2 * memfd_create system call and file sealing support
4 * Code was originally included in shmem.c, and broken out to facilitate
5 * use by hugetlbfs as well as tmpfs.
7 * This file is released under the GPL.
11 #include <linux/vfs.h>
12 #include <linux/pagemap.h>
13 #include <linux/file.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>
24 * We need a tag: a new tag would expand every radix_tree_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.
28 #define MEMFD_TAG_PINNED PAGECACHE_TAG_TOWRITE
29 #define LAST_SCAN 4 /* about 150ms max */
31 static void memfd_tag_pins(struct address_space *mapping)
33 struct radix_tree_iter iter;
43 xa_lock_irq(&mapping->i_pages);
44 radix_tree_for_each_slot(slot, &mapping->i_pages, &iter, start) {
46 page = radix_tree_deref_slot_protected(slot, &mapping->i_pages.xa_lock);
47 if (!page || radix_tree_exception(page) || PageTail(page)) {
48 if (radix_tree_deref_retry(page)) {
49 slot = radix_tree_iter_retry(&iter);
53 if (PageTransHuge(page) && !PageHuge(page))
54 cache_count = HPAGE_PMD_NR;
56 page_count(page) - total_mapcount(page)) {
57 radix_tree_tag_set(&mapping->i_pages,
58 iter.index, MEMFD_TAG_PINNED);
62 latency += cache_count;
67 slot = radix_tree_iter_resume(slot, &iter);
68 xa_unlock_irq(&mapping->i_pages);
70 xa_lock_irq(&mapping->i_pages);
72 xa_unlock_irq(&mapping->i_pages);
76 * Setting SEAL_WRITE requires us to verify there's no pending writer. However,
77 * via get_user_pages(), drivers might have some pending I/O without any active
78 * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages
79 * and see whether it has an elevated ref-count. If so, we tag them and wait for
81 * The caller must guarantee that no new user will acquire writable references
82 * to those pages to avoid races.
84 static int memfd_wait_for_pins(struct address_space *mapping)
86 struct radix_tree_iter iter;
93 memfd_tag_pins(mapping);
96 for (scan = 0; scan <= LAST_SCAN; scan++) {
97 if (!radix_tree_tagged(&mapping->i_pages, MEMFD_TAG_PINNED))
102 else if (schedule_timeout_killable((HZ << scan) / 200))
107 radix_tree_for_each_tagged(slot, &mapping->i_pages, &iter,
108 start, MEMFD_TAG_PINNED) {
110 page = radix_tree_deref_slot(slot);
111 if (radix_tree_exception(page)) {
112 if (radix_tree_deref_retry(page)) {
113 slot = radix_tree_iter_retry(&iter);
121 if (page && PageTransHuge(page) && !PageHuge(page))
122 cache_count = HPAGE_PMD_NR;
124 if (page && cache_count !=
125 page_count(page) - total_mapcount(page)) {
126 if (scan < LAST_SCAN)
127 goto continue_resched;
130 * On the last scan, we clean up all those tags
131 * we inserted; but make a note that we still
132 * found pages pinned.
137 xa_lock_irq(&mapping->i_pages);
138 radix_tree_tag_clear(&mapping->i_pages,
139 iter.index, MEMFD_TAG_PINNED);
140 xa_unlock_irq(&mapping->i_pages);
142 if (need_resched()) {
143 slot = radix_tree_iter_resume(slot, &iter);
153 static unsigned int *memfd_file_seals_ptr(struct file *file)
155 if (shmem_file(file))
156 return &SHMEM_I(file_inode(file))->seals;
158 #ifdef CONFIG_HUGETLBFS
159 if (is_file_hugepages(file))
160 return &HUGETLBFS_I(file_inode(file))->seals;
166 #define F_ALL_SEALS (F_SEAL_SEAL | \
171 static int memfd_add_seals(struct file *file, unsigned int seals)
173 struct inode *inode = file_inode(file);
174 unsigned int *file_seals;
179 * Sealing allows multiple parties to share a tmpfs or hugetlbfs file
180 * but restrict access to a specific subset of file operations. Seals
181 * can only be added, but never removed. This way, mutually untrusted
182 * parties can share common memory regions with a well-defined policy.
183 * A malicious peer can thus never perform unwanted operations on a
186 * Seals are only supported on special tmpfs or hugetlbfs files and
187 * always affect the whole underlying inode. Once a seal is set, it
188 * may prevent some kinds of access to the file. Currently, the
189 * following seals are defined:
190 * SEAL_SEAL: Prevent further seals from being set on this file
191 * SEAL_SHRINK: Prevent the file from shrinking
192 * SEAL_GROW: Prevent the file from growing
193 * SEAL_WRITE: Prevent write access to the file
195 * As we don't require any trust relationship between two parties, we
196 * must prevent seals from being removed. Therefore, sealing a file
197 * only adds a given set of seals to the file, it never touches
198 * existing seals. Furthermore, the "setting seals"-operation can be
199 * sealed itself, which basically prevents any further seal from being
202 * Semantics of sealing are only defined on volatile files. Only
203 * anonymous tmpfs and hugetlbfs files support sealing. More
204 * importantly, seals are never written to disk. Therefore, there's
205 * no plan to support it on other file types.
208 if (!(file->f_mode & FMODE_WRITE))
210 if (seals & ~(unsigned int)F_ALL_SEALS)
215 file_seals = memfd_file_seals_ptr(file);
221 if (*file_seals & F_SEAL_SEAL) {
226 if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
227 error = mapping_deny_writable(file->f_mapping);
231 error = memfd_wait_for_pins(file->f_mapping);
233 mapping_allow_writable(file->f_mapping);
238 *file_seals |= seals;
246 static int memfd_get_seals(struct file *file)
248 unsigned int *seals = memfd_file_seals_ptr(file);
250 return seals ? *seals : -EINVAL;
253 long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
259 /* disallow upper 32bit */
263 error = memfd_add_seals(file, arg);
266 error = memfd_get_seals(file);
276 #define MFD_NAME_PREFIX "memfd:"
277 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
278 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
280 #define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB)
282 SYSCALL_DEFINE2(memfd_create,
283 const char __user *, uname,
286 unsigned int *file_seals;
292 if (!(flags & MFD_HUGETLB)) {
293 if (flags & ~(unsigned int)MFD_ALL_FLAGS)
296 /* Allow huge page size encoding in flags. */
297 if (flags & ~(unsigned int)(MFD_ALL_FLAGS |
298 (MFD_HUGE_MASK << MFD_HUGE_SHIFT)))
302 /* length includes terminating zero */
303 len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
306 if (len > MFD_NAME_MAX_LEN + 1)
309 name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL);
313 strcpy(name, MFD_NAME_PREFIX);
314 if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) {
319 /* terminating-zero may have changed after strnlen_user() returned */
320 if (name[len + MFD_NAME_PREFIX_LEN - 1]) {
325 fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0);
331 if (flags & MFD_HUGETLB) {
332 struct user_struct *user = NULL;
334 file = hugetlb_file_setup(name, 0, VM_NORESERVE, &user,
335 HUGETLB_ANONHUGE_INODE,
336 (flags >> MFD_HUGE_SHIFT) &
339 file = shmem_file_setup(name, 0, VM_NORESERVE);
341 error = PTR_ERR(file);
344 file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
345 file->f_flags |= O_LARGEFILE;
347 if (flags & MFD_ALLOW_SEALING) {
348 file_seals = memfd_file_seals_ptr(file);
349 *file_seals &= ~F_SEAL_SEAL;
352 fd_install(fd, file);