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 24 /* 25 * We need a tag: a new tag would expand every xa_node by 8 bytes, 26 * so reuse a tag which we firmly believe is never set or cleared on tmpfs 27 * or hugetlbfs because they are memory only filesystems. 28 */ 29 #define MEMFD_TAG_PINNED PAGECACHE_TAG_TOWRITE 30 #define LAST_SCAN 4 /* about 150ms max */ 31 32 static bool memfd_folio_has_extra_refs(struct folio *folio) 33 { 34 return folio_ref_count(folio) - folio_mapcount(folio) != 35 folio_nr_pages(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 int err; 74 75 if (is_file_hugepages(memfd)) { 76 /* 77 * The folio would most likely be accessed by a DMA driver, 78 * therefore, we have zone memory constraints where we can 79 * alloc from. Also, the folio will be pinned for an indefinite 80 * amount of time, so it is not expected to be migrated away. 81 */ 82 gfp_mask = htlb_alloc_mask(hstate_file(memfd)); 83 gfp_mask &= ~(__GFP_HIGHMEM | __GFP_MOVABLE); 84 85 folio = alloc_hugetlb_folio_nodemask(hstate_file(memfd), 86 numa_node_id(), 87 NULL, 88 gfp_mask, 89 false); 90 if (folio && folio_try_get(folio)) { 91 err = hugetlb_add_to_page_cache(folio, 92 memfd->f_mapping, 93 idx); 94 if (err) { 95 folio_put(folio); 96 free_huge_folio(folio); 97 return ERR_PTR(err); 98 } 99 return folio; 100 } 101 return ERR_PTR(-ENOMEM); 102 } 103 #endif 104 return shmem_read_folio(memfd->f_mapping, idx); 105 } 106 107 /* 108 * Setting SEAL_WRITE requires us to verify there's no pending writer. However, 109 * via get_user_pages(), drivers might have some pending I/O without any active 110 * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all folios 111 * and see whether it has an elevated ref-count. If so, we tag them and wait for 112 * them to be dropped. 113 * The caller must guarantee that no new user will acquire writable references 114 * to those folios to avoid races. 115 */ 116 static int memfd_wait_for_pins(struct address_space *mapping) 117 { 118 XA_STATE(xas, &mapping->i_pages, 0); 119 struct folio *folio; 120 int error, scan; 121 122 memfd_tag_pins(&xas); 123 124 error = 0; 125 for (scan = 0; scan <= LAST_SCAN; scan++) { 126 int latency = 0; 127 128 if (!xas_marked(&xas, MEMFD_TAG_PINNED)) 129 break; 130 131 if (!scan) 132 lru_add_drain_all(); 133 else if (schedule_timeout_killable((HZ << scan) / 200)) 134 scan = LAST_SCAN; 135 136 xas_set(&xas, 0); 137 xas_lock_irq(&xas); 138 xas_for_each_marked(&xas, folio, ULONG_MAX, MEMFD_TAG_PINNED) { 139 bool clear = true; 140 141 if (!xa_is_value(folio) && 142 memfd_folio_has_extra_refs(folio)) { 143 /* 144 * On the last scan, we clean up all those tags 145 * we inserted; but make a note that we still 146 * found folios pinned. 147 */ 148 if (scan == LAST_SCAN) 149 error = -EBUSY; 150 else 151 clear = false; 152 } 153 if (clear) 154 xas_clear_mark(&xas, MEMFD_TAG_PINNED); 155 156 if (++latency < XA_CHECK_SCHED) 157 continue; 158 latency = 0; 159 160 xas_pause(&xas); 161 xas_unlock_irq(&xas); 162 cond_resched(); 163 xas_lock_irq(&xas); 164 } 165 xas_unlock_irq(&xas); 166 } 167 168 return error; 169 } 170 171 static unsigned int *memfd_file_seals_ptr(struct file *file) 172 { 173 if (shmem_file(file)) 174 return &SHMEM_I(file_inode(file))->seals; 175 176 #ifdef CONFIG_HUGETLBFS 177 if (is_file_hugepages(file)) 178 return &HUGETLBFS_I(file_inode(file))->seals; 179 #endif 180 181 return NULL; 182 } 183 184 #define F_ALL_SEALS (F_SEAL_SEAL | \ 185 F_SEAL_EXEC | \ 186 F_SEAL_SHRINK | \ 187 F_SEAL_GROW | \ 188 F_SEAL_WRITE | \ 189 F_SEAL_FUTURE_WRITE) 190 191 static int memfd_add_seals(struct file *file, unsigned int seals) 192 { 193 struct inode *inode = file_inode(file); 194 unsigned int *file_seals; 195 int error; 196 197 /* 198 * SEALING 199 * Sealing allows multiple parties to share a tmpfs or hugetlbfs file 200 * but restrict access to a specific subset of file operations. Seals 201 * can only be added, but never removed. This way, mutually untrusted 202 * parties can share common memory regions with a well-defined policy. 203 * A malicious peer can thus never perform unwanted operations on a 204 * shared object. 205 * 206 * Seals are only supported on special tmpfs or hugetlbfs files and 207 * always affect the whole underlying inode. Once a seal is set, it 208 * may prevent some kinds of access to the file. Currently, the 209 * following seals are defined: 210 * SEAL_SEAL: Prevent further seals from being set on this file 211 * SEAL_SHRINK: Prevent the file from shrinking 212 * SEAL_GROW: Prevent the file from growing 213 * SEAL_WRITE: Prevent write access to the file 214 * SEAL_EXEC: Prevent modification of the exec bits in the file mode 215 * 216 * As we don't require any trust relationship between two parties, we 217 * must prevent seals from being removed. Therefore, sealing a file 218 * only adds a given set of seals to the file, it never touches 219 * existing seals. Furthermore, the "setting seals"-operation can be 220 * sealed itself, which basically prevents any further seal from being 221 * added. 222 * 223 * Semantics of sealing are only defined on volatile files. Only 224 * anonymous tmpfs and hugetlbfs files support sealing. More 225 * importantly, seals are never written to disk. Therefore, there's 226 * no plan to support it on other file types. 227 */ 228 229 if (!(file->f_mode & FMODE_WRITE)) 230 return -EPERM; 231 if (seals & ~(unsigned int)F_ALL_SEALS) 232 return -EINVAL; 233 234 inode_lock(inode); 235 236 file_seals = memfd_file_seals_ptr(file); 237 if (!file_seals) { 238 error = -EINVAL; 239 goto unlock; 240 } 241 242 if (*file_seals & F_SEAL_SEAL) { 243 error = -EPERM; 244 goto unlock; 245 } 246 247 if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) { 248 error = mapping_deny_writable(file->f_mapping); 249 if (error) 250 goto unlock; 251 252 error = memfd_wait_for_pins(file->f_mapping); 253 if (error) { 254 mapping_allow_writable(file->f_mapping); 255 goto unlock; 256 } 257 } 258 259 /* 260 * SEAL_EXEC implys SEAL_WRITE, making W^X from the start. 261 */ 262 if (seals & F_SEAL_EXEC && inode->i_mode & 0111) 263 seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE; 264 265 *file_seals |= seals; 266 error = 0; 267 268 unlock: 269 inode_unlock(inode); 270 return error; 271 } 272 273 static int memfd_get_seals(struct file *file) 274 { 275 unsigned int *seals = memfd_file_seals_ptr(file); 276 277 return seals ? *seals : -EINVAL; 278 } 279 280 long memfd_fcntl(struct file *file, unsigned int cmd, unsigned int arg) 281 { 282 long error; 283 284 switch (cmd) { 285 case F_ADD_SEALS: 286 error = memfd_add_seals(file, arg); 287 break; 288 case F_GET_SEALS: 289 error = memfd_get_seals(file); 290 break; 291 default: 292 error = -EINVAL; 293 break; 294 } 295 296 return error; 297 } 298 299 #define MFD_NAME_PREFIX "memfd:" 300 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1) 301 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN) 302 303 #define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB | MFD_NOEXEC_SEAL | MFD_EXEC) 304 305 static int check_sysctl_memfd_noexec(unsigned int *flags) 306 { 307 #ifdef CONFIG_SYSCTL 308 struct pid_namespace *ns = task_active_pid_ns(current); 309 int sysctl = pidns_memfd_noexec_scope(ns); 310 311 if (!(*flags & (MFD_EXEC | MFD_NOEXEC_SEAL))) { 312 if (sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL) 313 *flags |= MFD_NOEXEC_SEAL; 314 else 315 *flags |= MFD_EXEC; 316 } 317 318 if (!(*flags & MFD_NOEXEC_SEAL) && sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_ENFORCED) { 319 pr_err_ratelimited( 320 "%s[%d]: memfd_create() requires MFD_NOEXEC_SEAL with vm.memfd_noexec=%d\n", 321 current->comm, task_pid_nr(current), sysctl); 322 return -EACCES; 323 } 324 #endif 325 return 0; 326 } 327 328 SYSCALL_DEFINE2(memfd_create, 329 const char __user *, uname, 330 unsigned int, flags) 331 { 332 unsigned int *file_seals; 333 struct file *file; 334 int fd, error; 335 char *name; 336 long len; 337 338 if (!(flags & MFD_HUGETLB)) { 339 if (flags & ~(unsigned int)MFD_ALL_FLAGS) 340 return -EINVAL; 341 } else { 342 /* Allow huge page size encoding in flags. */ 343 if (flags & ~(unsigned int)(MFD_ALL_FLAGS | 344 (MFD_HUGE_MASK << MFD_HUGE_SHIFT))) 345 return -EINVAL; 346 } 347 348 /* Invalid if both EXEC and NOEXEC_SEAL are set.*/ 349 if ((flags & MFD_EXEC) && (flags & MFD_NOEXEC_SEAL)) 350 return -EINVAL; 351 352 error = check_sysctl_memfd_noexec(&flags); 353 if (error < 0) 354 return error; 355 356 /* length includes terminating zero */ 357 len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1); 358 if (len <= 0) 359 return -EFAULT; 360 if (len > MFD_NAME_MAX_LEN + 1) 361 return -EINVAL; 362 363 name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL); 364 if (!name) 365 return -ENOMEM; 366 367 strcpy(name, MFD_NAME_PREFIX); 368 if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) { 369 error = -EFAULT; 370 goto err_name; 371 } 372 373 /* terminating-zero may have changed after strnlen_user() returned */ 374 if (name[len + MFD_NAME_PREFIX_LEN - 1]) { 375 error = -EFAULT; 376 goto err_name; 377 } 378 379 fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0); 380 if (fd < 0) { 381 error = fd; 382 goto err_name; 383 } 384 385 if (flags & MFD_HUGETLB) { 386 file = hugetlb_file_setup(name, 0, VM_NORESERVE, 387 HUGETLB_ANONHUGE_INODE, 388 (flags >> MFD_HUGE_SHIFT) & 389 MFD_HUGE_MASK); 390 } else 391 file = shmem_file_setup(name, 0, VM_NORESERVE); 392 if (IS_ERR(file)) { 393 error = PTR_ERR(file); 394 goto err_fd; 395 } 396 file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE; 397 file->f_flags |= O_LARGEFILE; 398 399 if (flags & MFD_NOEXEC_SEAL) { 400 struct inode *inode = file_inode(file); 401 402 inode->i_mode &= ~0111; 403 file_seals = memfd_file_seals_ptr(file); 404 if (file_seals) { 405 *file_seals &= ~F_SEAL_SEAL; 406 *file_seals |= F_SEAL_EXEC; 407 } 408 } else if (flags & MFD_ALLOW_SEALING) { 409 /* MFD_EXEC and MFD_ALLOW_SEALING are set */ 410 file_seals = memfd_file_seals_ptr(file); 411 if (file_seals) 412 *file_seals &= ~F_SEAL_SEAL; 413 } 414 415 fd_install(fd, file); 416 kfree(name); 417 return fd; 418 419 err_fd: 420 put_unused_fd(fd); 421 err_name: 422 kfree(name); 423 return error; 424 } 425