linux/Documentation/filesystems/fuse.rst
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   1.. SPDX-License-Identifier: GPL-2.0
   2
   3====
   4FUSE
   5====
   6
   7Definitions
   8===========
   9
  10Userspace filesystem:
  11  A filesystem in which data and metadata are provided by an ordinary
  12  userspace process.  The filesystem can be accessed normally through
  13  the kernel interface.
  14
  15Filesystem daemon:
  16  The process(es) providing the data and metadata of the filesystem.
  17
  18Non-privileged mount (or user mount):
  19  A userspace filesystem mounted by a non-privileged (non-root) user.
  20  The filesystem daemon is running with the privileges of the mounting
  21  user.  NOTE: this is not the same as mounts allowed with the "user"
  22  option in /etc/fstab, which is not discussed here.
  23
  24Filesystem connection:
  25  A connection between the filesystem daemon and the kernel.  The
  26  connection exists until either the daemon dies, or the filesystem is
  27  umounted.  Note that detaching (or lazy umounting) the filesystem
  28  does *not* break the connection, in this case it will exist until
  29  the last reference to the filesystem is released.
  30
  31Mount owner:
  32  The user who does the mounting.
  33
  34User:
  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
  50`FUSE homepage: <https://github.com/libfuse/>`_
  51
  52Filesystem type
  53===============
  54
  55The filesystem type given to mount(2) can be one of the following:
  56
  57    fuse
  58      This is the usual way to mount a FUSE filesystem.  The first
  59      argument of the mount system call may contain an arbitrary string,
  60      which is not interpreted by the kernel.
  61
  62    fuseblk
  63      The filesystem is block device based.  The first argument of the
  64      mount system call is interpreted as the name of the device.
  65
  66Mount options
  67=============
  68
  69fd=N
  70  The file descriptor to use for communication between the userspace
  71  filesystem and the kernel.  The file descriptor must have been
  72  obtained by opening the FUSE device ('/dev/fuse').
  73
  74rootmode=M
  75  The file mode of the filesystem's root in octal representation.
  76
  77user_id=N
  78  The numeric user id of the mount owner.
  79
  80group_id=N
  81  The numeric group id of the mount owner.
  82
  83default_permissions
  84  By default FUSE doesn't check file access permissions, the
  85  filesystem is free to implement its access policy or leave it to
  86  the underlying file access mechanism (e.g. in case of network
  87  filesystems).  This option enables permission checking, restricting
  88  access based on file mode.  It is usually useful together with the
  89  'allow_other' mount option.
  90
  91allow_other
  92  This option overrides the security measure restricting file access
  93  to the user mounting the filesystem.  This option is by default only
  94  allowed to root, but this restriction can be removed with a
  95  (userspace) configuration option.
  96
  97max_read=N
  98  With this option the maximum size of read operations can be set.
  99  The default is infinite.  Note that the size of read requests is
 100  limited anyway to 32 pages (which is 128kbyte on i386).
 101
 102blksize=N
 103  Set the block size for the filesystem.  The default is 512.  This
 104  option is only valid for 'fuseblk' type mounts.
 105
 106Control filesystem
 107==================
 108
 109There's a control filesystem for FUSE, which can be mounted by::
 110
 111  mount -t fusectl none /sys/fs/fuse/connections
 112
 113Mounting it under the '/sys/fs/fuse/connections' directory makes it
 114backwards compatible with earlier versions.
 115
 116Under the fuse control filesystem each connection has a directory
 117named by a unique number.
 118
 119For each connection the following files exist within this directory:
 120
 121        waiting
 122          The number of requests which are waiting to be transferred to
 123          userspace or being processed by the filesystem daemon.  If there is
 124          no filesystem activity and 'waiting' is non-zero, then the
 125          filesystem is hung or deadlocked.
 126
 127        abort
 128          Writing anything into this file will abort the filesystem
 129          connection.  This means that all waiting requests will be aborted an
 130          error returned for all aborted and new requests.
 131
 132Only the owner of the mount may read or write these files.
 133
 134Interrupting filesystem operations
 135##################################
 136
 137If a process issuing a FUSE filesystem request is interrupted, the
 138following will happen:
 139
 140  -  If the request is not yet sent to userspace AND the signal is
 141     fatal (SIGKILL or unhandled fatal signal), then the request is
 142     dequeued and returns immediately.
 143
 144  -  If the request is not yet sent to userspace AND the signal is not
 145     fatal, then an interrupted flag is set for the request.  When
 146     the request has been successfully transferred to userspace and
 147     this flag is set, an INTERRUPT request is queued.
 148
 149  -  If the request is already sent to userspace, then an INTERRUPT
 150     request is queued.
 151
 152INTERRUPT requests take precedence over other requests, so the
 153userspace filesystem will receive queued INTERRUPTs before any others.
 154
 155The userspace filesystem may ignore the INTERRUPT requests entirely,
 156or may honor them by sending a reply to the *original* request, with
 157the error set to EINTR.
 158
 159It is also possible that there's a race between processing the
 160original request and its INTERRUPT request.  There are two possibilities:
 161
 162  1. The INTERRUPT request is processed before the original request is
 163     processed
 164
 165  2. The INTERRUPT request is processed after the original request has
 166     been answered
 167
 168If the filesystem cannot find the original request, it should wait for
 169some timeout and/or a number of new requests to arrive, after which it
 170should reply to the INTERRUPT request with an EAGAIN error.  In case
 1711) the INTERRUPT request will be requeued.  In case 2) the INTERRUPT
 172reply will be ignored.
 173
 174Aborting a filesystem connection
 175================================
 176
 177It is possible to get into certain situations where the filesystem is
 178not responding.  Reasons for this may be:
 179
 180  a) Broken userspace filesystem implementation
 181
 182  b) Network connection down
 183
 184  c) Accidental deadlock
 185
 186  d) Malicious deadlock
 187
 188(For more on c) and d) see later sections)
 189
 190In either of these cases it may be useful to abort the connection to
 191the filesystem.  There are several ways to do this:
 192
 193  - Kill the filesystem daemon.  Works in case of a) and b)
 194
 195  - Kill the filesystem daemon and all users of the filesystem.  Works
 196    in all cases except some malicious deadlocks
 197
 198  - Use forced umount (umount -f).  Works in all cases but only if
 199    filesystem is still attached (it hasn't been lazy unmounted)
 200
 201  - Abort filesystem through the FUSE control filesystem.  Most
 202    powerful method, always works.
 203
 204How do non-privileged mounts work?
 205==================================
 206
 207Since the mount() system call is a privileged operation, a helper
 208program (fusermount) is needed, which is installed setuid root.
 209
 210The implication of providing non-privileged mounts is that the mount
 211owner must not be able to use this capability to compromise the
 212system.  Obvious requirements arising from this are:
 213
 214 A) mount owner should not be able to get elevated privileges with the
 215    help of the mounted filesystem
 216
 217 B) mount owner should not get illegitimate access to information from
 218    other users' and the super user's processes
 219
 220 C) mount owner should not be able to induce undesired behavior in
 221    other users' or the super user's processes
 222
 223How are requirements fulfilled?
 224===============================
 225
 226 A) The mount owner could gain elevated privileges by either:
 227
 228    1. creating a filesystem containing a device file, then opening this device
 229
 230    2. creating a filesystem containing a suid or sgid application, then executing this application
 231
 232    The solution is not to allow opening device files and ignore
 233    setuid and setgid bits when executing programs.  To ensure this
 234    fusermount always adds "nosuid" and "nodev" to the mount options
 235    for non-privileged mounts.
 236
 237 B) If another user is accessing files or directories in the
 238    filesystem, the filesystem daemon serving requests can record the
 239    exact sequence and timing of operations performed.  This
 240    information is otherwise inaccessible to the mount owner, so this
 241    counts as an information leak.
 242
 243    The solution to this problem will be presented in point 2) of C).
 244
 245 C) There are several ways in which the mount owner can induce
 246    undesired behavior in other users' processes, such as:
 247
 248     1) mounting a filesystem over a file or directory which the mount
 249        owner could otherwise not be able to modify (or could only
 250        make limited modifications).
 251
 252        This is solved in fusermount, by checking the access
 253        permissions on the mountpoint and only allowing the mount if
 254        the mount owner can do unlimited modification (has write
 255        access to the mountpoint, and mountpoint is not a "sticky"
 256        directory)
 257
 258     2) Even if 1) is solved the mount owner can change the behavior
 259        of other users' processes.
 260
 261         i) It can slow down or indefinitely delay the execution of a
 262            filesystem operation creating a DoS against the user or the
 263            whole system.  For example a suid application locking a
 264            system file, and then accessing a file on the mount owner's
 265            filesystem could be stopped, and thus causing the system
 266            file to be locked forever.
 267
 268         ii) It can present files or directories of unlimited length, or
 269             directory structures of unlimited depth, possibly causing a
 270             system process to eat up diskspace, memory or other
 271             resources, again causing *DoS*.
 272
 273        The solution to this as well as B) is not to allow processes
 274        to access the filesystem, which could otherwise not be
 275        monitored or manipulated by the mount owner.  Since if the
 276        mount owner can ptrace a process, it can do all of the above
 277        without using a FUSE mount, the same criteria as used in
 278        ptrace can be used to check if a process is allowed to access
 279        the filesystem or not.
 280
 281        Note that the *ptrace* check is not strictly necessary to
 282        prevent B/2/i, it is enough to check if mount owner has enough
 283        privilege to send signal to the process accessing the
 284        filesystem, since *SIGSTOP* can be used to get a similar effect.
 285
 286I think these limitations are unacceptable?
 287===========================================
 288
 289If a sysadmin trusts the users enough, or can ensure through other
 290measures, that system processes will never enter non-privileged
 291mounts, it can relax the last limitation with a 'user_allow_other'
 292config option.  If this config option is set, the mounting user can
 293add the 'allow_other' mount option which disables the check for other
 294users' processes.
 295
 296Kernel - userspace interface
 297============================
 298
 299The following diagram shows how a filesystem operation (in this
 300example unlink) is performed in FUSE. ::
 301
 302
 303 |  "rm /mnt/fuse/file"               |  FUSE filesystem daemon
 304 |                                    |
 305 |                                    |  >sys_read()
 306 |                                    |    >fuse_dev_read()
 307 |                                    |      >request_wait()
 308 |                                    |        [sleep on fc->waitq]
 309 |                                    |
 310 |  >sys_unlink()                     |
 311 |    >fuse_unlink()                  |
 312 |      [get request from             |
 313 |       fc->unused_list]             |
 314 |      >request_send()               |
 315 |        [queue req on fc->pending]  |
 316 |        [wake up fc->waitq]         |        [woken up]
 317 |        >request_wait_answer()      |
 318 |          [sleep on req->waitq]     |
 319 |                                    |      <request_wait()
 320 |                                    |      [remove req from fc->pending]
 321 |                                    |      [copy req to read buffer]
 322 |                                    |      [add req to fc->processing]
 323 |                                    |    <fuse_dev_read()
 324 |                                    |  <sys_read()
 325 |                                    |
 326 |                                    |  [perform unlink]
 327 |                                    |
 328 |                                    |  >sys_write()
 329 |                                    |    >fuse_dev_write()
 330 |                                    |      [look up req in fc->processing]
 331 |                                    |      [remove from fc->processing]
 332 |                                    |      [copy write buffer to req]
 333 |          [woken up]                |      [wake up req->waitq]
 334 |                                    |    <fuse_dev_write()
 335 |                                    |  <sys_write()
 336 |        <request_wait_answer()      |
 337 |      <request_send()               |
 338 |      [add request to               |
 339 |       fc->unused_list]             |
 340 |    <fuse_unlink()                  |
 341 |  <sys_unlink()                     |
 342
 343.. note:: Everything in the description above is greatly simplified
 344
 345There are a couple of ways in which to deadlock a FUSE filesystem.
 346Since we are talking about unprivileged userspace programs,
 347something must be done about these.
 348
 349**Scenario 1 -  Simple deadlock**::
 350
 351 |  "rm /mnt/fuse/file"               |  FUSE filesystem daemon
 352 |                                    |
 353 |  >sys_unlink("/mnt/fuse/file")     |
 354 |    [acquire inode semaphore        |
 355 |     for "file"]                    |
 356 |    >fuse_unlink()                  |
 357 |      [sleep on req->waitq]         |
 358 |                                    |  <sys_read()
 359 |                                    |  >sys_unlink("/mnt/fuse/file")
 360 |                                    |    [acquire inode semaphore
 361 |                                    |     for "file"]
 362 |                                    |    *DEADLOCK*
 363
 364The solution for this is to allow the filesystem to be aborted.
 365
 366**Scenario 2 - Tricky deadlock**
 367
 368
 369This one needs a carefully crafted filesystem.  It's a variation on
 370the above, only the call back to the filesystem is not explicit,
 371but is caused by a pagefault. ::
 372
 373 |  Kamikaze filesystem thread 1      |  Kamikaze filesystem thread 2
 374 |                                    |
 375 |  [fd = open("/mnt/fuse/file")]     |  [request served normally]
 376 |  [mmap fd to 'addr']               |
 377 |  [close fd]                        |  [FLUSH triggers 'magic' flag]
 378 |  [read a byte from addr]           |
 379 |    >do_page_fault()                |
 380 |      [find or create page]         |
 381 |      [lock page]                   |
 382 |      >fuse_readpage()              |
 383 |         [queue READ request]       |
 384 |         [sleep on req->waitq]      |
 385 |                                    |  [read request to buffer]
 386 |                                    |  [create reply header before addr]
 387 |                                    |  >sys_write(addr - headerlength)
 388 |                                    |    >fuse_dev_write()
 389 |                                    |      [look up req in fc->processing]
 390 |                                    |      [remove from fc->processing]
 391 |                                    |      [copy write buffer to req]
 392 |                                    |        >do_page_fault()
 393 |                                    |           [find or create page]
 394 |                                    |           [lock page]
 395 |                                    |           * DEADLOCK *
 396
 397The solution is basically the same as above.
 398
 399An additional problem is that while the write buffer is being copied
 400to the request, the request must not be interrupted/aborted.  This is
 401because the destination address of the copy may not be valid after the
 402request has returned.
 403
 404This is solved with doing the copy atomically, and allowing abort
 405while the page(s) belonging to the write buffer are faulted with
 406get_user_pages().  The 'req->locked' flag indicates when the copy is
 407taking place, and abort is delayed until this flag is unset.
 408