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 xa_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 xa_state * xas)31 static void memfd_tag_pins(struct xa_state *xas)
32 {
33 	struct page *page;
34 	int latency = 0;
35 	int cache_count;
36 
37 	lru_add_drain();
38 
39 	xas_lock_irq(xas);
40 	xas_for_each(xas, page, ULONG_MAX) {
41 		cache_count = 1;
42 		if (!xa_is_value(page) &&
43 		    PageTransHuge(page) && !PageHuge(page))
44 			cache_count = HPAGE_PMD_NR;
45 
46 		if (!xa_is_value(page) &&
47 		    page_count(page) - total_mapcount(page) != cache_count)
48 			xas_set_mark(xas, MEMFD_TAG_PINNED);
49 		if (cache_count != 1)
50 			xas_set(xas, page->index + cache_count);
51 
52 		latency += cache_count;
53 		if (latency < XA_CHECK_SCHED)
54 			continue;
55 		latency = 0;
56 
57 		xas_pause(xas);
58 		xas_unlock_irq(xas);
59 		cond_resched();
60 		xas_lock_irq(xas);
61 	}
62 	xas_unlock_irq(xas);
63 }
64 
65 /*
66  * Setting SEAL_WRITE requires us to verify there's no pending writer. However,
67  * via get_user_pages(), drivers might have some pending I/O without any active
68  * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages
69  * and see whether it has an elevated ref-count. If so, we tag them and wait for
70  * them to be dropped.
71  * The caller must guarantee that no new user will acquire writable references
72  * to those pages to avoid races.
73  */
memfd_wait_for_pins(struct address_space * mapping)74 static int memfd_wait_for_pins(struct address_space *mapping)
75 {
76 	XA_STATE(xas, &mapping->i_pages, 0);
77 	struct page *page;
78 	int error, scan;
79 
80 	memfd_tag_pins(&xas);
81 
82 	error = 0;
83 	for (scan = 0; scan <= LAST_SCAN; scan++) {
84 		int latency = 0;
85 		int cache_count;
86 
87 		if (!xas_marked(&xas, MEMFD_TAG_PINNED))
88 			break;
89 
90 		if (!scan)
91 			lru_add_drain_all();
92 		else if (schedule_timeout_killable((HZ << scan) / 200))
93 			scan = LAST_SCAN;
94 
95 		xas_set(&xas, 0);
96 		xas_lock_irq(&xas);
97 		xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) {
98 			bool clear = true;
99 
100 			cache_count = 1;
101 			if (!xa_is_value(page) &&
102 			    PageTransHuge(page) && !PageHuge(page))
103 				cache_count = HPAGE_PMD_NR;
104 
105 			if (!xa_is_value(page) && cache_count !=
106 			    page_count(page) - total_mapcount(page)) {
107 				/*
108 				 * On the last scan, we clean up all those tags
109 				 * we inserted; but make a note that we still
110 				 * found pages pinned.
111 				 */
112 				if (scan == LAST_SCAN)
113 					error = -EBUSY;
114 				else
115 					clear = false;
116 			}
117 			if (clear)
118 				xas_clear_mark(&xas, MEMFD_TAG_PINNED);
119 
120 			latency += cache_count;
121 			if (latency < XA_CHECK_SCHED)
122 				continue;
123 			latency = 0;
124 
125 			xas_pause(&xas);
126 			xas_unlock_irq(&xas);
127 			cond_resched();
128 			xas_lock_irq(&xas);
129 		}
130 		xas_unlock_irq(&xas);
131 	}
132 
133 	return error;
134 }
135 
memfd_file_seals_ptr(struct file * file)136 static unsigned int *memfd_file_seals_ptr(struct file *file)
137 {
138 	if (shmem_file(file))
139 		return &SHMEM_I(file_inode(file))->seals;
140 
141 #ifdef CONFIG_HUGETLBFS
142 	if (is_file_hugepages(file))
143 		return &HUGETLBFS_I(file_inode(file))->seals;
144 #endif
145 
146 	return NULL;
147 }
148 
149 #define F_ALL_SEALS (F_SEAL_SEAL | \
150 		     F_SEAL_SHRINK | \
151 		     F_SEAL_GROW | \
152 		     F_SEAL_WRITE | \
153 		     F_SEAL_FUTURE_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 		file = hugetlb_file_setup(name, 0, VM_NORESERVE,
317 					HUGETLB_ANONHUGE_INODE,
318 					(flags >> MFD_HUGE_SHIFT) &
319 					MFD_HUGE_MASK);
320 	} else
321 		file = shmem_file_setup(name, 0, VM_NORESERVE);
322 	if (IS_ERR(file)) {
323 		error = PTR_ERR(file);
324 		goto err_fd;
325 	}
326 	file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
327 	file->f_flags |= O_LARGEFILE;
328 
329 	if (flags & MFD_ALLOW_SEALING) {
330 		file_seals = memfd_file_seals_ptr(file);
331 		*file_seals &= ~F_SEAL_SEAL;
332 	}
333 
334 	fd_install(fd, file);
335 	kfree(name);
336 	return fd;
337 
338 err_fd:
339 	put_unused_fd(fd);
340 err_name:
341 	kfree(name);
342 	return error;
343 }
344