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 struct hstate *h = hstate_file(memfd); 83 84 gfp_mask = htlb_alloc_mask(h); 85 gfp_mask &= ~(__GFP_HIGHMEM | __GFP_MOVABLE); 86 idx >>= huge_page_order(h); 87 88 folio = alloc_hugetlb_folio_reserve(h, 89 numa_node_id(), 90 NULL, 91 gfp_mask); 92 if (folio) { 93 err = hugetlb_add_to_page_cache(folio, 94 memfd->f_mapping, 95 idx); 96 if (err) { 97 folio_put(folio); 98 return ERR_PTR(err); 99 } 100 folio_unlock(folio); 101 return folio; 102 } 103 return ERR_PTR(-ENOMEM); 104 } 105 #endif 106 return shmem_read_folio(memfd->f_mapping, idx); 107 } 108 109 /* 110 * Setting SEAL_WRITE requires us to verify there's no pending writer. However, 111 * via get_user_pages(), drivers might have some pending I/O without any active 112 * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all folios 113 * and see whether it has an elevated ref-count. If so, we tag them and wait for 114 * them to be dropped. 115 * The caller must guarantee that no new user will acquire writable references 116 * to those folios to avoid races. 117 */ 118 static int memfd_wait_for_pins(struct address_space *mapping) 119 { 120 XA_STATE(xas, &mapping->i_pages, 0); 121 struct folio *folio; 122 int error, scan; 123 124 memfd_tag_pins(&xas); 125 126 error = 0; 127 for (scan = 0; scan <= LAST_SCAN; scan++) { 128 int latency = 0; 129 130 if (!xas_marked(&xas, MEMFD_TAG_PINNED)) 131 break; 132 133 if (!scan) 134 lru_add_drain_all(); 135 else if (schedule_timeout_killable((HZ << scan) / 200)) 136 scan = LAST_SCAN; 137 138 xas_set(&xas, 0); 139 xas_lock_irq(&xas); 140 xas_for_each_marked(&xas, folio, ULONG_MAX, MEMFD_TAG_PINNED) { 141 bool clear = true; 142 143 if (!xa_is_value(folio) && 144 memfd_folio_has_extra_refs(folio)) { 145 /* 146 * On the last scan, we clean up all those tags 147 * we inserted; but make a note that we still 148 * found folios pinned. 149 */ 150 if (scan == LAST_SCAN) 151 error = -EBUSY; 152 else 153 clear = false; 154 } 155 if (clear) 156 xas_clear_mark(&xas, MEMFD_TAG_PINNED); 157 158 if (++latency < XA_CHECK_SCHED) 159 continue; 160 latency = 0; 161 162 xas_pause(&xas); 163 xas_unlock_irq(&xas); 164 cond_resched(); 165 xas_lock_irq(&xas); 166 } 167 xas_unlock_irq(&xas); 168 } 169 170 return error; 171 } 172 173 static unsigned int *memfd_file_seals_ptr(struct file *file) 174 { 175 if (shmem_file(file)) 176 return &SHMEM_I(file_inode(file))->seals; 177 178 #ifdef CONFIG_HUGETLBFS 179 if (is_file_hugepages(file)) 180 return &HUGETLBFS_I(file_inode(file))->seals; 181 #endif 182 183 return NULL; 184 } 185 186 #define F_ALL_SEALS (F_SEAL_SEAL | \ 187 F_SEAL_EXEC | \ 188 F_SEAL_SHRINK | \ 189 F_SEAL_GROW | \ 190 F_SEAL_WRITE | \ 191 F_SEAL_FUTURE_WRITE) 192 193 static int memfd_add_seals(struct file *file, unsigned int seals) 194 { 195 struct inode *inode = file_inode(file); 196 unsigned int *file_seals; 197 int error; 198 199 /* 200 * SEALING 201 * Sealing allows multiple parties to share a tmpfs or hugetlbfs file 202 * but restrict access to a specific subset of file operations. Seals 203 * can only be added, but never removed. This way, mutually untrusted 204 * parties can share common memory regions with a well-defined policy. 205 * A malicious peer can thus never perform unwanted operations on a 206 * shared object. 207 * 208 * Seals are only supported on special tmpfs or hugetlbfs files and 209 * always affect the whole underlying inode. Once a seal is set, it 210 * may prevent some kinds of access to the file. Currently, the 211 * following seals are defined: 212 * SEAL_SEAL: Prevent further seals from being set on this file 213 * SEAL_SHRINK: Prevent the file from shrinking 214 * SEAL_GROW: Prevent the file from growing 215 * SEAL_WRITE: Prevent write access to the file 216 * SEAL_EXEC: Prevent modification of the exec bits in the file mode 217 * 218 * As we don't require any trust relationship between two parties, we 219 * must prevent seals from being removed. Therefore, sealing a file 220 * only adds a given set of seals to the file, it never touches 221 * existing seals. Furthermore, the "setting seals"-operation can be 222 * sealed itself, which basically prevents any further seal from being 223 * added. 224 * 225 * Semantics of sealing are only defined on volatile files. Only 226 * anonymous tmpfs and hugetlbfs files support sealing. More 227 * importantly, seals are never written to disk. Therefore, there's 228 * no plan to support it on other file types. 229 */ 230 231 if (!(file->f_mode & FMODE_WRITE)) 232 return -EPERM; 233 if (seals & ~(unsigned int)F_ALL_SEALS) 234 return -EINVAL; 235 236 inode_lock(inode); 237 238 file_seals = memfd_file_seals_ptr(file); 239 if (!file_seals) { 240 error = -EINVAL; 241 goto unlock; 242 } 243 244 if (*file_seals & F_SEAL_SEAL) { 245 error = -EPERM; 246 goto unlock; 247 } 248 249 if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) { 250 error = mapping_deny_writable(file->f_mapping); 251 if (error) 252 goto unlock; 253 254 error = memfd_wait_for_pins(file->f_mapping); 255 if (error) { 256 mapping_allow_writable(file->f_mapping); 257 goto unlock; 258 } 259 } 260 261 /* 262 * SEAL_EXEC implys SEAL_WRITE, making W^X from the start. 263 */ 264 if (seals & F_SEAL_EXEC && inode->i_mode & 0111) 265 seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE; 266 267 *file_seals |= seals; 268 error = 0; 269 270 unlock: 271 inode_unlock(inode); 272 return error; 273 } 274 275 static int memfd_get_seals(struct file *file) 276 { 277 unsigned int *seals = memfd_file_seals_ptr(file); 278 279 return seals ? *seals : -EINVAL; 280 } 281 282 long memfd_fcntl(struct file *file, unsigned int cmd, unsigned int arg) 283 { 284 long error; 285 286 switch (cmd) { 287 case F_ADD_SEALS: 288 error = memfd_add_seals(file, arg); 289 break; 290 case F_GET_SEALS: 291 error = memfd_get_seals(file); 292 break; 293 default: 294 error = -EINVAL; 295 break; 296 } 297 298 return error; 299 } 300 301 #define MFD_NAME_PREFIX "memfd:" 302 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1) 303 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN) 304 305 #define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB | MFD_NOEXEC_SEAL | MFD_EXEC) 306 307 static int check_sysctl_memfd_noexec(unsigned int *flags) 308 { 309 #ifdef CONFIG_SYSCTL 310 struct pid_namespace *ns = task_active_pid_ns(current); 311 int sysctl = pidns_memfd_noexec_scope(ns); 312 313 if (!(*flags & (MFD_EXEC | MFD_NOEXEC_SEAL))) { 314 if (sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL) 315 *flags |= MFD_NOEXEC_SEAL; 316 else 317 *flags |= MFD_EXEC; 318 } 319 320 if (!(*flags & MFD_NOEXEC_SEAL) && sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_ENFORCED) { 321 pr_err_ratelimited( 322 "%s[%d]: memfd_create() requires MFD_NOEXEC_SEAL with vm.memfd_noexec=%d\n", 323 current->comm, task_pid_nr(current), sysctl); 324 return -EACCES; 325 } 326 #endif 327 return 0; 328 } 329 330 SYSCALL_DEFINE2(memfd_create, 331 const char __user *, uname, 332 unsigned int, flags) 333 { 334 unsigned int *file_seals; 335 struct file *file; 336 int fd, error; 337 char *name; 338 long len; 339 340 if (!(flags & MFD_HUGETLB)) { 341 if (flags & ~(unsigned int)MFD_ALL_FLAGS) 342 return -EINVAL; 343 } else { 344 /* Allow huge page size encoding in flags. */ 345 if (flags & ~(unsigned int)(MFD_ALL_FLAGS | 346 (MFD_HUGE_MASK << MFD_HUGE_SHIFT))) 347 return -EINVAL; 348 } 349 350 /* Invalid if both EXEC and NOEXEC_SEAL are set.*/ 351 if ((flags & MFD_EXEC) && (flags & MFD_NOEXEC_SEAL)) 352 return -EINVAL; 353 354 error = check_sysctl_memfd_noexec(&flags); 355 if (error < 0) 356 return error; 357 358 /* length includes terminating zero */ 359 len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1); 360 if (len <= 0) 361 return -EFAULT; 362 if (len > MFD_NAME_MAX_LEN + 1) 363 return -EINVAL; 364 365 name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL); 366 if (!name) 367 return -ENOMEM; 368 369 strcpy(name, MFD_NAME_PREFIX); 370 if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) { 371 error = -EFAULT; 372 goto err_name; 373 } 374 375 /* terminating-zero may have changed after strnlen_user() returned */ 376 if (name[len + MFD_NAME_PREFIX_LEN - 1]) { 377 error = -EFAULT; 378 goto err_name; 379 } 380 381 fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0); 382 if (fd < 0) { 383 error = fd; 384 goto err_name; 385 } 386 387 if (flags & MFD_HUGETLB) { 388 file = hugetlb_file_setup(name, 0, VM_NORESERVE, 389 HUGETLB_ANONHUGE_INODE, 390 (flags >> MFD_HUGE_SHIFT) & 391 MFD_HUGE_MASK); 392 } else 393 file = shmem_file_setup(name, 0, VM_NORESERVE); 394 if (IS_ERR(file)) { 395 error = PTR_ERR(file); 396 goto err_fd; 397 } 398 file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE; 399 file->f_flags |= O_LARGEFILE; 400 401 if (flags & MFD_NOEXEC_SEAL) { 402 struct inode *inode = file_inode(file); 403 404 inode->i_mode &= ~0111; 405 file_seals = memfd_file_seals_ptr(file); 406 if (file_seals) { 407 *file_seals &= ~F_SEAL_SEAL; 408 *file_seals |= F_SEAL_EXEC; 409 } 410 } else if (flags & MFD_ALLOW_SEALING) { 411 /* MFD_EXEC and MFD_ALLOW_SEALING are set */ 412 file_seals = memfd_file_seals_ptr(file); 413 if (file_seals) 414 *file_seals &= ~F_SEAL_SEAL; 415 } 416 417 fd_install(fd, file); 418 kfree(name); 419 return fd; 420 421 err_fd: 422 put_unused_fd(fd); 423 err_name: 424 kfree(name); 425 return error; 426 } 427