xref: /linux/mm/memfd.c (revision bf36793fa260cb68cc817f311f1f683788261796)
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 
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 
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  */
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 		gfp_mask = htlb_alloc_mask(hstate_file(memfd));
83 		gfp_mask &= ~(__GFP_HIGHMEM | __GFP_MOVABLE);
84 
85 		folio = alloc_hugetlb_folio_nodemask(hstate_file(memfd),
86 						     numa_node_id(),
87 						     NULL,
88 						     gfp_mask,
89 						     false);
90 		if (folio && folio_try_get(folio)) {
91 			err = hugetlb_add_to_page_cache(folio,
92 							memfd->f_mapping,
93 							idx);
94 			if (err) {
95 				folio_put(folio);
96 				free_huge_folio(folio);
97 				return ERR_PTR(err);
98 			}
99 			return folio;
100 		}
101 		return ERR_PTR(-ENOMEM);
102 	}
103 #endif
104 	return shmem_read_folio(memfd->f_mapping, idx);
105 }
106 
107 /*
108  * Setting SEAL_WRITE requires us to verify there's no pending writer. However,
109  * via get_user_pages(), drivers might have some pending I/O without any active
110  * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all folios
111  * and see whether it has an elevated ref-count. If so, we tag them and wait for
112  * them to be dropped.
113  * The caller must guarantee that no new user will acquire writable references
114  * to those folios to avoid races.
115  */
116 static int memfd_wait_for_pins(struct address_space *mapping)
117 {
118 	XA_STATE(xas, &mapping->i_pages, 0);
119 	struct folio *folio;
120 	int error, scan;
121 
122 	memfd_tag_pins(&xas);
123 
124 	error = 0;
125 	for (scan = 0; scan <= LAST_SCAN; scan++) {
126 		int latency = 0;
127 
128 		if (!xas_marked(&xas, MEMFD_TAG_PINNED))
129 			break;
130 
131 		if (!scan)
132 			lru_add_drain_all();
133 		else if (schedule_timeout_killable((HZ << scan) / 200))
134 			scan = LAST_SCAN;
135 
136 		xas_set(&xas, 0);
137 		xas_lock_irq(&xas);
138 		xas_for_each_marked(&xas, folio, ULONG_MAX, MEMFD_TAG_PINNED) {
139 			bool clear = true;
140 
141 			if (!xa_is_value(folio) &&
142 			    memfd_folio_has_extra_refs(folio)) {
143 				/*
144 				 * On the last scan, we clean up all those tags
145 				 * we inserted; but make a note that we still
146 				 * found folios pinned.
147 				 */
148 				if (scan == LAST_SCAN)
149 					error = -EBUSY;
150 				else
151 					clear = false;
152 			}
153 			if (clear)
154 				xas_clear_mark(&xas, MEMFD_TAG_PINNED);
155 
156 			if (++latency < XA_CHECK_SCHED)
157 				continue;
158 			latency = 0;
159 
160 			xas_pause(&xas);
161 			xas_unlock_irq(&xas);
162 			cond_resched();
163 			xas_lock_irq(&xas);
164 		}
165 		xas_unlock_irq(&xas);
166 	}
167 
168 	return error;
169 }
170 
171 static unsigned int *memfd_file_seals_ptr(struct file *file)
172 {
173 	if (shmem_file(file))
174 		return &SHMEM_I(file_inode(file))->seals;
175 
176 #ifdef CONFIG_HUGETLBFS
177 	if (is_file_hugepages(file))
178 		return &HUGETLBFS_I(file_inode(file))->seals;
179 #endif
180 
181 	return NULL;
182 }
183 
184 #define F_ALL_SEALS (F_SEAL_SEAL | \
185 		     F_SEAL_EXEC | \
186 		     F_SEAL_SHRINK | \
187 		     F_SEAL_GROW | \
188 		     F_SEAL_WRITE | \
189 		     F_SEAL_FUTURE_WRITE)
190 
191 static int memfd_add_seals(struct file *file, unsigned int seals)
192 {
193 	struct inode *inode = file_inode(file);
194 	unsigned int *file_seals;
195 	int error;
196 
197 	/*
198 	 * SEALING
199 	 * Sealing allows multiple parties to share a tmpfs or hugetlbfs file
200 	 * but restrict access to a specific subset of file operations. Seals
201 	 * can only be added, but never removed. This way, mutually untrusted
202 	 * parties can share common memory regions with a well-defined policy.
203 	 * A malicious peer can thus never perform unwanted operations on a
204 	 * shared object.
205 	 *
206 	 * Seals are only supported on special tmpfs or hugetlbfs files and
207 	 * always affect the whole underlying inode. Once a seal is set, it
208 	 * may prevent some kinds of access to the file. Currently, the
209 	 * following seals are defined:
210 	 *   SEAL_SEAL: Prevent further seals from being set on this file
211 	 *   SEAL_SHRINK: Prevent the file from shrinking
212 	 *   SEAL_GROW: Prevent the file from growing
213 	 *   SEAL_WRITE: Prevent write access to the file
214 	 *   SEAL_EXEC: Prevent modification of the exec bits in the file mode
215 	 *
216 	 * As we don't require any trust relationship between two parties, we
217 	 * must prevent seals from being removed. Therefore, sealing a file
218 	 * only adds a given set of seals to the file, it never touches
219 	 * existing seals. Furthermore, the "setting seals"-operation can be
220 	 * sealed itself, which basically prevents any further seal from being
221 	 * added.
222 	 *
223 	 * Semantics of sealing are only defined on volatile files. Only
224 	 * anonymous tmpfs and hugetlbfs files support sealing. More
225 	 * importantly, seals are never written to disk. Therefore, there's
226 	 * no plan to support it on other file types.
227 	 */
228 
229 	if (!(file->f_mode & FMODE_WRITE))
230 		return -EPERM;
231 	if (seals & ~(unsigned int)F_ALL_SEALS)
232 		return -EINVAL;
233 
234 	inode_lock(inode);
235 
236 	file_seals = memfd_file_seals_ptr(file);
237 	if (!file_seals) {
238 		error = -EINVAL;
239 		goto unlock;
240 	}
241 
242 	if (*file_seals & F_SEAL_SEAL) {
243 		error = -EPERM;
244 		goto unlock;
245 	}
246 
247 	if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
248 		error = mapping_deny_writable(file->f_mapping);
249 		if (error)
250 			goto unlock;
251 
252 		error = memfd_wait_for_pins(file->f_mapping);
253 		if (error) {
254 			mapping_allow_writable(file->f_mapping);
255 			goto unlock;
256 		}
257 	}
258 
259 	/*
260 	 * SEAL_EXEC implys SEAL_WRITE, making W^X from the start.
261 	 */
262 	if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
263 		seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
264 
265 	*file_seals |= seals;
266 	error = 0;
267 
268 unlock:
269 	inode_unlock(inode);
270 	return error;
271 }
272 
273 static int memfd_get_seals(struct file *file)
274 {
275 	unsigned int *seals = memfd_file_seals_ptr(file);
276 
277 	return seals ? *seals : -EINVAL;
278 }
279 
280 long memfd_fcntl(struct file *file, unsigned int cmd, unsigned int arg)
281 {
282 	long error;
283 
284 	switch (cmd) {
285 	case F_ADD_SEALS:
286 		error = memfd_add_seals(file, arg);
287 		break;
288 	case F_GET_SEALS:
289 		error = memfd_get_seals(file);
290 		break;
291 	default:
292 		error = -EINVAL;
293 		break;
294 	}
295 
296 	return error;
297 }
298 
299 #define MFD_NAME_PREFIX "memfd:"
300 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
301 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
302 
303 #define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB | MFD_NOEXEC_SEAL | MFD_EXEC)
304 
305 static int check_sysctl_memfd_noexec(unsigned int *flags)
306 {
307 #ifdef CONFIG_SYSCTL
308 	struct pid_namespace *ns = task_active_pid_ns(current);
309 	int sysctl = pidns_memfd_noexec_scope(ns);
310 
311 	if (!(*flags & (MFD_EXEC | MFD_NOEXEC_SEAL))) {
312 		if (sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL)
313 			*flags |= MFD_NOEXEC_SEAL;
314 		else
315 			*flags |= MFD_EXEC;
316 	}
317 
318 	if (!(*flags & MFD_NOEXEC_SEAL) && sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_ENFORCED) {
319 		pr_err_ratelimited(
320 			"%s[%d]: memfd_create() requires MFD_NOEXEC_SEAL with vm.memfd_noexec=%d\n",
321 			current->comm, task_pid_nr(current), sysctl);
322 		return -EACCES;
323 	}
324 #endif
325 	return 0;
326 }
327 
328 SYSCALL_DEFINE2(memfd_create,
329 		const char __user *, uname,
330 		unsigned int, flags)
331 {
332 	unsigned int *file_seals;
333 	struct file *file;
334 	int fd, error;
335 	char *name;
336 	long len;
337 
338 	if (!(flags & MFD_HUGETLB)) {
339 		if (flags & ~(unsigned int)MFD_ALL_FLAGS)
340 			return -EINVAL;
341 	} else {
342 		/* Allow huge page size encoding in flags. */
343 		if (flags & ~(unsigned int)(MFD_ALL_FLAGS |
344 				(MFD_HUGE_MASK << MFD_HUGE_SHIFT)))
345 			return -EINVAL;
346 	}
347 
348 	/* Invalid if both EXEC and NOEXEC_SEAL are set.*/
349 	if ((flags & MFD_EXEC) && (flags & MFD_NOEXEC_SEAL))
350 		return -EINVAL;
351 
352 	error = check_sysctl_memfd_noexec(&flags);
353 	if (error < 0)
354 		return error;
355 
356 	/* length includes terminating zero */
357 	len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
358 	if (len <= 0)
359 		return -EFAULT;
360 	if (len > MFD_NAME_MAX_LEN + 1)
361 		return -EINVAL;
362 
363 	name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL);
364 	if (!name)
365 		return -ENOMEM;
366 
367 	strcpy(name, MFD_NAME_PREFIX);
368 	if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) {
369 		error = -EFAULT;
370 		goto err_name;
371 	}
372 
373 	/* terminating-zero may have changed after strnlen_user() returned */
374 	if (name[len + MFD_NAME_PREFIX_LEN - 1]) {
375 		error = -EFAULT;
376 		goto err_name;
377 	}
378 
379 	fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0);
380 	if (fd < 0) {
381 		error = fd;
382 		goto err_name;
383 	}
384 
385 	if (flags & MFD_HUGETLB) {
386 		file = hugetlb_file_setup(name, 0, VM_NORESERVE,
387 					HUGETLB_ANONHUGE_INODE,
388 					(flags >> MFD_HUGE_SHIFT) &
389 					MFD_HUGE_MASK);
390 	} else
391 		file = shmem_file_setup(name, 0, VM_NORESERVE);
392 	if (IS_ERR(file)) {
393 		error = PTR_ERR(file);
394 		goto err_fd;
395 	}
396 	file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
397 	file->f_flags |= O_LARGEFILE;
398 
399 	if (flags & MFD_NOEXEC_SEAL) {
400 		struct inode *inode = file_inode(file);
401 
402 		inode->i_mode &= ~0111;
403 		file_seals = memfd_file_seals_ptr(file);
404 		if (file_seals) {
405 			*file_seals &= ~F_SEAL_SEAL;
406 			*file_seals |= F_SEAL_EXEC;
407 		}
408 	} else if (flags & MFD_ALLOW_SEALING) {
409 		/* MFD_EXEC and MFD_ALLOW_SEALING are set */
410 		file_seals = memfd_file_seals_ptr(file);
411 		if (file_seals)
412 			*file_seals &= ~F_SEAL_SEAL;
413 	}
414 
415 	fd_install(fd, file);
416 	kfree(name);
417 	return fd;
418 
419 err_fd:
420 	put_unused_fd(fd);
421 err_name:
422 	kfree(name);
423 	return error;
424 }
425