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 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                      F_SEAL_FUTURE_WRITE)
 136
 137 static int memfd_add_seals(struct file *file, unsigned int seals)
 138 {
 139         struct inode *inode = file_inode(file);
 140         unsigned int *file_seals;
 141         int error;
 142
 143         /*
 144          * SEALING
 145          * Sealing allows multiple parties to share a tmpfs or hugetlbfs file
 146          * but restrict access to a specific subset of file operations. Seals
 147          * can only be added, but never removed. This way, mutually untrusted
 148          * parties can share common memory regions with a well-defined policy.
 149          * A malicious peer can thus never perform unwanted operations on a
 150          * shared object.
 151          *
 152          * Seals are only supported on special tmpfs or hugetlbfs files and
 153          * always affect the whole underlying inode. Once a seal is set, it
 154          * may prevent some kinds of access to the file. Currently, the
 155          * following seals are defined:
 156          *   SEAL_SEAL: Prevent further seals from being set on this file
 157          *   SEAL_SHRINK: Prevent the file from shrinking
 158          *   SEAL_GROW: Prevent the file from growing
 159          *   SEAL_WRITE: Prevent write access to the file
 160          *
 161          * As we don't require any trust relationship between two parties, we
 162          * must prevent seals from being removed. Therefore, sealing a file
 163          * only adds a given set of seals to the file, it never touches
 164          * existing seals. Furthermore, the "setting seals"-operation can be
 165          * sealed itself, which basically prevents any further seal from being
 166          * added.
 167          *
 168          * Semantics of sealing are only defined on volatile files. Only
 169          * anonymous tmpfs and hugetlbfs files support sealing. More
 170          * importantly, seals are never written to disk. Therefore, there's
 171          * no plan to support it on other file types.
 172          */
 173
 174         if (!(file->f_mode & FMODE_WRITE))
 175                 return -EPERM;
 176         if (seals & ~(unsigned int)F_ALL_SEALS)
 177                 return -EINVAL;
 178
 179         inode_lock(inode);
 180
 181         file_seals = memfd_file_seals_ptr(file);
 182         if (!file_seals) {
 183                 error = -EINVAL;
 184                 goto unlock;
 185         }
 186
 187         if (*file_seals & F_SEAL_SEAL) {
 188                 error = -EPERM;
 189                 goto unlock;
 190         }
 191
 192         if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
 193                 error = mapping_deny_writable(file->f_mapping);
 194                 if (error)
 195                         goto unlock;
 196
 197                 error = memfd_wait_for_pins(file->f_mapping);
 198                 if (error) {
 199                         mapping_allow_writable(file->f_mapping);
 200                         goto unlock;
 201                 }
 202         }
 203
 204         *file_seals |= seals;
 205         error = 0;
 206
 207 unlock:
 208         inode_unlock(inode);
 209         return error;
 210 }
 211
 212 static int memfd_get_seals(struct file *file)
 213 {
 214         unsigned int *seals = memfd_file_seals_ptr(file);
 215
 216         return seals ? *seals : -EINVAL;
 217 }
 218
 219 long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
 220 {
 221         long error;
 222
 223         switch (cmd) {
 224         case F_ADD_SEALS:
 225                 /* disallow upper 32bit */
 226                 if (arg > UINT_MAX)
 227                         return -EINVAL;
 228
 229                 error = memfd_add_seals(file, arg);
 230                 break;
 231         case F_GET_SEALS:
 232                 error = memfd_get_seals(file);
 233                 break;
 234         default:
 235                 error = -EINVAL;
 236                 break;
 237         }
 238
 239         return error;
 240 }
 241
 242 #define MFD_NAME_PREFIX "memfd:"
 243 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
 244 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
 245
 246 #define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB)
 247
 248 SYSCALL_DEFINE2(memfd_create,
 249                 const char __user *, uname,
 250                 unsigned int, flags)
 251 {
 252         unsigned int *file_seals;
 253         struct file *file;
 254         int fd, error;
 255         char *name;
 256         long len;
 257
 258         if (!(flags & MFD_HUGETLB)) {
 259                 if (flags & ~(unsigned int)MFD_ALL_FLAGS)
 260                         return -EINVAL;
 261         } else {
 262                 /* Allow huge page size encoding in flags. */
 263                 if (flags & ~(unsigned int)(MFD_ALL_FLAGS |
 264                                 (MFD_HUGE_MASK << MFD_HUGE_SHIFT)))
 265                         return -EINVAL;
 266         }
 267
 268         /* length includes terminating zero */
 269         len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
 270         if (len <= 0)
 271                 return -EFAULT;
 272         if (len > MFD_NAME_MAX_LEN + 1)
 273                 return -EINVAL;
 274
 275         name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL);
 276         if (!name)
 277                 return -ENOMEM;
 278
 279         strcpy(name, MFD_NAME_PREFIX);
 280         if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) {
 281                 error = -EFAULT;
 282                 goto err_name;
 283         }
 284
 285         /* terminating-zero may have changed after strnlen_user() returned */
 286         if (name[len + MFD_NAME_PREFIX_LEN - 1]) {
 287                 error = -EFAULT;
 288                 goto err_name;
 289         }
 290
 291         fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0);
 292         if (fd < 0) {
 293                 error = fd;
 294                 goto err_name;
 295         }
 296
 297         if (flags & MFD_HUGETLB) {
 298                 struct user_struct *user = NULL;
 299
 300                 file = hugetlb_file_setup(name, 0, VM_NORESERVE, &user,
 301                                         HUGETLB_ANONHUGE_INODE,
 302                                         (flags >> MFD_HUGE_SHIFT) &
 303                                         MFD_HUGE_MASK);
 304         } else
 305                 file = shmem_file_setup(name, 0, VM_NORESERVE);
 306         if (IS_ERR(file)) {
 307                 error = PTR_ERR(file);
 308                 goto err_fd;
 309         }
 310         file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
 311         file->f_flags |= O_LARGEFILE;
 312
 313         if (flags & MFD_ALLOW_SEALING) {
 314                 file_seals = memfd_file_seals_ptr(file);
 315                 *file_seals &= ~F_SEAL_SEAL;
 316         }
 317
 318         fd_install(fd, file);
 319         kfree(name);
 320         return fd;
 321
 322 err_fd:
 323         put_unused_fd(fd);
 324 err_name:
 325         kfree(name);
 326         return error;
 327 }