Line data Source code
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 :
31 0 : static void memfd_tag_pins(struct xa_state *xas)
32 : {
33 0 : struct page *page;
34 0 : unsigned int tagged = 0;
35 :
36 0 : lru_add_drain();
37 :
38 0 : xas_lock_irq(xas);
39 0 : xas_for_each(xas, page, ULONG_MAX) {
40 0 : if (xa_is_value(page))
41 0 : continue;
42 0 : page = find_subpage(page, xas->xa_index);
43 0 : if (page_count(page) - page_mapcount(page) > 1)
44 0 : xas_set_mark(xas, MEMFD_TAG_PINNED);
45 :
46 0 : if (++tagged % XA_CHECK_SCHED)
47 0 : continue;
48 :
49 0 : xas_pause(xas);
50 0 : xas_unlock_irq(xas);
51 0 : cond_resched();
52 0 : xas_lock_irq(xas);
53 : }
54 0 : xas_unlock_irq(xas);
55 0 : }
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 : */
66 0 : static int memfd_wait_for_pins(struct address_space *mapping)
67 : {
68 0 : XA_STATE(xas, &mapping->i_pages, 0);
69 0 : struct page *page;
70 0 : int error, scan;
71 :
72 0 : memfd_tag_pins(&xas);
73 :
74 0 : error = 0;
75 0 : for (scan = 0; scan <= LAST_SCAN; scan++) {
76 0 : unsigned int tagged = 0;
77 :
78 0 : if (!xas_marked(&xas, MEMFD_TAG_PINNED))
79 : break;
80 :
81 0 : if (!scan)
82 0 : lru_add_drain_all();
83 0 : else if (schedule_timeout_killable((HZ << scan) / 200))
84 0 : scan = LAST_SCAN;
85 :
86 0 : xas_set(&xas, 0);
87 0 : xas_lock_irq(&xas);
88 0 : xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) {
89 0 : bool clear = true;
90 0 : if (xa_is_value(page))
91 0 : continue;
92 0 : page = find_subpage(page, xas.xa_index);
93 0 : 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 0 : if (scan == LAST_SCAN)
100 : error = -EBUSY;
101 : else
102 : clear = false;
103 : }
104 : if (clear)
105 0 : xas_clear_mark(&xas, MEMFD_TAG_PINNED);
106 0 : if (++tagged % XA_CHECK_SCHED)
107 0 : continue;
108 :
109 0 : xas_pause(&xas);
110 0 : xas_unlock_irq(&xas);
111 0 : cond_resched();
112 0 : xas_lock_irq(&xas);
113 : }
114 0 : xas_unlock_irq(&xas);
115 : }
116 :
117 0 : return error;
118 : }
119 :
120 0 : static unsigned int *memfd_file_seals_ptr(struct file *file)
121 : {
122 0 : if (shmem_file(file))
123 0 : 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 :
139 0 : static int memfd_add_seals(struct file *file, unsigned int seals)
140 : {
141 0 : struct inode *inode = file_inode(file);
142 0 : unsigned int *file_seals;
143 0 : 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 0 : if (!(file->f_mode & FMODE_WRITE))
177 : return -EPERM;
178 0 : if (seals & ~(unsigned int)F_ALL_SEALS)
179 : return -EINVAL;
180 :
181 0 : inode_lock(inode);
182 :
183 0 : file_seals = memfd_file_seals_ptr(file);
184 0 : if (!file_seals) {
185 0 : error = -EINVAL;
186 0 : goto unlock;
187 : }
188 :
189 0 : if (*file_seals & F_SEAL_SEAL) {
190 0 : error = -EPERM;
191 0 : goto unlock;
192 : }
193 :
194 0 : if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
195 0 : error = mapping_deny_writable(file->f_mapping);
196 0 : if (error)
197 0 : goto unlock;
198 :
199 0 : error = memfd_wait_for_pins(file->f_mapping);
200 0 : if (error) {
201 0 : mapping_allow_writable(file->f_mapping);
202 0 : goto unlock;
203 : }
204 : }
205 :
206 0 : *file_seals |= seals;
207 0 : error = 0;
208 :
209 0 : unlock:
210 0 : inode_unlock(inode);
211 0 : return error;
212 : }
213 :
214 0 : static int memfd_get_seals(struct file *file)
215 : {
216 0 : unsigned int *seals = memfd_file_seals_ptr(file);
217 :
218 0 : return seals ? *seals : -EINVAL;
219 : }
220 :
221 0 : long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
222 : {
223 0 : long error;
224 :
225 0 : switch (cmd) {
226 0 : case F_ADD_SEALS:
227 : /* disallow upper 32bit */
228 0 : if (arg > UINT_MAX)
229 : return -EINVAL;
230 :
231 0 : error = memfd_add_seals(file, arg);
232 0 : break;
233 0 : case F_GET_SEALS:
234 0 : error = memfd_get_seals(file);
235 0 : 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 :
250 6 : SYSCALL_DEFINE2(memfd_create,
251 : const char __user *, uname,
252 : unsigned int, flags)
253 : {
254 3 : unsigned int *file_seals;
255 3 : struct file *file;
256 3 : int fd, error;
257 3 : char *name;
258 3 : long len;
259 :
260 3 : if (!(flags & MFD_HUGETLB)) {
261 3 : if (flags & ~(unsigned int)MFD_ALL_FLAGS)
262 : return -EINVAL;
263 : } else {
264 : /* Allow huge page size encoding in flags. */
265 0 : 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 3 : len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
272 3 : if (len <= 0)
273 : return -EFAULT;
274 3 : if (len > MFD_NAME_MAX_LEN + 1)
275 : return -EINVAL;
276 :
277 3 : name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL);
278 3 : if (!name)
279 : return -ENOMEM;
280 :
281 3 : strcpy(name, MFD_NAME_PREFIX);
282 6 : if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) {
283 0 : error = -EFAULT;
284 0 : goto err_name;
285 : }
286 :
287 : /* terminating-zero may have changed after strnlen_user() returned */
288 3 : if (name[len + MFD_NAME_PREFIX_LEN - 1]) {
289 0 : error = -EFAULT;
290 0 : goto err_name;
291 : }
292 :
293 3 : fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0);
294 3 : if (fd < 0) {
295 0 : error = fd;
296 0 : goto err_name;
297 : }
298 :
299 3 : if (flags & MFD_HUGETLB) {
300 3 : struct user_struct *user = NULL;
301 :
302 3 : file = hugetlb_file_setup(name, 0, VM_NORESERVE, &user,
303 : HUGETLB_ANONHUGE_INODE,
304 3 : (flags >> MFD_HUGE_SHIFT) &
305 : MFD_HUGE_MASK);
306 : } else
307 3 : file = shmem_file_setup(name, 0, VM_NORESERVE);
308 3 : if (IS_ERR(file)) {
309 0 : error = PTR_ERR(file);
310 0 : goto err_fd;
311 : }
312 3 : file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
313 3 : file->f_flags |= O_LARGEFILE;
314 :
315 3 : if (flags & MFD_ALLOW_SEALING) {
316 0 : file_seals = memfd_file_seals_ptr(file);
317 0 : *file_seals &= ~F_SEAL_SEAL;
318 : }
319 :
320 3 : fd_install(fd, file);
321 3 : kfree(name);
322 3 : return fd;
323 :
324 0 : err_fd:
325 0 : put_unused_fd(fd);
326 0 : err_name:
327 0 : kfree(name);
328 0 : return error;
329 : }
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