }
59 xas_unlock_irq(xas);
60 }
61
62 /*
63 * This is a helper function used by memfd_pin_user_pages() in GUP (gup.c).
64 * It is mainly called to allocate a folio in a memfd when the caller
65 * (memfd_pin_folios()) cannot find a folio in the page cache at a given
66 * index in the mapping.
67 */
memfd_alloc_folio(struct file * memfd,pgoff_t idx)68 struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
69 {
70 #ifdef CONFIG_HUGETLB_PAGE
71 struct folio *folio;
72 gfp_t gfp_mask;
73 int err;
74
75 if (is_file_hugepages(memfd)) {
76 /*
77 * The folio would most likely be accessed by a DMA driver,
78 * therefore, we have zone memory constraints where we can
79 * alloc from. Also, the folio will be pinned for an indefinite
80 * amount of time, so it is not expected to be migrated away.
81 */
82 struct hstate *h = hstate_file(memfd);
83
84 gfp_mask = htlb_alloc_mask(h);
85 gfp_mask &= ~(__GFP_HIGHMEM | __GFP_MOVABLE);
86 idx >>= huge_page_order(h);
87
88 folio = alloc_hugetlb_folio_reserve(h,
89 numa_node_id(),
90 NULL,
91 gfp_mask);
92 if (folio) {
93 err = hugetlb_add_to_page_cache(folio,
94 memfd->f_mapping,
95 idx);
96 if (err) {
97 folio_put(folio);
98 return ERR_PTR(err);
99 }
100 folio_unlock(folio);
101 return folio;
102 }
103 return ERR_PTR(-ENOMEM);
104 }
105 #endif
106 return shmem_read_folio(memfd->f_mapping, idx);
107 }
108
109 /*
110 * Setting SEAL_WRITE requires us to verify there's no pending writer. However,
111 * via get_user_pages(), drivers might have some pending I/O without any active
112 * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all folios
113 * and see whether it has an elevated ref-count. If so, we tag them and wait for
114 * them to be dropped.
115 * The caller must guarantee that no new user will acquire writable references
116 * to those folios to avoid races.
117 */
memfd_wait_for_pins(struct address_space * mapping)118 static int memfd_wait_for_pins(struct address_space *mapping)
119 {
120 XA_STATE(xas, &mapping->i_pages, 0);
121 struct folio *folio;
122 int error, scan;
123
124 memfd_tag_pins(&xas);
125
126 error = 0;
127 for (scan = 0; scan <= LAST_SCAN; scan++) {
128 int latency = 0;
129
130 if (!xas_marked(&xas, MEMFD_TAG_PINNED))
131 break;
132
133 if (!scan)
134 lru_add_drain_all();
135 else if (schedule_timeout_killable((HZ << scan) / 200))
136 scan = LAST_SCAN;
137
138 xas_set(&xas, 0);
139 xas_lock_irq(&xas);
140 xas_for_each_marked(&xas, folio, ULONG_MAX, MEMFD_TAG_PINNED) {
141 bool clear = true;
142
143 if (!xa_is_value(folio) &&
144 memfd_folio_has_extra_refs(folio)) {
145 /*
146 * On the last scan, we clean up all those tags
147 * we inserted; but make a note that we still
148 * found folios pinned.
149 */
150 if (scan == LAST_SCAN)
151 error = -EBUSY;
152 else
153 clear = false;
154 }
155 if (clear)
156 xas_clear_mark(&xas, MEMFD_TAG_PINNED);
157
158 if (++latency < XA_CHECK_SCHED)
159 continue;
160 latency = 0;
161
162 xas_pause(&xas);
163 xas_unlock_irq(&xas);
164 cond_resched();
165 xas_lock_irq(&xas);
166 }
167 xas_unlock_irq(&xas);
168 }
169
170 return error;
171 }
172
memfd_file_seals_ptr(struct file * file)173 static unsigned int *memfd_file_seals_ptr(struct file *file)
174 {
175 if (shmem_file(file))
176 return &SHMEM_I(file_inode(file))->seals;
177
178 #ifdef CONFIG_HUGETLBFS
179 if (is_file_hugepages(file))
180 return &HUGETLBFS_I(file_inode(file))->seals;
181 #endif
182
183 return NULL;
184 }
185
186 #define F_ALL_SEALS (F_SEAL_SEAL | \
187 F_SEAL_EXEC | \
188 F_SEAL_SHRINK | \
189 F_SEAL_GROW | \
190 F_SEAL_WRITE | \
191 F_SEAL_FUTURE_WRITE)
192
memfd_add_seals(struct file * file,unsigned int seals)193 static int memfd_add_seals(struct file *file, unsigned int seals)
194 {
195 struct inode *inode = file_inode(file);
196 unsigned int *file_seals;
197 int error;
198
199 /*
200 * SEALING
201 * Sealing allows multiple parties to share a tmpfs or hugetlbfs file
202 * but restrict access to a specific subset of file operations. Seals
203 * can only be added, but never removed. This way, mutually untrusted
204 * parties can share common memory regions with a well-defined policy.
205 * A malicious peer can thus never perform unwanted operations on a
206 * shared object.
207 *
208 * Seals are only supported on special tmpfs or hugetlbfs files and
209 * always affect the whole underlying inode. Once a seal is set, it
210 * may prevent some kinds of access to the file. Currently, the
211 * following seals are defined:
212 * SEAL_SEAL: Prevent further seals from being set on this file
213 * SEAL_SHRINK: Prevent the file from shrinking
214 * SEAL_GROW: Prevent the file from growing
215 * SEAL_WRITE: Prevent write access to the file
216 * SEAL_EXEC: Prevent modification of the exec bits in the file mode
217 *
218 * As we don't require any trust relationship between two parties, we
219 * must prevent seals from being removed. Therefore, sealing a file
220 * only adds a given set of seals to the file, it never touches
221 * existing seals. Furthermore, the "setting seals"-operation can be
222 * sealed itself, which basically prevents any further seal from being
223 * added.
224 *
225 * Semantics of sealing are only defined on volatile files. Only
226 * anonymous tmpfs and hugetlbfs files support sealing. More
227 * importantly, seals are never written to disk. Therefore, there's
228 * no plan to support it on other file types.
229 */
230
231 if (!(file->f_mode & FMODE_WRITE))
232 return -EPERM;
233 if (seals & ~(unsigned int)F_ALL_SEALS)
234 return -EINVAL;
235
236 inode_lock(inode);
237
238 file_seals = memfd_file_seals_ptr(file);
239 if (!file_seals) {
240 error = -EINVAL;
241 goto unlock;
242 }
243
244 if (*file_seals & F_SEAL_SEAL) {
245 error = -EPERM;
246 goto unlock;
247 }
248
249 if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
250 error = mapping_deny_writable(file->f_mapping);
251 if (error)
252 goto unlock;
253
254 error = memfd_wait_for_pins(file->f_mapping);
255 if (error) {
256 mapping_allow_writable(file->f_mapping);
257 goto unlock;
258 }
259 }
260
261 /*
262 * SEAL_EXEC implys SEAL_WRITE, making W^X from the start.
263 */
264 if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
265 seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
266
267 *file_seals |= seals;
268 error = 0;
269
270 unlock:
271 inode_unlock(inode);
272 return error;
273 }
274
memfd_get_seals(struct file * file)275 static int memfd_get_seals(struct file *file)
276 {
277 unsigned int *seals = memfd_file_seals_ptr(file);
278
279 return seals ? *seals : -EINVAL;
280 }
281
memfd_fcntl(struct file * file,unsigned int cmd,unsigned int arg)282 long memfd_fcntl(struct file *file, unsigned int cmd, unsigned int arg)
283 {
284 long error;
285
286 switch (cmd) {
287 case F_ADD_SEALS:
288 error = memfd_add_seals(file, arg);
289 break;
290 case F_GET_SEALS:
291 error = memfd_get_seals(file);
292 break;
293 default:
294 error = -EINVAL;
295 break;
296 }
297
298 return error;
299 }
300
301 #define MFD_NAME_PREFIX "memfd:"
302 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
303 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
304
305 #define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB | MFD_NOEXEC_SEAL | MFD_EXEC)
306
check_sysctl_memfd_noexec(unsigned int * flags)307 static int check_sysctl_memfd_noexec(unsigned int *flags)
308 {
309 #ifdef CONFIG_SYSCTL
310 struct pid_namespace *ns = task_active_pid_ns(current);
311 int sysctl = pidns_memfd_noexec_scope(ns);
312
313 if (!(*flags & (MFD_EXEC | MFD_NOEXEC_SEAL))) {
314 if (sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL)
315 *flags |= MFD_NOEXEC_SEAL;
316 else
317 *flags |= MFD_EXEC;
318 }
319
320 if (!(*flags & MFD_NOEXEC_SEAL) && sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_ENFORCED) {
321 pr_err_ratelimited(
322 "%s[%d]: memfd_create() requires MFD_NOEXEC_SEAL with vm.memfd_noexec=%d\n",
323 current->comm, task_pid_nr(current), sysctl);
324 return -EACCES;
325 }
326 #endif
327 return 0;
328 }
329
SYSCALL_DEFINE2(memfd_create,const char __user *,uname,unsigned int,flags)330 SYSCALL_DEFINE2(memfd_create,
331 const char __user *, uname,
332 unsigned int, flags)
333 {
334 unsigned int *file_seals;
335 struct file *file;
336 int fd, error;
337 char *name;
338 long len;
339
340 if (!(flags & MFD_HUGETLB)) {
341 if (flags & ~(unsigned int)MFD_ALL_FLAGS)
342 return -EINVAL;
343 } else {
344 /* Allow huge page size encoding in flags. */
345 if (flags & ~(unsigned int)(MFD_ALL_FLAGS |
346 (MFD_HUGE_MASK << MFD_HUGE_SHIFT)))
347 return -EINVAL;
348 }
349
350 /* Invalid if both EXEC and NOEXEC_SEAL are set.*/
351 if ((flags & MFD_EXEC) && (flags & MFD_NOEXEC_SEAL))
352 return -EINVAL;
353
354 error = check_sysctl_memfd_noexec(&flags);
355 if (error < 0)
356 return error;
357
358 /* length includes terminating zero */
359 len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
360 if (len <= 0)
361 return -EFAULT;
362 if (len > MFD_NAME_MAX_LEN + 1)
363 return -EINVAL;
364
365 name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL);
366 if (!name)
367 return -ENOMEM;
368
369 strcpy(name, MFD_NAME_PREFIX);
370 if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) {
371 error = -EFAULT;
372 goto err_name;
373 }
374
375 /* terminating-zero may have changed after strnlen_user() returned */
376 if (name[len + MFD_NAME_PREFIX_LEN - 1]) {
377 error = -EFAULT;
378 goto err_name;
379 }
380
381 fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0);
382 if (fd < 0) {
383 error = fd;
384 goto err_name;
385 }
386
387 if (flags & MFD_HUGETLB) {
388 file = hugetlb_file_setup(name, 0, VM_NORESERVE,
389 HUGETLB_ANONHUGE_INODE,
390 (flags >> MFD_HUGE_SHIFT) &
391 MFD_HUGE_MASK);
392 } else
393 file = shmem_file_setup(name, 0, VM_NORESERVE);
394 if (IS_ERR(file)) {
395 error = PTR_ERR(file);
396 goto err_fd;
397 }
398 file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
399 file->f_flags |= O_LARGEFILE;
400
401 if (flags & MFD_NOEXEC_SEAL) {
402 struct inode *inode = file_inode(file);
403
404 inode->i_mode &= ~0111;
405 file_seals = memfd_file_seals_ptr(file);
406 if (file_seals) {
407 *file_seals &= ~F_SEAL_SEAL;
408 *file_seals |= F_SEAL_EXEC;
409 }
410 } else if (flags & MFD_ALLOW_SEALING) {
411 /* MFD_EXEC and MFD_ALLOW_SEALING are set */
412 file_seals = memfd_file_seals_ptr(file);
413 if (file_seals)
414 *file_seals &= ~F_SEAL_SEAL;
415 }
416
417 fd_install(fd, file);
418 kfree(name);
419 return fd;
420
421 err_fd:
422 put_unused_fd(fd);
423 err_name:
424 kfree(name);
425 return error;
426 }
427