root/mm/memfd.c

DEFINITIONS

1. memfd_tag_pins
2. memfd_wait_for_pins
3. memfd_file_seals_ptr
4. memfd_add_seals
5. memfd_get_seals
6. memfd_fcntl
7. SYSCALL_DEFINE2

   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                 page = find_subpage(page, xas->xa_index);
  43                 if (page_count(page) - page_mapcount(page) > 1)
  44                         xas_set_mark(xas, MEMFD_TAG_PINNED);
  45 
  46                 if (++tagged % XA_CHECK_SCHED)
  47                         continue;
  48 
  49                 xas_pause(xas);
  50                 xas_unlock_irq(xas);
  51                 cond_resched();
  52                 xas_lock_irq(xas);
  53         }
  54         xas_unlock_irq(xas);
  55 }
  56 
  57 /*
  58  * Setting SEAL_WRITE requires us to verify there's no pending writer. However,
  59  * via get_user_pages(), drivers might have some pending I/O without any active
  60  * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages
  61  * and see whether it has an elevated ref-count. If so, we tag them and wait for
  62  * them to be dropped.
  63  * The caller must guarantee that no new user will acquire writable references
  64  * to those pages to avoid races.
  65  */
  66 static int memfd_wait_for_pins(struct address_space *mapping)
  67 {
  68         XA_STATE(xas, &mapping->i_pages, 0);
  69         struct page *page;
  70         int error, scan;
  71 
  72         memfd_tag_pins(&xas);
  73 
  74         error = 0;
  75         for (scan = 0; scan <= LAST_SCAN; scan++) {
  76                 unsigned int tagged = 0;
  77 
  78                 if (!xas_marked(&xas, MEMFD_TAG_PINNED))
  79                         break;
  80 
  81                 if (!scan)
  82                         lru_add_drain_all();
  83                 else if (schedule_timeout_killable((HZ << scan) / 200))
  84                         scan = LAST_SCAN;
  85 
  86                 xas_set(&xas, 0);
  87                 xas_lock_irq(&xas);
  88                 xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) {
  89                         bool clear = true;
  90                         if (xa_is_value(page))
  91                                 continue;
  92                         page = find_subpage(page, xas.xa_index);
  93                         if (page_count(page) - page_mapcount(page) != 1) {
  94                                 /*
  95                                  * On the last scan, we clean up all those tags
  96                                  * we inserted; but make a note that we still
  97                                  * found pages pinned.
  98                                  */
  99                                 if (scan == LAST_SCAN)
 100                                         error = -EBUSY;
 101                                 else
 102                                         clear = false;
 103                         }
 104                         if (clear)
 105                                 xas_clear_mark(&xas, MEMFD_TAG_PINNED);
 106                         if (++tagged % XA_CHECK_SCHED)
 107                                 continue;
 108 
 109                         xas_pause(&xas);
 110                         xas_unlock_irq(&xas);
 111                         cond_resched();
 112                         xas_lock_irq(&xas);
 113                 }
 114                 xas_unlock_irq(&xas);
 115         }
 116 
 117         return error;
 118 }
 119 
 120 static unsigned int *memfd_file_seals_ptr(struct file *file)
 121 {
 122         if (shmem_file(file))
 123                 return &SHMEM_I(file_inode(file))->seals;
 124 
 125 #ifdef CONFIG_HUGETLBFS
 126         if (is_file_hugepages(file))
 127                 return &HUGETLBFS_I(file_inode(file))->seals;
 128 #endif
 129 
 130         return NULL;
 131 }
 132 
 133 #define F_ALL_SEALS (F_SEAL_SEAL | \
 134                      F_SEAL_SHRINK | \
 135                      F_SEAL_GROW | \
 136                      F_SEAL_WRITE | \
 137                      F_SEAL_FUTURE_WRITE)
 138 
 139 static int memfd_add_seals(struct file *file, unsigned int seals)
 140 {
 141         struct inode *inode = file_inode(file);
 142         unsigned int *file_seals;
 143         int error;
 144 
 145         /*
 146          * SEALING
 147          * Sealing allows multiple parties to share a tmpfs or hugetlbfs file
 148          * but restrict access to a specific subset of file operations. Seals
 149          * can only be added, but never removed. This way, mutually untrusted
 150          * parties can share common memory regions with a well-defined policy.
 151          * A malicious peer can thus never perform unwanted operations on a
 152          * shared object.
 153          *
 154          * Seals are only supported on special tmpfs or hugetlbfs files and
 155          * always affect the whole underlying inode. Once a seal is set, it
 156          * may prevent some kinds of access to the file. Currently, the
 157          * following seals are defined:
 158          *   SEAL_SEAL: Prevent further seals from being set on this file
 159          *   SEAL_SHRINK: Prevent the file from shrinking
 160          *   SEAL_GROW: Prevent the file from growing
 161          *   SEAL_WRITE: Prevent write access to the file
 162          *
 163          * As we don't require any trust relationship between two parties, we
 164          * must prevent seals from being removed. Therefore, sealing a file
 165          * only adds a given set of seals to the file, it never touches
 166          * existing seals. Furthermore, the "setting seals"-operation can be
 167          * sealed itself, which basically prevents any further seal from being
 168          * added.
 169          *
 170          * Semantics of sealing are only defined on volatile files. Only
 171          * anonymous tmpfs and hugetlbfs files support sealing. More
 172          * importantly, seals are never written to disk. Therefore, there's
 173          * no plan to support it on other file types.
 174          */
 175 
 176         if (!(file->f_mode & FMODE_WRITE))
 177                 return -EPERM;
 178         if (seals & ~(unsigned int)F_ALL_SEALS)
 179                 return -EINVAL;
 180 
 181         inode_lock(inode);
 182 
 183         file_seals = memfd_file_seals_ptr(file);
 184         if (!file_seals) {
 185                 error = -EINVAL;
 186                 goto unlock;
 187         }
 188 
 189         if (*file_seals & F_SEAL_SEAL) {
 190                 error = -EPERM;
 191                 goto unlock;
 192         }
 193 
 194         if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
 195                 error = mapping_deny_writable(file->f_mapping);
 196                 if (error)
 197                         goto unlock;
 198 
 199                 error = memfd_wait_for_pins(file->f_mapping);
 200                 if (error) {
 201                         mapping_allow_writable(file->f_mapping);
 202                         goto unlock;
 203                 }
 204         }
 205 
 206         *file_seals |= seals;
 207         error = 0;
 208 
 209 unlock:
 210         inode_unlock(inode);
 211         return error;
 212 }
 213 
 214 static int memfd_get_seals(struct file *file)
 215 {
 216         unsigned int *seals = memfd_file_seals_ptr(file);
 217 
 218         return seals ? *seals : -EINVAL;
 219 }
 220 
 221 long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
 222 {
 223         long error;
 224 
 225         switch (cmd) {
 226         case F_ADD_SEALS:
 227                 /* disallow upper 32bit */
 228                 if (arg > UINT_MAX)
 229                         return -EINVAL;
 230 
 231                 error = memfd_add_seals(file, arg);
 232                 break;
 233         case F_GET_SEALS:
 234                 error = memfd_get_seals(file);
 235                 break;
 236         default:
 237                 error = -EINVAL;
 238                 break;
 239         }
 240 
 241         return error;
 242 }
 243 
 244 #define MFD_NAME_PREFIX "memfd:"
 245 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
 246 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
 247 
 248 #define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB)
 249 
 250 SYSCALL_DEFINE2(memfd_create,
 251                 const char __user *, uname,
 252                 unsigned int, flags)
 253 {
 254         unsigned int *file_seals;
 255         struct file *file;
 256         int fd, error;
 257         char *name;
 258         long len;
 259 
 260         if (!(flags & MFD_HUGETLB)) {
 261                 if (flags & ~(unsigned int)MFD_ALL_FLAGS)
 262                         return -EINVAL;
 263         } else {
 264                 /* Allow huge page size encoding in flags. */
 265                 if (flags & ~(unsigned int)(MFD_ALL_FLAGS |
 266                                 (MFD_HUGE_MASK << MFD_HUGE_SHIFT)))
 267                         return -EINVAL;
 268         }
 269 
 270         /* length includes terminating zero */
 271         len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
 272         if (len <= 0)
 273                 return -EFAULT;
 274         if (len > MFD_NAME_MAX_LEN + 1)
 275                 return -EINVAL;
 276 
 277         name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL);
 278         if (!name)
 279                 return -ENOMEM;
 280 
 281         strcpy(name, MFD_NAME_PREFIX);
 282         if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) {
 283                 error = -EFAULT;
 284                 goto err_name;
 285         }
 286 
 287         /* terminating-zero may have changed after strnlen_user() returned */
 288         if (name[len + MFD_NAME_PREFIX_LEN - 1]) {
 289                 error = -EFAULT;
 290                 goto err_name;
 291         }
 292 
 293         fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0);
 294         if (fd < 0) {
 295                 error = fd;
 296                 goto err_name;
 297         }
 298 
 299         if (flags & MFD_HUGETLB) {
 300                 struct user_struct *user = NULL;
 301 
 302                 file = hugetlb_file_setup(name, 0, VM_NORESERVE, &user,
 303                                         HUGETLB_ANONHUGE_INODE,
 304                                         (flags >> MFD_HUGE_SHIFT) &
 305                                         MFD_HUGE_MASK);
 306         } else
 307                 file = shmem_file_setup(name, 0, VM_NORESERVE);
 308         if (IS_ERR(file)) {
 309                 error = PTR_ERR(file);
 310                 goto err_fd;
 311         }
 312         file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
 313         file->f_flags |= O_LARGEFILE;
 314 
 315         if (flags & MFD_ALLOW_SEALING) {
 316                 file_seals = memfd_file_seals_ptr(file);
 317                 *file_seals &= ~F_SEAL_SEAL;
 318         }
 319 
 320         fd_install(fd, file);
 321         kfree(name);
 322         return fd;
 323 
 324 err_fd:
 325         put_unused_fd(fd);
 326 err_name:
 327         kfree(name);
 328         return error;
 329 }