linux/Documentation/filesystems/fuse.txt
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   1Definitions
   2~~~~~~~~~~~
   3
   4Userspace filesystem:
   5
   6  A filesystem in which data and metadata are provided by an ordinary
   7  userspace process.  The filesystem can be accessed normally through
   8  the kernel interface.
   9
  10Filesystem daemon:
  11
  12  The process(es) providing the data and metadata of the filesystem.
  13
  14Non-privileged mount (or user mount):
  15
  16  A userspace filesystem mounted by a non-privileged (non-root) user.
  17  The filesystem daemon is running with the privileges of the mounting
  18  user.  NOTE: this is not the same as mounts allowed with the "user"
  19  option in /etc/fstab, which is not discussed here.
  20
  21Filesystem connection:
  22
  23  A connection between the filesystem daemon and the kernel.  The
  24  connection exists until either the daemon dies, or the filesystem is
  25  umounted.  Note that detaching (or lazy umounting) the filesystem
  26  does _not_ break the connection, in this case it will exist until
  27  the last reference to the filesystem is released.
  28
  29Mount owner:
  30
  31  The user who does the mounting.
  32
  33User:
  34
  35  The user who is performing filesystem operations.
  36
  37What is FUSE?
  38~~~~~~~~~~~~~
  39
  40FUSE is a userspace filesystem framework.  It consists of a kernel
  41module (fuse.ko), a userspace library (libfuse.*) and a mount utility
  42(fusermount).
  43
  44One of the most important features of FUSE is allowing secure,
  45non-privileged mounts.  This opens up new possibilities for the use of
  46filesystems.  A good example is sshfs: a secure network filesystem
  47using the sftp protocol.
  48
  49The userspace library and utilities are available from the FUSE
  50homepage:
  51
  52  http://fuse.sourceforge.net/
  53
  54Filesystem type
  55~~~~~~~~~~~~~~~
  56
  57The filesystem type given to mount(2) can be one of the following:
  58
  59'fuse'
  60
  61  This is the usual way to mount a FUSE filesystem.  The first
  62  argument of the mount system call may contain an arbitrary string,
  63  which is not interpreted by the kernel.
  64
  65'fuseblk'
  66
  67  The filesystem is block device based.  The first argument of the
  68  mount system call is interpreted as the name of the device.
  69
  70Mount options
  71~~~~~~~~~~~~~
  72
  73'fd=N'
  74
  75  The file descriptor to use for communication between the userspace
  76  filesystem and the kernel.  The file descriptor must have been
  77  obtained by opening the FUSE device ('/dev/fuse').
  78
  79'rootmode=M'
  80
  81  The file mode of the filesystem's root in octal representation.
  82
  83'user_id=N'
  84
  85  The numeric user id of the mount owner.
  86
  87'group_id=N'
  88
  89  The numeric group id of the mount owner.
  90
  91'default_permissions'
  92
  93  By default FUSE doesn't check file access permissions, the
  94  filesystem is free to implement its access policy or leave it to
  95  the underlying file access mechanism (e.g. in case of network
  96  filesystems).  This option enables permission checking, restricting
  97  access based on file mode.  It is usually useful together with the
  98  'allow_other' mount option.
  99
 100'allow_other'
 101
 102  This option overrides the security measure restricting file access
 103  to the user mounting the filesystem.  This option is by default only
 104  allowed to root, but this restriction can be removed with a
 105  (userspace) configuration option.
 106
 107'max_read=N'
 108
 109  With this option the maximum size of read operations can be set.
 110  The default is infinite.  Note that the size of read requests is
 111  limited anyway to 32 pages (which is 128kbyte on i386).
 112
 113'blksize=N'
 114
 115  Set the block size for the filesystem.  The default is 512.  This
 116  option is only valid for 'fuseblk' type mounts.
 117
 118Control filesystem
 119~~~~~~~~~~~~~~~~~~
 120
 121There's a control filesystem for FUSE, which can be mounted by:
 122
 123  mount -t fusectl none /sys/fs/fuse/connections
 124
 125Mounting it under the '/sys/fs/fuse/connections' directory makes it
 126backwards compatible with earlier versions.
 127
 128Under the fuse control filesystem each connection has a directory
 129named by a unique number.
 130
 131For each connection the following files exist within this directory:
 132
 133 'waiting'
 134
 135  The number of requests which are waiting to be transferred to
 136  userspace or being processed by the filesystem daemon.  If there is
 137  no filesystem activity and 'waiting' is non-zero, then the
 138  filesystem is hung or deadlocked.
 139
 140 'abort'
 141
 142  Writing anything into this file will abort the filesystem
 143  connection.  This means that all waiting requests will be aborted an
 144  error returned for all aborted and new requests.
 145
 146Only the owner of the mount may read or write these files.
 147
 148Interrupting filesystem operations
 149~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 150
 151If a process issuing a FUSE filesystem request is interrupted, the
 152following will happen:
 153
 154  1) If the request is not yet sent to userspace AND the signal is
 155     fatal (SIGKILL or unhandled fatal signal), then the request is
 156     dequeued and returns immediately.
 157
 158  2) If the request is not yet sent to userspace AND the signal is not
 159     fatal, then an 'interrupted' flag is set for the request.  When
 160     the request has been successfully transferred to userspace and
 161     this flag is set, an INTERRUPT request is queued.
 162
 163  3) If the request is already sent to userspace, then an INTERRUPT
 164     request is queued.
 165
 166INTERRUPT requests take precedence over other requests, so the
 167userspace filesystem will receive queued INTERRUPTs before any others.
 168
 169The userspace filesystem may ignore the INTERRUPT requests entirely,
 170or may honor them by sending a reply to the _original_ request, with
 171the error set to EINTR.
 172
 173It is also possible that there's a race between processing the
 174original request and its INTERRUPT request.  There are two possibilities:
 175
 176  1) The INTERRUPT request is processed before the original request is
 177     processed
 178
 179  2) The INTERRUPT request is processed after the original request has
 180     been answered
 181
 182If the filesystem cannot find the original request, it should wait for
 183some timeout and/or a number of new requests to arrive, after which it
 184should reply to the INTERRUPT request with an EAGAIN error.  In case
 1851) the INTERRUPT request will be requeued.  In case 2) the INTERRUPT
 186reply will be ignored.
 187
 188Aborting a filesystem connection
 189~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 190
 191It is possible to get into certain situations where the filesystem is
 192not responding.  Reasons for this may be:
 193
 194  a) Broken userspace filesystem implementation
 195
 196  b) Network connection down
 197
 198  c) Accidental deadlock
 199
 200  d) Malicious deadlock
 201
 202(For more on c) and d) see later sections)
 203
 204In either of these cases it may be useful to abort the connection to
 205the filesystem.  There are several ways to do this:
 206
 207  - Kill the filesystem daemon.  Works in case of a) and b)
 208
 209  - Kill the filesystem daemon and all users of the filesystem.  Works
 210    in all cases except some malicious deadlocks
 211
 212  - Use forced umount (umount -f).  Works in all cases but only if
 213    filesystem is still attached (it hasn't been lazy unmounted)
 214
 215  - Abort filesystem through the FUSE control filesystem.  Most
 216    powerful method, always works.
 217
 218How do non-privileged mounts work?
 219~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 220
 221Since the mount() system call is a privileged operation, a helper
 222program (fusermount) is needed, which is installed setuid root.
 223
 224The implication of providing non-privileged mounts is that the mount
 225owner must not be able to use this capability to compromise the
 226system.  Obvious requirements arising from this are:
 227
 228 A) mount owner should not be able to get elevated privileges with the
 229    help of the mounted filesystem
 230
 231 B) mount owner should not get illegitimate access to information from
 232    other users' and the super user's processes
 233
 234 C) mount owner should not be able to induce undesired behavior in
 235    other users' or the super user's processes
 236
 237How are requirements fulfilled?
 238~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 239
 240 A) The mount owner could gain elevated privileges by either:
 241
 242     1) creating a filesystem containing a device file, then opening
 243        this device
 244
 245     2) creating a filesystem containing a suid or sgid application,
 246        then executing this application
 247
 248    The solution is not to allow opening device files and ignore
 249    setuid and setgid bits when executing programs.  To ensure this
 250    fusermount always adds "nosuid" and "nodev" to the mount options
 251    for non-privileged mounts.
 252
 253 B) If another user is accessing files or directories in the
 254    filesystem, the filesystem daemon serving requests can record the
 255    exact sequence and timing of operations performed.  This
 256    information is otherwise inaccessible to the mount owner, so this
 257    counts as an information leak.
 258
 259    The solution to this problem will be presented in point 2) of C).
 260
 261 C) There are several ways in which the mount owner can induce
 262    undesired behavior in other users' processes, such as:
 263
 264     1) mounting a filesystem over a file or directory which the mount
 265        owner could otherwise not be able to modify (or could only
 266        make limited modifications).
 267
 268        This is solved in fusermount, by checking the access
 269        permissions on the mountpoint and only allowing the mount if
 270        the mount owner can do unlimited modification (has write
 271        access to the mountpoint, and mountpoint is not a "sticky"
 272        directory)
 273
 274     2) Even if 1) is solved the mount owner can change the behavior
 275        of other users' processes.
 276
 277         i) It can slow down or indefinitely delay the execution of a
 278           filesystem operation creating a DoS against the user or the
 279           whole system.  For example a suid application locking a
 280           system file, and then accessing a file on the mount owner's
 281           filesystem could be stopped, and thus causing the system
 282           file to be locked forever.
 283
 284         ii) It can present files or directories of unlimited length, or
 285           directory structures of unlimited depth, possibly causing a
 286           system process to eat up diskspace, memory or other
 287           resources, again causing DoS.
 288
 289        The solution to this as well as B) is not to allow processes
 290        to access the filesystem, which could otherwise not be
 291        monitored or manipulated by the mount owner.  Since if the
 292        mount owner can ptrace a process, it can do all of the above
 293        without using a FUSE mount, the same criteria as used in
 294        ptrace can be used to check if a process is allowed to access
 295        the filesystem or not.
 296
 297        Note that the ptrace check is not strictly necessary to
 298        prevent B/2/i, it is enough to check if mount owner has enough
 299        privilege to send signal to the process accessing the
 300        filesystem, since SIGSTOP can be used to get a similar effect.
 301
 302I think these limitations are unacceptable?
 303~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 304
 305If a sysadmin trusts the users enough, or can ensure through other
 306measures, that system processes will never enter non-privileged
 307mounts, it can relax the last limitation with a "user_allow_other"
 308config option.  If this config option is set, the mounting user can
 309add the "allow_other" mount option which disables the check for other
 310users' processes.
 311
 312Kernel - userspace interface
 313~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 314
 315The following diagram shows how a filesystem operation (in this
 316example unlink) is performed in FUSE.
 317
 318NOTE: everything in this description is greatly simplified
 319
 320 |  "rm /mnt/fuse/file"               |  FUSE filesystem daemon
 321 |                                    |
 322 |                                    |  >sys_read()
 323 |                                    |    >fuse_dev_read()
 324 |                                    |      >request_wait()
 325 |                                    |        [sleep on fc->waitq]
 326 |                                    |
 327 |  >sys_unlink()                     |
 328 |    >fuse_unlink()                  |
 329 |      [get request from             |
 330 |       fc->unused_list]             |
 331 |      >request_send()               |
 332 |        [queue req on fc->pending]  |
 333 |        [wake up fc->waitq]         |        [woken up]
 334 |        >request_wait_answer()      |
 335 |          [sleep on req->waitq]     |
 336 |                                    |      <request_wait()
 337 |                                    |      [remove req from fc->pending]
 338 |                                    |      [copy req to read buffer]
 339 |                                    |      [add req to fc->processing]
 340 |                                    |    <fuse_dev_read()
 341 |                                    |  <sys_read()
 342 |                                    |
 343 |                                    |  [perform unlink]
 344 |                                    |
 345 |                                    |  >sys_write()
 346 |                                    |    >fuse_dev_write()
 347 |                                    |      [look up req in fc->processing]
 348 |                                    |      [remove from fc->processing]
 349 |                                    |      [copy write buffer to req]
 350 |          [woken up]                |      [wake up req->waitq]
 351 |                                    |    <fuse_dev_write()
 352 |                                    |  <sys_write()
 353 |        <request_wait_answer()      |
 354 |      <request_send()               |
 355 |      [add request to               |
 356 |       fc->unused_list]             |
 357 |    <fuse_unlink()                  |
 358 |  <sys_unlink()                     |
 359
 360There are a couple of ways in which to deadlock a FUSE filesystem.
 361Since we are talking about unprivileged userspace programs,
 362something must be done about these.
 363
 364Scenario 1 -  Simple deadlock
 365-----------------------------
 366
 367 |  "rm /mnt/fuse/file"               |  FUSE filesystem daemon
 368 |                                    |
 369 |  >sys_unlink("/mnt/fuse/file")     |
 370 |    [acquire inode semaphore        |
 371 |     for "file"]                    |
 372 |    >fuse_unlink()                  |
 373 |      [sleep on req->waitq]         |
 374 |                                    |  <sys_read()
 375 |                                    |  >sys_unlink("/mnt/fuse/file")
 376 |                                    |    [acquire inode semaphore
 377 |                                    |     for "file"]
 378 |                                    |    *DEADLOCK*
 379
 380The solution for this is to allow the filesystem to be aborted.
 381
 382Scenario 2 - Tricky deadlock
 383----------------------------
 384
 385This one needs a carefully crafted filesystem.  It's a variation on
 386the above, only the call back to the filesystem is not explicit,
 387but is caused by a pagefault.
 388
 389 |  Kamikaze filesystem thread 1      |  Kamikaze filesystem thread 2
 390 |                                    |
 391 |  [fd = open("/mnt/fuse/file")]     |  [request served normally]
 392 |  [mmap fd to 'addr']               |
 393 |  [close fd]                        |  [FLUSH triggers 'magic' flag]
 394 |  [read a byte from addr]           |
 395 |    >do_page_fault()                |
 396 |      [find or create page]         |
 397 |      [lock page]                   |
 398 |      >fuse_readpage()              |
 399 |         [queue READ request]       |
 400 |         [sleep on req->waitq]      |
 401 |                                    |  [read request to buffer]
 402 |                                    |  [create reply header before addr]
 403 |                                    |  >sys_write(addr - headerlength)
 404 |                                    |    >fuse_dev_write()
 405 |                                    |      [look up req in fc->processing]
 406 |                                    |      [remove from fc->processing]
 407 |                                    |      [copy write buffer to req]
 408 |                                    |        >do_page_fault()
 409 |                                    |           [find or create page]
 410 |                                    |           [lock page]
 411 |                                    |           * DEADLOCK *
 412
 413Solution is basically the same as above.
 414
 415An additional problem is that while the write buffer is being copied
 416to the request, the request must not be interrupted/aborted.  This is
 417because the destination address of the copy may not be valid after the
 418request has returned.
 419
 420This is solved with doing the copy atomically, and allowing abort
 421while the page(s) belonging to the write buffer are faulted with
 422get_user_pages().  The 'req->locked' flag indicates when the copy is
 423taking place, and abort is delayed until this flag is unset.
 424
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