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 void memfd_tag_pins(struct xa_state *xas) 33 { 34 struct page *page; 35 int latency = 0; 36 int cache_count; 37 38 lru_add_drain(); 39 40 xas_lock_irq(xas); 41 xas_for_each(xas, page, ULONG_MAX) { 42 cache_count = 1; 43 if (!xa_is_value(page) && 44 PageTransHuge(page) && !PageHuge(page)) 45 cache_count = HPAGE_PMD_NR; 46 47 if (!xa_is_value(page) && 48 page_count(page) - total_mapcount(page) != cache_count) 49 xas_set_mark(xas, MEMFD_TAG_PINNED); 50 if (cache_count != 1) 51 xas_set(xas, page->index + cache_count); 52 53 latency += cache_count; 54 if (latency < XA_CHECK_SCHED) 55 continue; 56 latency = 0; 57 58 xas_pause(xas); 59 xas_unlock_irq(xas); 60 cond_resched(); 61 xas_lock_irq(xas); 62 } 63 xas_unlock_irq(xas); 64 } 65 66 /* 67 * Setting SEAL_WRITE requires us to verify there's no pending writer. However, 68 * via get_user_pages(), drivers might have some pending I/O without any active 69 * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages 70 * and see whether it has an elevated ref-count. If so, we tag them and wait for 71 * them to be dropped. 72 * The caller must guarantee that no new user will acquire writable references 73 * to those pages to avoid races. 74 */ 75 static int memfd_wait_for_pins(struct address_space *mapping) 76 { 77 XA_STATE(xas, &mapping->i_pages, 0); 78 struct page *page; 79 int error, scan; 80 81 memfd_tag_pins(&xas); 82 83 error = 0; 84 for (scan = 0; scan <= LAST_SCAN; scan++) { 85 int latency = 0; 86 int cache_count; 87 88 if (!xas_marked(&xas, MEMFD_TAG_PINNED)) 89 break; 90 91 if (!scan) 92 lru_add_drain_all(); 93 else if (schedule_timeout_killable((HZ << scan) / 200)) 94 scan = LAST_SCAN; 95 96 xas_set(&xas, 0); 97 xas_lock_irq(&xas); 98 xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) { 99 bool clear = true; 100 101 cache_count = 1; 102 if (!xa_is_value(page) && 103 PageTransHuge(page) && !PageHuge(page)) 104 cache_count = HPAGE_PMD_NR; 105 106 if (!xa_is_value(page) && cache_count != 107 page_count(page) - total_mapcount(page)) { 108 /* 109 * On the last scan, we clean up all those tags 110 * we inserted; but make a note that we still 111 * found pages pinned. 112 */ 113 if (scan == LAST_SCAN) 114 error = -EBUSY; 115 else 116 clear = false; 117 } 118 if (clear) 119 xas_clear_mark(&xas, MEMFD_TAG_PINNED); 120 121 latency += cache_count; 122 if (latency < XA_CHECK_SCHED) 123 continue; 124 latency = 0; 125 126 xas_pause(&xas); 127 xas_unlock_irq(&xas); 128 cond_resched(); 129 xas_lock_irq(&xas); 130 } 131 xas_unlock_irq(&xas); 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_EXEC | \ 152 F_SEAL_SHRINK | \ 153 F_SEAL_GROW | \ 154 F_SEAL_WRITE | \ 155 F_SEAL_FUTURE_WRITE) 156 157 static int memfd_add_seals(struct file *file, unsigned int seals) 158 { 159 struct inode *inode = file_inode(file); 160 unsigned int *file_seals; 161 int error; 162 163 /* 164 * SEALING 165 * Sealing allows multiple parties to share a tmpfs or hugetlbfs file 166 * but restrict access to a specific subset of file operations. Seals 167 * can only be added, but never removed. This way, mutually untrusted 168 * parties can share common memory regions with a well-defined policy. 169 * A malicious peer can thus never perform unwanted operations on a 170 * shared object. 171 * 172 * Seals are only supported on special tmpfs or hugetlbfs files and 173 * always affect the whole underlying inode. Once a seal is set, it 174 * may prevent some kinds of access to the file. Currently, the 175 * following seals are defined: 176 * SEAL_SEAL: Prevent further seals from being set on this file 177 * SEAL_SHRINK: Prevent the file from shrinking 178 * SEAL_GROW: Prevent the file from growing 179 * SEAL_WRITE: Prevent write access to the file 180 * SEAL_EXEC: Prevent modification of the exec bits in the file mode 181 * 182 * As we don't require any trust relationship between two parties, we 183 * must prevent seals from being removed. Therefore, sealing a file 184 * only adds a given set of seals to the file, it never touches 185 * existing seals. Furthermore, the "setting seals"-operation can be 186 * sealed itself, which basically prevents any further seal from being 187 * added. 188 * 189 * Semantics of sealing are only defined on volatile files. Only 190 * anonymous tmpfs and hugetlbfs files support sealing. More 191 * importantly, seals are never written to disk. Therefore, there's 192 * no plan to support it on other file types. 193 */ 194 195 if (!(file->f_mode & FMODE_WRITE)) 196 return -EPERM; 197 if (seals & ~(unsigned int)F_ALL_SEALS) 198 return -EINVAL; 199 200 inode_lock(inode); 201 202 file_seals = memfd_file_seals_ptr(file); 203 if (!file_seals) { 204 error = -EINVAL; 205 goto unlock; 206 } 207 208 if (*file_seals & F_SEAL_SEAL) { 209 error = -EPERM; 210 goto unlock; 211 } 212 213 if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) { 214 error = mapping_deny_writable(file->f_mapping); 215 if (error) 216 goto unlock; 217 218 error = memfd_wait_for_pins(file->f_mapping); 219 if (error) { 220 mapping_allow_writable(file->f_mapping); 221 goto unlock; 222 } 223 } 224 225 /* 226 * SEAL_EXEC implys SEAL_WRITE, making W^X from the start. 227 */ 228 if (seals & F_SEAL_EXEC && inode->i_mode & 0111) 229 seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE; 230 231 *file_seals |= seals; 232 error = 0; 233 234 unlock: 235 inode_unlock(inode); 236 return error; 237 } 238 239 static int memfd_get_seals(struct file *file) 240 { 241 unsigned int *seals = memfd_file_seals_ptr(file); 242 243 return seals ? *seals : -EINVAL; 244 } 245 246 long memfd_fcntl(struct file *file, unsigned int cmd, unsigned int arg) 247 { 248 long error; 249 250 switch (cmd) { 251 case F_ADD_SEALS: 252 error = memfd_add_seals(file, arg); 253 break; 254 case F_GET_SEALS: 255 error = memfd_get_seals(file); 256 break; 257 default: 258 error = -EINVAL; 259 break; 260 } 261 262 return error; 263 } 264 265 #define MFD_NAME_PREFIX "memfd:" 266 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1) 267 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN) 268 269 #define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB | MFD_NOEXEC_SEAL | MFD_EXEC) 270 271 SYSCALL_DEFINE2(memfd_create, 272 const char __user *, uname, 273 unsigned int, flags) 274 { 275 char comm[TASK_COMM_LEN]; 276 unsigned int *file_seals; 277 struct file *file; 278 int fd, error; 279 char *name; 280 long len; 281 282 if (!(flags & MFD_HUGETLB)) { 283 if (flags & ~(unsigned int)MFD_ALL_FLAGS) 284 return -EINVAL; 285 } else { 286 /* Allow huge page size encoding in flags. */ 287 if (flags & ~(unsigned int)(MFD_ALL_FLAGS | 288 (MFD_HUGE_MASK << MFD_HUGE_SHIFT))) 289 return -EINVAL; 290 } 291 292 /* Invalid if both EXEC and NOEXEC_SEAL are set.*/ 293 if ((flags & MFD_EXEC) && (flags & MFD_NOEXEC_SEAL)) 294 return -EINVAL; 295 296 if (!(flags & (MFD_EXEC | MFD_NOEXEC_SEAL))) { 297 #ifdef CONFIG_SYSCTL 298 int sysctl = MEMFD_NOEXEC_SCOPE_EXEC; 299 struct pid_namespace *ns; 300 301 ns = task_active_pid_ns(current); 302 if (ns) 303 sysctl = ns->memfd_noexec_scope; 304 305 switch (sysctl) { 306 case MEMFD_NOEXEC_SCOPE_EXEC: 307 flags |= MFD_EXEC; 308 break; 309 case MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL: 310 flags |= MFD_NOEXEC_SEAL; 311 break; 312 default: 313 pr_warn_once( 314 "memfd_create(): MFD_NOEXEC_SEAL is enforced, pid=%d '%s'\n", 315 task_pid_nr(current), get_task_comm(comm, current)); 316 return -EINVAL; 317 } 318 #else 319 flags |= MFD_EXEC; 320 #endif 321 pr_warn_once( 322 "memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL, pid=%d '%s'\n", 323 task_pid_nr(current), get_task_comm(comm, current)); 324 } 325 326 /* length includes terminating zero */ 327 len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1); 328 if (len <= 0) 329 return -EFAULT; 330 if (len > MFD_NAME_MAX_LEN + 1) 331 return -EINVAL; 332 333 name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL); 334 if (!name) 335 return -ENOMEM; 336 337 strcpy(name, MFD_NAME_PREFIX); 338 if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) { 339 error = -EFAULT; 340 goto err_name; 341 } 342 343 /* terminating-zero may have changed after strnlen_user() returned */ 344 if (name[len + MFD_NAME_PREFIX_LEN - 1]) { 345 error = -EFAULT; 346 goto err_name; 347 } 348 349 fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0); 350 if (fd < 0) { 351 error = fd; 352 goto err_name; 353 } 354 355 if (flags & MFD_HUGETLB) { 356 file = hugetlb_file_setup(name, 0, VM_NORESERVE, 357 HUGETLB_ANONHUGE_INODE, 358 (flags >> MFD_HUGE_SHIFT) & 359 MFD_HUGE_MASK); 360 } else 361 file = shmem_file_setup(name, 0, VM_NORESERVE); 362 if (IS_ERR(file)) { 363 error = PTR_ERR(file); 364 goto err_fd; 365 } 366 file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE; 367 file->f_flags |= O_LARGEFILE; 368 369 if (flags & MFD_NOEXEC_SEAL) { 370 struct inode *inode = file_inode(file); 371 372 inode->i_mode &= ~0111; 373 file_seals = memfd_file_seals_ptr(file); 374 if (file_seals) { 375 *file_seals &= ~F_SEAL_SEAL; 376 *file_seals |= F_SEAL_EXEC; 377 } 378 } else if (flags & MFD_ALLOW_SEALING) { 379 /* MFD_EXEC and MFD_ALLOW_SEALING are set */ 380 file_seals = memfd_file_seals_ptr(file); 381 if (file_seals) 382 *file_seals &= ~F_SEAL_SEAL; 383 } 384 385 fd_install(fd, file); 386 kfree(name); 387 return fd; 388 389 err_fd: 390 put_unused_fd(fd); 391 err_name: 392 kfree(name); 393 return error; 394 } 395