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 * Setting SEAL_WRITE requires us to verify there's no pending writer. However, 64 * via get_user_pages(), drivers might have some pending I/O without any active 65 * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all folios 66 * and see whether it has an elevated ref-count. If so, we tag them and wait for 67 * them to be dropped. 68 * The caller must guarantee that no new user will acquire writable references 69 * to those folios to avoid races. 70 */ 71 static int memfd_wait_for_pins(struct address_space *mapping) 72 { 73 XA_STATE(xas, &mapping->i_pages, 0); 74 struct folio *folio; 75 int error, scan; 76 77 memfd_tag_pins(&xas); 78 79 error = 0; 80 for (scan = 0; scan <= LAST_SCAN; scan++) { 81 int latency = 0; 82 83 if (!xas_marked(&xas, MEMFD_TAG_PINNED)) 84 break; 85 86 if (!scan) 87 lru_add_drain_all(); 88 else if (schedule_timeout_killable((HZ << scan) / 200)) 89 scan = LAST_SCAN; 90 91 xas_set(&xas, 0); 92 xas_lock_irq(&xas); 93 xas_for_each_marked(&xas, folio, ULONG_MAX, MEMFD_TAG_PINNED) { 94 bool clear = true; 95 96 if (!xa_is_value(folio) && 97 memfd_folio_has_extra_refs(folio)) { 98 /* 99 * On the last scan, we clean up all those tags 100 * we inserted; but make a note that we still 101 * found folios pinned. 102 */ 103 if (scan == LAST_SCAN) 104 error = -EBUSY; 105 else 106 clear = false; 107 } 108 if (clear) 109 xas_clear_mark(&xas, MEMFD_TAG_PINNED); 110 111 if (++latency < XA_CHECK_SCHED) 112 continue; 113 latency = 0; 114 115 xas_pause(&xas); 116 xas_unlock_irq(&xas); 117 cond_resched(); 118 xas_lock_irq(&xas); 119 } 120 xas_unlock_irq(&xas); 121 } 122 123 return error; 124 } 125 126 static unsigned int *memfd_file_seals_ptr(struct file *file) 127 { 128 if (shmem_file(file)) 129 return &SHMEM_I(file_inode(file))->seals; 130 131 #ifdef CONFIG_HUGETLBFS 132 if (is_file_hugepages(file)) 133 return &HUGETLBFS_I(file_inode(file))->seals; 134 #endif 135 136 return NULL; 137 } 138 139 #define F_ALL_SEALS (F_SEAL_SEAL | \ 140 F_SEAL_EXEC | \ 141 F_SEAL_SHRINK | \ 142 F_SEAL_GROW | \ 143 F_SEAL_WRITE | \ 144 F_SEAL_FUTURE_WRITE) 145 146 static int memfd_add_seals(struct file *file, unsigned int seals) 147 { 148 struct inode *inode = file_inode(file); 149 unsigned int *file_seals; 150 int error; 151 152 /* 153 * SEALING 154 * Sealing allows multiple parties to share a tmpfs or hugetlbfs file 155 * but restrict access to a specific subset of file operations. Seals 156 * can only be added, but never removed. This way, mutually untrusted 157 * parties can share common memory regions with a well-defined policy. 158 * A malicious peer can thus never perform unwanted operations on a 159 * shared object. 160 * 161 * Seals are only supported on special tmpfs or hugetlbfs files and 162 * always affect the whole underlying inode. Once a seal is set, it 163 * may prevent some kinds of access to the file. Currently, the 164 * following seals are defined: 165 * SEAL_SEAL: Prevent further seals from being set on this file 166 * SEAL_SHRINK: Prevent the file from shrinking 167 * SEAL_GROW: Prevent the file from growing 168 * SEAL_WRITE: Prevent write access to the file 169 * SEAL_EXEC: Prevent modification of the exec bits in the file mode 170 * 171 * As we don't require any trust relationship between two parties, we 172 * must prevent seals from being removed. Therefore, sealing a file 173 * only adds a given set of seals to the file, it never touches 174 * existing seals. Furthermore, the "setting seals"-operation can be 175 * sealed itself, which basically prevents any further seal from being 176 * added. 177 * 178 * Semantics of sealing are only defined on volatile files. Only 179 * anonymous tmpfs and hugetlbfs files support sealing. More 180 * importantly, seals are never written to disk. Therefore, there's 181 * no plan to support it on other file types. 182 */ 183 184 if (!(file->f_mode & FMODE_WRITE)) 185 return -EPERM; 186 if (seals & ~(unsigned int)F_ALL_SEALS) 187 return -EINVAL; 188 189 inode_lock(inode); 190 191 file_seals = memfd_file_seals_ptr(file); 192 if (!file_seals) { 193 error = -EINVAL; 194 goto unlock; 195 } 196 197 if (*file_seals & F_SEAL_SEAL) { 198 error = -EPERM; 199 goto unlock; 200 } 201 202 if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) { 203 error = mapping_deny_writable(file->f_mapping); 204 if (error) 205 goto unlock; 206 207 error = memfd_wait_for_pins(file->f_mapping); 208 if (error) { 209 mapping_allow_writable(file->f_mapping); 210 goto unlock; 211 } 212 } 213 214 /* 215 * SEAL_EXEC implys SEAL_WRITE, making W^X from the start. 216 */ 217 if (seals & F_SEAL_EXEC && inode->i_mode & 0111) 218 seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE; 219 220 *file_seals |= seals; 221 error = 0; 222 223 unlock: 224 inode_unlock(inode); 225 return error; 226 } 227 228 static int memfd_get_seals(struct file *file) 229 { 230 unsigned int *seals = memfd_file_seals_ptr(file); 231 232 return seals ? *seals : -EINVAL; 233 } 234 235 long memfd_fcntl(struct file *file, unsigned int cmd, unsigned int arg) 236 { 237 long error; 238 239 switch (cmd) { 240 case F_ADD_SEALS: 241 error = memfd_add_seals(file, arg); 242 break; 243 case F_GET_SEALS: 244 error = memfd_get_seals(file); 245 break; 246 default: 247 error = -EINVAL; 248 break; 249 } 250 251 return error; 252 } 253 254 #define MFD_NAME_PREFIX "memfd:" 255 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1) 256 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN) 257 258 #define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB | MFD_NOEXEC_SEAL | MFD_EXEC) 259 260 static int check_sysctl_memfd_noexec(unsigned int *flags) 261 { 262 #ifdef CONFIG_SYSCTL 263 struct pid_namespace *ns = task_active_pid_ns(current); 264 int sysctl = pidns_memfd_noexec_scope(ns); 265 266 if (!(*flags & (MFD_EXEC | MFD_NOEXEC_SEAL))) { 267 if (sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL) 268 *flags |= MFD_NOEXEC_SEAL; 269 else 270 *flags |= MFD_EXEC; 271 } 272 273 if (!(*flags & MFD_NOEXEC_SEAL) && sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_ENFORCED) { 274 pr_err_ratelimited( 275 "%s[%d]: memfd_create() requires MFD_NOEXEC_SEAL with vm.memfd_noexec=%d\n", 276 current->comm, task_pid_nr(current), sysctl); 277 return -EACCES; 278 } 279 #endif 280 return 0; 281 } 282 283 SYSCALL_DEFINE2(memfd_create, 284 const char __user *, uname, 285 unsigned int, flags) 286 { 287 unsigned int *file_seals; 288 struct file *file; 289 int fd, error; 290 char *name; 291 long len; 292 293 if (!(flags & MFD_HUGETLB)) { 294 if (flags & ~(unsigned int)MFD_ALL_FLAGS) 295 return -EINVAL; 296 } else { 297 /* Allow huge page size encoding in flags. */ 298 if (flags & ~(unsigned int)(MFD_ALL_FLAGS | 299 (MFD_HUGE_MASK << MFD_HUGE_SHIFT))) 300 return -EINVAL; 301 } 302 303 /* Invalid if both EXEC and NOEXEC_SEAL are set.*/ 304 if ((flags & MFD_EXEC) && (flags & MFD_NOEXEC_SEAL)) 305 return -EINVAL; 306 307 error = check_sysctl_memfd_noexec(&flags); 308 if (error < 0) 309 return error; 310 311 /* length includes terminating zero */ 312 len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1); 313 if (len <= 0) 314 return -EFAULT; 315 if (len > MFD_NAME_MAX_LEN + 1) 316 return -EINVAL; 317 318 name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL); 319 if (!name) 320 return -ENOMEM; 321 322 strcpy(name, MFD_NAME_PREFIX); 323 if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) { 324 error = -EFAULT; 325 goto err_name; 326 } 327 328 /* terminating-zero may have changed after strnlen_user() returned */ 329 if (name[len + MFD_NAME_PREFIX_LEN - 1]) { 330 error = -EFAULT; 331 goto err_name; 332 } 333 334 fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0); 335 if (fd < 0) { 336 error = fd; 337 goto err_name; 338 } 339 340 if (flags & MFD_HUGETLB) { 341 file = hugetlb_file_setup(name, 0, VM_NORESERVE, 342 HUGETLB_ANONHUGE_INODE, 343 (flags >> MFD_HUGE_SHIFT) & 344 MFD_HUGE_MASK); 345 } else 346 file = shmem_file_setup(name, 0, VM_NORESERVE); 347 if (IS_ERR(file)) { 348 error = PTR_ERR(file); 349 goto err_fd; 350 } 351 file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE; 352 file->f_flags |= O_LARGEFILE; 353 354 if (flags & MFD_NOEXEC_SEAL) { 355 struct inode *inode = file_inode(file); 356 357 inode->i_mode &= ~0111; 358 file_seals = memfd_file_seals_ptr(file); 359 if (file_seals) { 360 *file_seals &= ~F_SEAL_SEAL; 361 *file_seals |= F_SEAL_EXEC; 362 } 363 } else if (flags & MFD_ALLOW_SEALING) { 364 /* MFD_EXEC and MFD_ALLOW_SEALING are set */ 365 file_seals = memfd_file_seals_ptr(file); 366 if (file_seals) 367 *file_seals &= ~F_SEAL_SEAL; 368 } 369 370 fd_install(fd, file); 371 kfree(name); 372 return fd; 373 374 err_fd: 375 put_unused_fd(fd); 376 err_name: 377 kfree(name); 378 return error; 379 } 380