mm/memfd.c

   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                 *file_seals &= ~F_SEAL_SEAL;
 375                 *file_seals |= F_SEAL_EXEC;
 376         } else if (flags & MFD_ALLOW_SEALING) {
 377                 /* MFD_EXEC and MFD_ALLOW_SEALING are set */
 378                 file_seals = memfd_file_seals_ptr(file);
 379                 *file_seals &= ~F_SEAL_SEAL;
 380         }
 381
 382         fd_install(fd, file);
 383         kfree(name);
 384         return fd;
 385
 386 err_fd:
 387         put_unused_fd(fd);
 388 err_name:
 389         kfree(name);
 390         return error;
 391 }