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 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. 27 */ 28 #define MEMFD_TAG_PINNED PAGECACHE_TAG_TOWRITE 29 #define LAST_SCAN 4 /* about 150ms max */ 30 31 static void memfd_tag_pins(struct address_space *mapping) 32 { 33 struct radix_tree_iter iter; 34 void __rcu **slot; 35 pgoff_t start; 36 struct page *page; 37 38 lru_add_drain(); 39 start = 0; 40 rcu_read_lock(); 41 42 radix_tree_for_each_slot(slot, &mapping->i_pages, &iter, start) { 43 page = radix_tree_deref_slot(slot); 44 if (!page || radix_tree_exception(page)) { 45 if (radix_tree_deref_retry(page)) { 46 slot = radix_tree_iter_retry(&iter); 47 continue; 48 } 49 } else if (page_count(page) - page_mapcount(page) > 1) { 50 xa_lock_irq(&mapping->i_pages); 51 radix_tree_tag_set(&mapping->i_pages, iter.index, 52 MEMFD_TAG_PINNED); 53 xa_unlock_irq(&mapping->i_pages); 54 } 55 56 if (need_resched()) { 57 slot = radix_tree_iter_resume(slot, &iter); 58 cond_resched_rcu(); 59 } 60 } 61 rcu_read_unlock(); 62 } 63 64 /* 65 * Setting SEAL_WRITE requires us to verify there's no pending writer. However, 66 * via get_user_pages(), drivers might have some pending I/O without any active 67 * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages 68 * and see whether it has an elevated ref-count. If so, we tag them and wait for 69 * them to be dropped. 70 * The caller must guarantee that no new user will acquire writable references 71 * to those pages to avoid races. 72 */ 73 static int memfd_wait_for_pins(struct address_space *mapping) 74 { 75 struct radix_tree_iter iter; 76 void __rcu **slot; 77 pgoff_t start; 78 struct page *page; 79 int error, scan; 80 81 memfd_tag_pins(mapping); 82 83 error = 0; 84 for (scan = 0; scan <= LAST_SCAN; scan++) { 85 if (!radix_tree_tagged(&mapping->i_pages, MEMFD_TAG_PINNED)) 86 break; 87 88 if (!scan) 89 lru_add_drain_all(); 90 else if (schedule_timeout_killable((HZ << scan) / 200)) 91 scan = LAST_SCAN; 92 93 start = 0; 94 rcu_read_lock(); 95 radix_tree_for_each_tagged(slot, &mapping->i_pages, &iter, 96 start, MEMFD_TAG_PINNED) { 97 98 page = radix_tree_deref_slot(slot); 99 if (radix_tree_exception(page)) { 100 if (radix_tree_deref_retry(page)) { 101 slot = radix_tree_iter_retry(&iter); 102 continue; 103 } 104 105 page = NULL; 106 } 107 108 if (page && 109 page_count(page) - page_mapcount(page) != 1) { 110 if (scan < LAST_SCAN) 111 goto continue_resched; 112 113 /* 114 * On the last scan, we clean up all those tags 115 * we inserted; but make a note that we still 116 * found pages pinned. 117 */ 118 error = -EBUSY; 119 } 120 121 xa_lock_irq(&mapping->i_pages); 122 radix_tree_tag_clear(&mapping->i_pages, 123 iter.index, MEMFD_TAG_PINNED); 124 xa_unlock_irq(&mapping->i_pages); 125 continue_resched: 126 if (need_resched()) { 127 slot = radix_tree_iter_resume(slot, &iter); 128 cond_resched_rcu(); 129 } 130 } 131 rcu_read_unlock(); 132 } 133 134 return error; 135 } 136 137 static unsigned int *memfd_file_seals_ptr(struct file *file) 138 { 139 if (shmem_file(file)) 140 return &SHMEM_I(file_inode(file))->seals; 141 142 #ifdef CONFIG_HUGETLBFS 143 if (is_file_hugepages(file)) 144 return &HUGETLBFS_I(file_inode(file))->seals; 145 #endif 146 147 return NULL; 148 } 149 150 #define F_ALL_SEALS (F_SEAL_SEAL | \ 151 F_SEAL_SHRINK | \ 152 F_SEAL_GROW | \ 153 F_SEAL_WRITE) 154 155 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 } 229 230 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 } 236 237 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) 265 266 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 struct user_struct *user = NULL; 317 318 file = hugetlb_file_setup(name, 0, VM_NORESERVE, &user, 319 HUGETLB_ANONHUGE_INODE, 320 (flags >> MFD_HUGE_SHIFT) & 321 MFD_HUGE_MASK); 322 } else 323 file = shmem_file_setup(name, 0, VM_NORESERVE); 324 if (IS_ERR(file)) { 325 error = PTR_ERR(file); 326 goto err_fd; 327 } 328 file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE; 329 file->f_flags |= O_RDWR | O_LARGEFILE; 330 331 if (flags & MFD_ALLOW_SEALING) { 332 file_seals = memfd_file_seals_ptr(file); 333 *file_seals &= ~F_SEAL_SEAL; 334 } 335 336 fd_install(fd, file); 337 kfree(name); 338 return fd; 339 340 err_fd: 341 put_unused_fd(fd); 342 err_name: 343 kfree(name); 344 return error; 345 } 346