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 <linux/pid_namespace.h> 22 #include <uapi/linux/memfd.h> 23 #include "swap.h" 24 25 /* 26 * We need a tag: a new tag would expand every xa_node by 8 bytes, 27 * so reuse a tag which we firmly believe is never set or cleared on tmpfs 28 * or hugetlbfs because they are memory only filesystems. 29 */ 30 #define MEMFD_TAG_PINNED PAGECACHE_TAG_TOWRITE 31 #define LAST_SCAN 4 /* about 150ms max */ 32 33 static bool memfd_folio_has_extra_refs(struct folio *folio) 34 { 35 return folio_ref_count(folio) != folio_expected_ref_count(folio); 36 } 37 38 static void memfd_tag_pins(struct xa_state *xas) 39 { 40 struct folio *folio; 41 int latency = 0; 42 43 lru_add_drain(); 44 45 xas_lock_irq(xas); 46 xas_for_each(xas, folio, ULONG_MAX) { 47 if (!xa_is_value(folio) && memfd_folio_has_extra_refs(folio)) 48 xas_set_mark(xas, MEMFD_TAG_PINNED); 49 50 if (++latency < XA_CHECK_SCHED) 51 continue; 52 latency = 0; 53 54 xas_pause(xas); 55 xas_unlock_irq(xas); 56 cond_resched(); 57 xas_lock_irq(xas); 58 } 59 xas_unlock_irq(xas); 60 } 61 62 /* 63 * This is a helper function used by memfd_pin_user_pages() in GUP (gup.c). 64 * It is mainly called to allocate a folio in a memfd when the caller 65 * (memfd_pin_folios()) cannot find a folio in the page cache at a given 66 * index in the mapping. 67 */ 68 struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx) 69 { 70 #ifdef CONFIG_HUGETLB_PAGE 71 struct folio *folio; 72 gfp_t gfp_mask; 73 74 if (is_file_hugepages(memfd)) { 75 /* 76 * The folio would most likely be accessed by a DMA driver, 77 * therefore, we have zone memory constraints where we can 78 * alloc from. Also, the folio will be pinned for an indefinite 79 * amount of time, so it is not expected to be migrated away. 80 */ 81 struct inode *inode = file_inode(memfd); 82 struct hstate *h = hstate_file(memfd); 83 int err = -ENOMEM; 84 long nr_resv; 85 86 gfp_mask = htlb_alloc_mask(h); 87 gfp_mask &= ~(__GFP_HIGHMEM | __GFP_MOVABLE); 88 idx >>= huge_page_order(h); 89 90 nr_resv = hugetlb_reserve_pages(inode, idx, idx + 1, NULL, 0); 91 if (nr_resv < 0) 92 return ERR_PTR(nr_resv); 93 94 folio = alloc_hugetlb_folio_reserve(h, 95 numa_node_id(), 96 NULL, 97 gfp_mask); 98 if (folio) { 99 err = hugetlb_add_to_page_cache(folio, 100 memfd->f_mapping, 101 idx); 102 if (err) { 103 folio_put(folio); 104 goto err_unresv; 105 } 106 107 hugetlb_set_folio_subpool(folio, subpool_inode(inode)); 108 folio_unlock(folio); 109 return folio; 110 } 111 err_unresv: 112 if (nr_resv > 0) 113 hugetlb_unreserve_pages(inode, idx, idx + 1, 0); 114 return ERR_PTR(err); 115 } 116 #endif 117 return shmem_read_folio(memfd->f_mapping, idx); 118 } 119 120 /* 121 * Setting SEAL_WRITE requires us to verify there's no pending writer. However, 122 * via get_user_pages(), drivers might have some pending I/O without any active 123 * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all folios 124 * and see whether it has an elevated ref-count. If so, we tag them and wait for 125 * them to be dropped. 126 * The caller must guarantee that no new user will acquire writable references 127 * to those folios to avoid races. 128 */ 129 static int memfd_wait_for_pins(struct address_space *mapping) 130 { 131 XA_STATE(xas, &mapping->i_pages, 0); 132 struct folio *folio; 133 int error, scan; 134 135 memfd_tag_pins(&xas); 136 137 error = 0; 138 for (scan = 0; scan <= LAST_SCAN; scan++) { 139 int latency = 0; 140 141 if (!xas_marked(&xas, MEMFD_TAG_PINNED)) 142 break; 143 144 if (!scan) 145 lru_add_drain_all(); 146 else if (schedule_timeout_killable((HZ << scan) / 200)) 147 scan = LAST_SCAN; 148 149 xas_set(&xas, 0); 150 xas_lock_irq(&xas); 151 xas_for_each_marked(&xas, folio, ULONG_MAX, MEMFD_TAG_PINNED) { 152 bool clear = true; 153 154 if (!xa_is_value(folio) && 155 memfd_folio_has_extra_refs(folio)) { 156 /* 157 * On the last scan, we clean up all those tags 158 * we inserted; but make a note that we still 159 * found folios pinned. 160 */ 161 if (scan == LAST_SCAN) 162 error = -EBUSY; 163 else 164 clear = false; 165 } 166 if (clear) 167 xas_clear_mark(&xas, MEMFD_TAG_PINNED); 168 169 if (++latency < XA_CHECK_SCHED) 170 continue; 171 latency = 0; 172 173 xas_pause(&xas); 174 xas_unlock_irq(&xas); 175 cond_resched(); 176 xas_lock_irq(&xas); 177 } 178 xas_unlock_irq(&xas); 179 } 180 181 return error; 182 } 183 184 static unsigned int *memfd_file_seals_ptr(struct file *file) 185 { 186 if (shmem_file(file)) 187 return &SHMEM_I(file_inode(file))->seals; 188 189 #ifdef CONFIG_HUGETLBFS 190 if (is_file_hugepages(file)) 191 return &HUGETLBFS_I(file_inode(file))->seals; 192 #endif 193 194 return NULL; 195 } 196 197 #define F_ALL_SEALS (F_SEAL_SEAL | \ 198 F_SEAL_EXEC | \ 199 F_SEAL_SHRINK | \ 200 F_SEAL_GROW | \ 201 F_SEAL_WRITE | \ 202 F_SEAL_FUTURE_WRITE) 203 204 static int memfd_add_seals(struct file *file, unsigned int seals) 205 { 206 struct inode *inode = file_inode(file); 207 unsigned int *file_seals; 208 int error; 209 210 /* 211 * SEALING 212 * Sealing allows multiple parties to share a tmpfs or hugetlbfs file 213 * but restrict access to a specific subset of file operations. Seals 214 * can only be added, but never removed. This way, mutually untrusted 215 * parties can share common memory regions with a well-defined policy. 216 * A malicious peer can thus never perform unwanted operations on a 217 * shared object. 218 * 219 * Seals are only supported on special tmpfs or hugetlbfs files and 220 * always affect the whole underlying inode. Once a seal is set, it 221 * may prevent some kinds of access to the file. Currently, the 222 * following seals are defined: 223 * SEAL_SEAL: Prevent further seals from being set on this file 224 * SEAL_SHRINK: Prevent the file from shrinking 225 * SEAL_GROW: Prevent the file from growing 226 * SEAL_WRITE: Prevent write access to the file 227 * SEAL_EXEC: Prevent modification of the exec bits in the file mode 228 * 229 * As we don't require any trust relationship between two parties, we 230 * must prevent seals from being removed. Therefore, sealing a file 231 * only adds a given set of seals to the file, it never touches 232 * existing seals. Furthermore, the "setting seals"-operation can be 233 * sealed itself, which basically prevents any further seal from being 234 * added. 235 * 236 * Semantics of sealing are only defined on volatile files. Only 237 * anonymous tmpfs and hugetlbfs files support sealing. More 238 * importantly, seals are never written to disk. Therefore, there's 239 * no plan to support it on other file types. 240 */ 241 242 if (!(file->f_mode & FMODE_WRITE)) 243 return -EPERM; 244 if (seals & ~(unsigned int)F_ALL_SEALS) 245 return -EINVAL; 246 247 inode_lock(inode); 248 249 file_seals = memfd_file_seals_ptr(file); 250 if (!file_seals) { 251 error = -EINVAL; 252 goto unlock; 253 } 254 255 if (*file_seals & F_SEAL_SEAL) { 256 error = -EPERM; 257 goto unlock; 258 } 259 260 if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) { 261 error = mapping_deny_writable(file->f_mapping); 262 if (error) 263 goto unlock; 264 265 error = memfd_wait_for_pins(file->f_mapping); 266 if (error) { 267 mapping_allow_writable(file->f_mapping); 268 goto unlock; 269 } 270 } 271 272 /* 273 * SEAL_EXEC implies SEAL_WRITE, making W^X from the start. 274 */ 275 if (seals & F_SEAL_EXEC && inode->i_mode & 0111) 276 seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE; 277 278 *file_seals |= seals; 279 error = 0; 280 281 unlock: 282 inode_unlock(inode); 283 return error; 284 } 285 286 static int memfd_get_seals(struct file *file) 287 { 288 unsigned int *seals = memfd_file_seals_ptr(file); 289 290 return seals ? *seals : -EINVAL; 291 } 292 293 long memfd_fcntl(struct file *file, unsigned int cmd, unsigned int arg) 294 { 295 long error; 296 297 switch (cmd) { 298 case F_ADD_SEALS: 299 error = memfd_add_seals(file, arg); 300 break; 301 case F_GET_SEALS: 302 error = memfd_get_seals(file); 303 break; 304 default: 305 error = -EINVAL; 306 break; 307 } 308 309 return error; 310 } 311 312 #define MFD_NAME_PREFIX "memfd:" 313 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1) 314 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN) 315 316 #define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB | MFD_NOEXEC_SEAL | MFD_EXEC) 317 318 static int check_sysctl_memfd_noexec(unsigned int *flags) 319 { 320 #ifdef CONFIG_SYSCTL 321 struct pid_namespace *ns = task_active_pid_ns(current); 322 int sysctl = pidns_memfd_noexec_scope(ns); 323 324 if (!(*flags & (MFD_EXEC | MFD_NOEXEC_SEAL))) { 325 if (sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL) 326 *flags |= MFD_NOEXEC_SEAL; 327 else 328 *flags |= MFD_EXEC; 329 } 330 331 if (!(*flags & MFD_NOEXEC_SEAL) && sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_ENFORCED) { 332 pr_err_ratelimited( 333 "%s[%d]: memfd_create() requires MFD_NOEXEC_SEAL with vm.memfd_noexec=%d\n", 334 current->comm, task_pid_nr(current), sysctl); 335 return -EACCES; 336 } 337 #endif 338 return 0; 339 } 340 341 static inline bool is_write_sealed(unsigned int seals) 342 { 343 return seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE); 344 } 345 346 static int check_write_seal(vm_flags_t *vm_flags_ptr) 347 { 348 vm_flags_t vm_flags = *vm_flags_ptr; 349 vm_flags_t mask = vm_flags & (VM_SHARED | VM_WRITE); 350 351 /* If a private mapping then writability is irrelevant. */ 352 if (!(mask & VM_SHARED)) 353 return 0; 354 355 /* 356 * New PROT_WRITE and MAP_SHARED mmaps are not allowed when 357 * write seals are active. 358 */ 359 if (mask & VM_WRITE) 360 return -EPERM; 361 362 /* 363 * This is a read-only mapping, disallow mprotect() from making a 364 * write-sealed mapping writable in future. 365 */ 366 *vm_flags_ptr &= ~VM_MAYWRITE; 367 368 return 0; 369 } 370 371 int memfd_check_seals_mmap(struct file *file, vm_flags_t *vm_flags_ptr) 372 { 373 int err = 0; 374 unsigned int *seals_ptr = memfd_file_seals_ptr(file); 375 unsigned int seals = seals_ptr ? *seals_ptr : 0; 376 377 if (is_write_sealed(seals)) 378 err = check_write_seal(vm_flags_ptr); 379 380 return err; 381 } 382 383 static int sanitize_flags(unsigned int *flags_ptr) 384 { 385 unsigned int flags = *flags_ptr; 386 387 if (!(flags & MFD_HUGETLB)) { 388 if (flags & ~(unsigned int)MFD_ALL_FLAGS) 389 return -EINVAL; 390 } else { 391 /* Allow huge page size encoding in flags. */ 392 if (flags & ~(unsigned int)(MFD_ALL_FLAGS | 393 (MFD_HUGE_MASK << MFD_HUGE_SHIFT))) 394 return -EINVAL; 395 } 396 397 /* Invalid if both EXEC and NOEXEC_SEAL are set.*/ 398 if ((flags & MFD_EXEC) && (flags & MFD_NOEXEC_SEAL)) 399 return -EINVAL; 400 401 return check_sysctl_memfd_noexec(flags_ptr); 402 } 403 404 static char *alloc_name(const char __user *uname) 405 { 406 int error; 407 char *name; 408 long len; 409 410 name = kmalloc(NAME_MAX + 1, GFP_KERNEL); 411 if (!name) 412 return ERR_PTR(-ENOMEM); 413 414 memcpy(name, MFD_NAME_PREFIX, MFD_NAME_PREFIX_LEN); 415 /* returned length does not include terminating zero */ 416 len = strncpy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, MFD_NAME_MAX_LEN + 1); 417 if (len < 0) { 418 error = -EFAULT; 419 goto err_name; 420 } else if (len > MFD_NAME_MAX_LEN) { 421 error = -EINVAL; 422 goto err_name; 423 } 424 425 return name; 426 427 err_name: 428 kfree(name); 429 return ERR_PTR(error); 430 } 431 432 static struct file *alloc_file(const char *name, unsigned int flags) 433 { 434 unsigned int *file_seals; 435 struct file *file; 436 437 if (flags & MFD_HUGETLB) { 438 file = hugetlb_file_setup(name, 0, VM_NORESERVE, 439 HUGETLB_ANONHUGE_INODE, 440 (flags >> MFD_HUGE_SHIFT) & 441 MFD_HUGE_MASK); 442 } else { 443 file = shmem_file_setup(name, 0, VM_NORESERVE); 444 } 445 if (IS_ERR(file)) 446 return file; 447 file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE; 448 file->f_flags |= O_LARGEFILE; 449 450 if (flags & MFD_NOEXEC_SEAL) { 451 struct inode *inode = file_inode(file); 452 453 inode->i_mode &= ~0111; 454 file_seals = memfd_file_seals_ptr(file); 455 if (file_seals) { 456 *file_seals &= ~F_SEAL_SEAL; 457 *file_seals |= F_SEAL_EXEC; 458 } 459 } else if (flags & MFD_ALLOW_SEALING) { 460 /* MFD_EXEC and MFD_ALLOW_SEALING are set */ 461 file_seals = memfd_file_seals_ptr(file); 462 if (file_seals) 463 *file_seals &= ~F_SEAL_SEAL; 464 } 465 466 return file; 467 } 468 469 SYSCALL_DEFINE2(memfd_create, 470 const char __user *, uname, 471 unsigned int, flags) 472 { 473 struct file *file; 474 int fd, error; 475 char *name; 476 477 error = sanitize_flags(&flags); 478 if (error < 0) 479 return error; 480 481 name = alloc_name(uname); 482 if (IS_ERR(name)) 483 return PTR_ERR(name); 484 485 fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0); 486 if (fd < 0) { 487 error = fd; 488 goto err_free_name; 489 } 490 491 file = alloc_file(name, flags); 492 if (IS_ERR(file)) { 493 error = PTR_ERR(file); 494 goto err_free_fd; 495 } 496 497 fd_install(fd, file); 498 kfree(name); 499 return fd; 500 501 err_free_fd: 502 put_unused_fd(fd); 503 err_free_name: 504 kfree(name); 505 return error; 506 } 507