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