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 <uapi/linux/memfd.h>
  22
  23 /*
  24  * We need a tag: a new tag would expand every radix_tree_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 address_space *mapping)
  32 {
  33         struct radix_tree_iter iter;
  34         void __rcu **slot;
  35         pgoff_t start;
  36         struct page *page;
  37
  38         lru_add_drain();
  39         start = 0;
  40         rcu_read_lock();
  41
  42         radix_tree_for_each_slot(slot, &mapping->i_pages, &iter, start) {
  43                 page = radix_tree_deref_slot(slot);
  44                 if (!page || radix_tree_exception(page)) {
  45                         if (radix_tree_deref_retry(page)) {
  46                                 slot = radix_tree_iter_retry(&iter);
  47                                 continue;
  48                         }
  49                 } else if (page_count(page) - page_mapcount(page) > 1) {
  50                         xa_lock_irq(&mapping->i_pages);
  51                         radix_tree_tag_set(&mapping->i_pages, iter.index,
  52                                            MEMFD_TAG_PINNED);
  53                         xa_unlock_irq(&mapping->i_pages);
  54                 }
  55
  56                 if (need_resched()) {
  57                         slot = radix_tree_iter_resume(slot, &iter);
  58                         cond_resched_rcu();
  59                 }
  60         }
  61         rcu_read_unlock();
  62 }
  63
  64 /*
  65  * Setting SEAL_WRITE requires us to verify there's no pending writer. However,
  66  * via get_user_pages(), drivers might have some pending I/O without any active
  67  * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages
  68  * and see whether it has an elevated ref-count. If so, we tag them and wait for
  69  * them to be dropped.
  70  * The caller must guarantee that no new user will acquire writable references
  71  * to those pages to avoid races.
  72  */
  73 static int memfd_wait_for_pins(struct address_space *mapping)
  74 {
  75         struct radix_tree_iter iter;
  76         void __rcu **slot;
  77         pgoff_t start;
  78         struct page *page;
  79         int error, scan;
  80
  81         memfd_tag_pins(mapping);
  82
  83         error = 0;
  84         for (scan = 0; scan <= LAST_SCAN; scan++) {
  85                 if (!radix_tree_tagged(&mapping->i_pages, MEMFD_TAG_PINNED))
  86                         break;
  87
  88                 if (!scan)
  89                         lru_add_drain_all();
  90                 else if (schedule_timeout_killable((HZ << scan) / 200))
  91                         scan = LAST_SCAN;
  92
  93                 start = 0;
  94                 rcu_read_lock();
  95                 radix_tree_for_each_tagged(slot, &mapping->i_pages, &iter,
  96                                            start, MEMFD_TAG_PINNED) {
  97
  98                         page = radix_tree_deref_slot(slot);
  99                         if (radix_tree_exception(page)) {
 100                                 if (radix_tree_deref_retry(page)) {
 101                                         slot = radix_tree_iter_retry(&iter);
 102                                         continue;
 103                                 }
 104
 105                                 page = NULL;
 106                         }
 107
 108                         if (page &&
 109                             page_count(page) - page_mapcount(page) != 1) {
 110                                 if (scan < LAST_SCAN)
 111                                         goto continue_resched;
 112
 113                                 /*
 114                                  * On the last scan, we clean up all those tags
 115                                  * we inserted; but make a note that we still
 116                                  * found pages pinned.
 117                                  */
 118                                 error = -EBUSY;
 119                         }
 120
 121                         xa_lock_irq(&mapping->i_pages);
 122                         radix_tree_tag_clear(&mapping->i_pages,
 123                                              iter.index, MEMFD_TAG_PINNED);
 124                         xa_unlock_irq(&mapping->i_pages);
 125 continue_resched:
 126                         if (need_resched()) {
 127                                 slot = radix_tree_iter_resume(slot, &iter);
 128                                 cond_resched_rcu();
 129                         }
 130                 }
 131                 rcu_read_unlock();
 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_SHRINK | \
 152                      F_SEAL_GROW | \
 153                      F_SEAL_WRITE)
 154
 155 static int memfd_add_seals(struct file *file, unsigned int seals)
 156 {
 157         struct inode *inode = file_inode(file);
 158         unsigned int *file_seals;
 159         int error;
 160
 161         /*
 162          * SEALING
 163          * Sealing allows multiple parties to share a tmpfs or hugetlbfs file
 164          * but restrict access to a specific subset of file operations. Seals
 165          * can only be added, but never removed. This way, mutually untrusted
 166          * parties can share common memory regions with a well-defined policy.
 167          * A malicious peer can thus never perform unwanted operations on a
 168          * shared object.
 169          *
 170          * Seals are only supported on special tmpfs or hugetlbfs files and
 171          * always affect the whole underlying inode. Once a seal is set, it
 172          * may prevent some kinds of access to the file. Currently, the
 173          * following seals are defined:
 174          *   SEAL_SEAL: Prevent further seals from being set on this file
 175          *   SEAL_SHRINK: Prevent the file from shrinking
 176          *   SEAL_GROW: Prevent the file from growing
 177          *   SEAL_WRITE: Prevent write access to the file
 178          *
 179          * As we don't require any trust relationship between two parties, we
 180          * must prevent seals from being removed. Therefore, sealing a file
 181          * only adds a given set of seals to the file, it never touches
 182          * existing seals. Furthermore, the "setting seals"-operation can be
 183          * sealed itself, which basically prevents any further seal from being
 184          * added.
 185          *
 186          * Semantics of sealing are only defined on volatile files. Only
 187          * anonymous tmpfs and hugetlbfs files support sealing. More
 188          * importantly, seals are never written to disk. Therefore, there's
 189          * no plan to support it on other file types.
 190          */
 191
 192         if (!(file->f_mode & FMODE_WRITE))
 193                 return -EPERM;
 194         if (seals & ~(unsigned int)F_ALL_SEALS)
 195                 return -EINVAL;
 196
 197         inode_lock(inode);
 198
 199         file_seals = memfd_file_seals_ptr(file);
 200         if (!file_seals) {
 201                 error = -EINVAL;
 202                 goto unlock;
 203         }
 204
 205         if (*file_seals & F_SEAL_SEAL) {
 206                 error = -EPERM;
 207                 goto unlock;
 208         }
 209
 210         if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
 211                 error = mapping_deny_writable(file->f_mapping);
 212                 if (error)
 213                         goto unlock;
 214
 215                 error = memfd_wait_for_pins(file->f_mapping);
 216                 if (error) {
 217                         mapping_allow_writable(file->f_mapping);
 218                         goto unlock;
 219                 }
 220         }
 221
 222         *file_seals |= seals;
 223         error = 0;
 224
 225 unlock:
 226         inode_unlock(inode);
 227         return error;
 228 }
 229
 230 static int memfd_get_seals(struct file *file)
 231 {
 232         unsigned int *seals = memfd_file_seals_ptr(file);
 233
 234         return seals ? *seals : -EINVAL;
 235 }
 236
 237 long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
 238 {
 239         long error;
 240
 241         switch (cmd) {
 242         case F_ADD_SEALS:
 243                 /* disallow upper 32bit */
 244                 if (arg > UINT_MAX)
 245                         return -EINVAL;
 246
 247                 error = memfd_add_seals(file, arg);
 248                 break;
 249         case F_GET_SEALS:
 250                 error = memfd_get_seals(file);
 251                 break;
 252         default:
 253                 error = -EINVAL;
 254                 break;
 255         }
 256
 257         return error;
 258 }
 259
 260 #define MFD_NAME_PREFIX "memfd:"
 261 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
 262 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
 263
 264 #define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB)
 265
 266 SYSCALL_DEFINE2(memfd_create,
 267                 const char __user *, uname,
 268                 unsigned int, flags)
 269 {
 270         unsigned int *file_seals;
 271         struct file *file;
 272         int fd, error;
 273         char *name;
 274         long len;
 275
 276         if (!(flags & MFD_HUGETLB)) {
 277                 if (flags & ~(unsigned int)MFD_ALL_FLAGS)
 278                         return -EINVAL;
 279         } else {
 280                 /* Allow huge page size encoding in flags. */
 281                 if (flags & ~(unsigned int)(MFD_ALL_FLAGS |
 282                                 (MFD_HUGE_MASK << MFD_HUGE_SHIFT)))
 283                         return -EINVAL;
 284         }
 285
 286         /* length includes terminating zero */
 287         len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
 288         if (len <= 0)
 289                 return -EFAULT;
 290         if (len > MFD_NAME_MAX_LEN + 1)
 291                 return -EINVAL;
 292
 293         name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL);
 294         if (!name)
 295                 return -ENOMEM;
 296
 297         strcpy(name, MFD_NAME_PREFIX);
 298         if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) {
 299                 error = -EFAULT;
 300                 goto err_name;
 301         }
 302
 303         /* terminating-zero may have changed after strnlen_user() returned */
 304         if (name[len + MFD_NAME_PREFIX_LEN - 1]) {
 305                 error = -EFAULT;
 306                 goto err_name;
 307         }
 308
 309         fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0);
 310         if (fd < 0) {
 311                 error = fd;
 312                 goto err_name;
 313         }
 314
 315         if (flags & MFD_HUGETLB) {
 316                 struct user_struct *user = NULL;
 317
 318                 file = hugetlb_file_setup(name, 0, VM_NORESERVE, &user,
 319                                         HUGETLB_ANONHUGE_INODE,
 320                                         (flags >> MFD_HUGE_SHIFT) &
 321                                         MFD_HUGE_MASK);
 322         } else
 323                 file = shmem_file_setup(name, 0, VM_NORESERVE);
 324         if (IS_ERR(file)) {
 325                 error = PTR_ERR(file);
 326                 goto err_fd;
 327         }
 328         file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
 329         file->f_flags |= O_LARGEFILE;
 330
 331         if (flags & MFD_ALLOW_SEALING) {
 332                 file_seals = memfd_file_seals_ptr(file);
 333                 *file_seals &= ~F_SEAL_SEAL;
 334         }
 335
 336         fd_install(fd, file);
 337         kfree(name);
 338         return fd;
 339
 340 err_fd:
 341         put_unused_fd(fd);
 342 err_name:
 343         kfree(name);
 344         return error;
 345 }