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 <uapi/linux/memfd.h> 22 23 /* 24 * We need a tag: a new tag would expand every xa_node by 8 bytes, 25 * so reuse a tag which we firmly believe is never set or cleared on tmpfs 26 * or hugetlbfs because they are memory only filesystems. 27 */ 28 #define MEMFD_TAG_PINNED PAGECACHE_TAG_TOWRITE 29 #define LAST_SCAN 4 /* about 150ms max */ 30 31 static void memfd_tag_pins(struct xa_state *xas) 32 { 33 struct page *page; 34 unsigned int tagged = 0; 35 36 lru_add_drain(); 37 38 xas_lock_irq(xas); 39 xas_for_each(xas, page, ULONG_MAX) { 40 if (xa_is_value(page)) 41 continue; 42 page = find_subpage(page, xas->xa_index); 43 if (page_count(page) - page_mapcount(page) > 1) 44 xas_set_mark(xas, MEMFD_TAG_PINNED); 45 46 if (++tagged % XA_CHECK_SCHED) 47 continue; 48 49 xas_pause(xas); 50 xas_unlock_irq(xas); 51 cond_resched(); 52 xas_lock_irq(xas); 53 } 54 xas_unlock_irq(xas); 55 } 56 57 /* 58 * Setting SEAL_WRITE requires us to verify there's no pending writer. However, 59 * via get_user_pages(), drivers might have some pending I/O without any active 60 * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages 61 * and see whether it has an elevated ref-count. If so, we tag them and wait for 62 * them to be dropped. 63 * The caller must guarantee that no new user will acquire writable references 64 * to those pages to avoid races. 65 */ 66 static int memfd_wait_for_pins(struct address_space *mapping) 67 { 68 XA_STATE(xas, &mapping->i_pages, 0); 69 struct page *page; 70 int error, scan; 71 72 memfd_tag_pins(&xas); 73 74 error = 0; 75 for (scan = 0; scan <= LAST_SCAN; scan++) { 76 unsigned int tagged = 0; 77 78 if (!xas_marked(&xas, MEMFD_TAG_PINNED)) 79 break; 80 81 if (!scan) 82 lru_add_drain_all(); 83 else if (schedule_timeout_killable((HZ << scan) / 200)) 84 scan = LAST_SCAN; 85 86 xas_set(&xas, 0); 87 xas_lock_irq(&xas); 88 xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) { 89 bool clear = true; 90 if (xa_is_value(page)) 91 continue; 92 page = find_subpage(page, xas.xa_index); 93 if (page_count(page) - page_mapcount(page) != 1) { 94 /* 95 * On the last scan, we clean up all those tags 96 * we inserted; but make a note that we still 97 * found pages pinned. 98 */ 99 if (scan == LAST_SCAN) 100 error = -EBUSY; 101 else 102 clear = false; 103 } 104 if (clear) 105 xas_clear_mark(&xas, MEMFD_TAG_PINNED); 106 if (++tagged % XA_CHECK_SCHED) 107 continue; 108 109 xas_pause(&xas); 110 xas_unlock_irq(&xas); 111 cond_resched(); 112 xas_lock_irq(&xas); 113 } 114 xas_unlock_irq(&xas); 115 } 116 117 return error; 118 } 119 120 static unsigned int *memfd_file_seals_ptr(struct file *file) 121 { 122 if (shmem_file(file)) 123 return &SHMEM_I(file_inode(file))->seals; 124 125 #ifdef CONFIG_HUGETLBFS 126 if (is_file_hugepages(file)) 127 return &HUGETLBFS_I(file_inode(file))->seals; 128 #endif 129 130 return NULL; 131 } 132 133 #define F_ALL_SEALS (F_SEAL_SEAL | \ 134 F_SEAL_SHRINK | \ 135 F_SEAL_GROW | \ 136 F_SEAL_WRITE | \ 137 F_SEAL_FUTURE_WRITE) 138 139 static int memfd_add_seals(struct file *file, unsigned int seals) 140 { 141 struct inode *inode = file_inode(file); 142 unsigned int *file_seals; 143 int error; 144 145 /* 146 * SEALING 147 * Sealing allows multiple parties to share a tmpfs or hugetlbfs file 148 * but restrict access to a specific subset of file operations. Seals 149 * can only be added, but never removed. This way, mutually untrusted 150 * parties can share common memory regions with a well-defined policy. 151 * A malicious peer can thus never perform unwanted operations on a 152 * shared object. 153 * 154 * Seals are only supported on special tmpfs or hugetlbfs files and 155 * always affect the whole underlying inode. Once a seal is set, it 156 * may prevent some kinds of access to the file. Currently, the 157 * following seals are defined: 158 * SEAL_SEAL: Prevent further seals from being set on this file 159 * SEAL_SHRINK: Prevent the file from shrinking 160 * SEAL_GROW: Prevent the file from growing 161 * SEAL_WRITE: Prevent write access to the file 162 * 163 * As we don't require any trust relationship between two parties, we 164 * must prevent seals from being removed. Therefore, sealing a file 165 * only adds a given set of seals to the file, it never touches 166 * existing seals. Furthermore, the "setting seals"-operation can be 167 * sealed itself, which basically prevents any further seal from being 168 * added. 169 * 170 * Semantics of sealing are only defined on volatile files. Only 171 * anonymous tmpfs and hugetlbfs files support sealing. More 172 * importantly, seals are never written to disk. Therefore, there's 173 * no plan to support it on other file types. 174 */ 175 176 if (!(file->f_mode & FMODE_WRITE)) 177 return -EPERM; 178 if (seals & ~(unsigned int)F_ALL_SEALS) 179 return -EINVAL; 180 181 inode_lock(inode); 182 183 file_seals = memfd_file_seals_ptr(file); 184 if (!file_seals) { 185 error = -EINVAL; 186 goto unlock; 187 } 188 189 if (*file_seals & F_SEAL_SEAL) { 190 error = -EPERM; 191 goto unlock; 192 } 193 194 if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) { 195 error = mapping_deny_writable(file->f_mapping); 196 if (error) 197 goto unlock; 198 199 error = memfd_wait_for_pins(file->f_mapping); 200 if (error) { 201 mapping_allow_writable(file->f_mapping); 202 goto unlock; 203 } 204 } 205 206 *file_seals |= seals; 207 error = 0; 208 209 unlock: 210 inode_unlock(inode); 211 return error; 212 } 213 214 static int memfd_get_seals(struct file *file) 215 { 216 unsigned int *seals = memfd_file_seals_ptr(file); 217 218 return seals ? *seals : -EINVAL; 219 } 220 221 long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg) 222 { 223 long error; 224 225 switch (cmd) { 226 case F_ADD_SEALS: 227 /* disallow upper 32bit */ 228 if (arg > UINT_MAX) 229 return -EINVAL; 230 231 error = memfd_add_seals(file, arg); 232 break; 233 case F_GET_SEALS: 234 error = memfd_get_seals(file); 235 break; 236 default: 237 error = -EINVAL; 238 break; 239 } 240 241 return error; 242 } 243 244 #define MFD_NAME_PREFIX "memfd:" 245 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1) 246 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN) 247 248 #define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB) 249 250 SYSCALL_DEFINE2(memfd_create, 251 const char __user *, uname, 252 unsigned int, flags) 253 { 254 unsigned int *file_seals; 255 struct file *file; 256 int fd, error; 257 char *name; 258 long len; 259 260 if (!(flags & MFD_HUGETLB)) { 261 if (flags & ~(unsigned int)MFD_ALL_FLAGS) 262 return -EINVAL; 263 } else { 264 /* Allow huge page size encoding in flags. */ 265 if (flags & ~(unsigned int)(MFD_ALL_FLAGS | 266 (MFD_HUGE_MASK << MFD_HUGE_SHIFT))) 267 return -EINVAL; 268 } 269 270 /* length includes terminating zero */ 271 len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1); 272 if (len <= 0) 273 return -EFAULT; 274 if (len > MFD_NAME_MAX_LEN + 1) 275 return -EINVAL; 276 277 name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL); 278 if (!name) 279 return -ENOMEM; 280 281 strcpy(name, MFD_NAME_PREFIX); 282 if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) { 283 error = -EFAULT; 284 goto err_name; 285 } 286 287 /* terminating-zero may have changed after strnlen_user() returned */ 288 if (name[len + MFD_NAME_PREFIX_LEN - 1]) { 289 error = -EFAULT; 290 goto err_name; 291 } 292 293 fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0); 294 if (fd < 0) { 295 error = fd; 296 goto err_name; 297 } 298 299 if (flags & MFD_HUGETLB) { 300 struct ucounts *ucounts = NULL; 301 302 file = hugetlb_file_setup(name, 0, VM_NORESERVE, &ucounts, 303 HUGETLB_ANONHUGE_INODE, 304 (flags >> MFD_HUGE_SHIFT) & 305 MFD_HUGE_MASK); 306 } else 307 file = shmem_file_setup(name, 0, VM_NORESERVE); 308 if (IS_ERR(file)) { 309 error = PTR_ERR(file); 310 goto err_fd; 311 } 312 file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE; 313 file->f_flags |= O_LARGEFILE; 314 315 if (flags & MFD_ALLOW_SEALING) { 316 file_seals = memfd_file_seals_ptr(file); 317 *file_seals &= ~F_SEAL_SEAL; 318 } 319 320 fd_install(fd, file); 321 kfree(name); 322 return fd; 323 324 err_fd: 325 put_unused_fd(fd); 326 err_name: 327 kfree(name); 328 return error; 329 } 330