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