Definitions
  ~~~~~~~~~~~
  
  Userspace filesystem:
  
    A filesystem in which data and metadata are provided by an ordinary
    userspace process.  The filesystem can be accessed normally through
    the kernel interface.
  
  Filesystem daemon:
  
    The process(es) providing the data and metadata of the filesystem.
  
  Non-privileged mount (or user mount):
  
    A userspace filesystem mounted by a non-privileged (non-root) user.
    The filesystem daemon is running with the privileges of the mounting
    user.  NOTE: this is not the same as mounts allowed with the "user"
    option in /etc/fstab, which is not discussed here.

  Filesystem connection:
  
    A connection between the filesystem daemon and the kernel.  The
    connection exists until either the daemon dies, or the filesystem is
    umounted.  Note that detaching (or lazy umounting) the filesystem
    does _not_ break the connection, in this case it will exist until
    the last reference to the filesystem is released.

  Mount owner:
  
    The user who does the mounting.
  
  User:
  
    The user who is performing filesystem operations.
  
  What is FUSE?
  ~~~~~~~~~~~~~
  
  FUSE is a userspace filesystem framework.  It consists of a kernel
  module (fuse.ko), a userspace library (libfuse.*) and a mount utility
  (fusermount).
  
  One of the most important features of FUSE is allowing secure,
  non-privileged mounts.  This opens up new possibilities for the use of
  filesystems.  A good example is sshfs: a secure network filesystem
  using the sftp protocol.
  
  The userspace library and utilities are available from the FUSE
  homepage:
  
    http://fuse.sourceforge.net/

  Filesystem type
  ~~~~~~~~~~~~~~~
  
  The filesystem type given to mount(2) can be one of the following:
  
  'fuse'
  
    This is the usual way to mount a FUSE filesystem.  The first
    argument of the mount system call may contain an arbitrary string,
    which is not interpreted by the kernel.
  
  'fuseblk'
  
    The filesystem is block device based.  The first argument of the
    mount system call is interpreted as the name of the device.

  Mount options
  ~~~~~~~~~~~~~
  
  'fd=N'
  
    The file descriptor to use for communication between the userspace
    filesystem and the kernel.  The file descriptor must have been
    obtained by opening the FUSE device ('/dev/fuse').
  
  'rootmode=M'
  
    The file mode of the filesystem's root in octal representation.
  
  'user_id=N'
  
    The numeric user id of the mount owner.
  
  'group_id=N'
  
    The numeric group id of the mount owner.
  
  'default_permissions'
  
    By default FUSE doesn't check file access permissions, the

    filesystem is free to implement its access policy or leave it to

    the underlying file access mechanism (e.g. in case of network
    filesystems).  This option enables permission checking, restricting

    access based on file mode.  It is usually useful together with the
    'allow_other' mount option.

  
  'allow_other'
  
    This option overrides the security measure restricting file access
    to the user mounting the filesystem.  This option is by default only
    allowed to root, but this restriction can be removed with a
    (userspace) configuration option.

  'max_read=N'
  
    With this option the maximum size of read operations can be set.
    The default is infinite.  Note that the size of read requests is
    limited anyway to 32 pages (which is 128kbyte on i386).

  'blksize=N'
  
    Set the block size for the filesystem.  The default is 512.  This
    option is only valid for 'fuseblk' type mounts.

  Control filesystem
  ~~~~~~~~~~~~~~~~~~
  
  There's a control filesystem for FUSE, which can be mounted by:

    mount -t fusectl none /sys/fs/fuse/connections

  Mounting it under the '/sys/fs/fuse/connections' directory makes it
  backwards compatible with earlier versions.

  Under the fuse control filesystem each connection has a directory
  named by a unique number.

  For each connection the following files exist within this directory:

  
   'waiting'

    The number of requests which are waiting to be transferred to

    userspace or being processed by the filesystem daemon.  If there is
    no filesystem activity and 'waiting' is non-zero, then the
    filesystem is hung or deadlocked.
  
   'abort'
  
    Writing anything into this file will abort the filesystem
    connection.  This means that all waiting requests will be aborted an
    error returned for all aborted and new requests.

  Only the owner of the mount may read or write these files.

  Interrupting filesystem operations
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
  If a process issuing a FUSE filesystem request is interrupted, the
  following will happen:
  
    1) If the request is not yet sent to userspace AND the signal is
       fatal (SIGKILL or unhandled fatal signal), then the request is
       dequeued and returns immediately.
  
    2) If the request is not yet sent to userspace AND the signal is not
       fatal, then an 'interrupted' flag is set for the request.  When

       the request has been successfully transferred to userspace and

       this flag is set, an INTERRUPT request is queued.
  
    3) If the request is already sent to userspace, then an INTERRUPT
       request is queued.
  
  INTERRUPT requests take precedence over other requests, so the
  userspace filesystem will receive queued INTERRUPTs before any others.
  
  The userspace filesystem may ignore the INTERRUPT requests entirely,
  or may honor them by sending a reply to the _original_ request, with
  the error set to EINTR.
  
  It is also possible that there's a race between processing the

  original request and its INTERRUPT request.  There are two possibilities:

  
    1) The INTERRUPT request is processed before the original request is
       processed
  
    2) The INTERRUPT request is processed after the original request has
       been answered
  
  If the filesystem cannot find the original request, it should wait for
  some timeout and/or a number of new requests to arrive, after which it
  should reply to the INTERRUPT request with an EAGAIN error.  In case
  1) the INTERRUPT request will be requeued.  In case 2) the INTERRUPT
  reply will be ignored.

  Aborting a filesystem connection
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
  It is possible to get into certain situations where the filesystem is
  not responding.  Reasons for this may be:
  
    a) Broken userspace filesystem implementation
  
    b) Network connection down
  
    c) Accidental deadlock
  
    d) Malicious deadlock
  
  (For more on c) and d) see later sections)
  
  In either of these cases it may be useful to abort the connection to
  the filesystem.  There are several ways to do this:
  
    - Kill the filesystem daemon.  Works in case of a) and b)
  
    - Kill the filesystem daemon and all users of the filesystem.  Works
      in all cases except some malicious deadlocks
  
    - Use forced umount (umount -f).  Works in all cases but only if
      filesystem is still attached (it hasn't been lazy unmounted)

    - Abort filesystem through the FUSE control filesystem.  Most
      powerful method, always works.

  How do non-privileged mounts work?
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
  Since the mount() system call is a privileged operation, a helper
  program (fusermount) is needed, which is installed setuid root.
  
  The implication of providing non-privileged mounts is that the mount
  owner must not be able to use this capability to compromise the
  system.  Obvious requirements arising from this are:
  
   A) mount owner should not be able to get elevated privileges with the
      help of the mounted filesystem
  
   B) mount owner should not get illegitimate access to information from
      other users' and the super user's processes
  
   C) mount owner should not be able to induce undesired behavior in
      other users' or the super user's processes
  
  How are requirements fulfilled?
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
   A) The mount owner could gain elevated privileges by either:
  
       1) creating a filesystem containing a device file, then opening
  	this device
  
       2) creating a filesystem containing a suid or sgid application,
  	then executing this application
  
      The solution is not to allow opening device files and ignore
      setuid and setgid bits when executing programs.  To ensure this
      fusermount always adds "nosuid" and "nodev" to the mount options
      for non-privileged mounts.
  
   B) If another user is accessing files or directories in the
      filesystem, the filesystem daemon serving requests can record the
      exact sequence and timing of operations performed.  This
      information is otherwise inaccessible to the mount owner, so this
      counts as an information leak.
  
      The solution to this problem will be presented in point 2) of C).
  
   C) There are several ways in which the mount owner can induce
      undesired behavior in other users' processes, such as:
  
       1) mounting a filesystem over a file or directory which the mount
          owner could otherwise not be able to modify (or could only
          make limited modifications).
  
          This is solved in fusermount, by checking the access
          permissions on the mountpoint and only allowing the mount if
          the mount owner can do unlimited modification (has write
          access to the mountpoint, and mountpoint is not a "sticky"
          directory)
  
       2) Even if 1) is solved the mount owner can change the behavior
          of other users' processes.
  
           i) It can slow down or indefinitely delay the execution of a
             filesystem operation creating a DoS against the user or the
             whole system.  For example a suid application locking a
             system file, and then accessing a file on the mount owner's
             filesystem could be stopped, and thus causing the system
             file to be locked forever.
  
           ii) It can present files or directories of unlimited length, or
             directory structures of unlimited depth, possibly causing a
             system process to eat up diskspace, memory or other
             resources, again causing DoS.
  
  	The solution to this as well as B) is not to allow processes
  	to access the filesystem, which could otherwise not be
  	monitored or manipulated by the mount owner.  Since if the
  	mount owner can ptrace a process, it can do all of the above
  	without using a FUSE mount, the same criteria as used in
  	ptrace can be used to check if a process is allowed to access
  	the filesystem or not.
  
  	Note that the ptrace check is not strictly necessary to
  	prevent B/2/i, it is enough to check if mount owner has enough
  	privilege to send signal to the process accessing the
  	filesystem, since SIGSTOP can be used to get a similar effect.
  
  I think these limitations are unacceptable?
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
  If a sysadmin trusts the users enough, or can ensure through other
  measures, that system processes will never enter non-privileged
  mounts, it can relax the last limitation with a "user_allow_other"
  config option.  If this config option is set, the mounting user can
  add the "allow_other" mount option which disables the check for other
  users' processes.
  
  Kernel - userspace interface
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
  The following diagram shows how a filesystem operation (in this
  example unlink) is performed in FUSE.
  
  NOTE: everything in this description is greatly simplified
  
   |  "rm /mnt/fuse/file"               |  FUSE filesystem daemon
   |                                    |
   |                                    |  >sys_read()
   |                                    |    >fuse_dev_read()
   |                                    |      >request_wait()
   |                                    |        [sleep on fc->waitq]
   |                                    |
   |  >sys_unlink()                     |
   |    >fuse_unlink()                  |
   |      [get request from             |
   |       fc->unused_list]             |
   |      >request_send()               |
   |        [queue req on fc->pending]  |
   |        [wake up fc->waitq]         |        [woken up]
   |        >request_wait_answer()      |
   |          [sleep on req->waitq]     |
   |                                    |      <request_wait()
   |                                    |      [remove req from fc->pending]
   |                                    |      [copy req to read buffer]
   |                                    |      [add req to fc->processing]
   |                                    |    <fuse_dev_read()
   |                                    |  <sys_read()
   |                                    |
   |                                    |  [perform unlink]
   |                                    |
   |                                    |  >sys_write()
   |                                    |    >fuse_dev_write()
   |                                    |      [look up req in fc->processing]
   |                                    |      [remove from fc->processing]
   |                                    |      [copy write buffer to req]
   |          [woken up]                |      [wake up req->waitq]
   |                                    |    <fuse_dev_write()
   |                                    |  <sys_write()
   |        <request_wait_answer()      |
   |      <request_send()               |
   |      [add request to               |
   |       fc->unused_list]             |
   |    <fuse_unlink()                  |
   |  <sys_unlink()                     |
  
  There are a couple of ways in which to deadlock a FUSE filesystem.
  Since we are talking about unprivileged userspace programs,
  something must be done about these.
  
  Scenario 1 -  Simple deadlock
  -----------------------------
  
   |  "rm /mnt/fuse/file"               |  FUSE filesystem daemon
   |                                    |
   |  >sys_unlink("/mnt/fuse/file")     |
   |    [acquire inode semaphore        |
   |     for "file"]                    |
   |    >fuse_unlink()                  |
   |      [sleep on req->waitq]         |
   |                                    |  <sys_read()
   |                                    |  >sys_unlink("/mnt/fuse/file")
   |                                    |    [acquire inode semaphore
   |                                    |     for "file"]
   |                                    |    *DEADLOCK*

  The solution for this is to allow the filesystem to be aborted.

  
  Scenario 2 - Tricky deadlock
  ----------------------------
  
  This one needs a carefully crafted filesystem.  It's a variation on
  the above, only the call back to the filesystem is not explicit,
  but is caused by a pagefault.
  
   |  Kamikaze filesystem thread 1      |  Kamikaze filesystem thread 2
   |                                    |
   |  [fd = open("/mnt/fuse/file")]     |  [request served normally]
   |  [mmap fd to 'addr']               |
   |  [close fd]                        |  [FLUSH triggers 'magic' flag]
   |  [read a byte from addr]           |
   |    >do_page_fault()                |
   |      [find or create page]         |
   |      [lock page]                   |
   |      >fuse_readpage()              |
   |         [queue READ request]       |
   |         [sleep on req->waitq]      |
   |                                    |  [read request to buffer]
   |                                    |  [create reply header before addr]
   |                                    |  >sys_write(addr - headerlength)
   |                                    |    >fuse_dev_write()
   |                                    |      [look up req in fc->processing]
   |                                    |      [remove from fc->processing]
   |                                    |      [copy write buffer to req]
   |                                    |        >do_page_fault()
   |                                    |           [find or create page]
   |                                    |           [lock page]
   |                                    |           * DEADLOCK *

  Solution is basically the same as above.

  An additional problem is that while the write buffer is being copied
  to the request, the request must not be interrupted/aborted.  This is
  because the destination address of the copy may not be valid after the
  request has returned.

  This is solved with doing the copy atomically, and allowing abort
  while the page(s) belonging to the write buffer are faulted with
  get_user_pages().  The 'req->locked' flag indicates when the copy is
  taking place, and abort is delayed until this flag is unset.