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
 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 }