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