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
(fusermount3).

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:

  https://github.com/libfuse/libfuse/

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
~~~~~~~~~~~~~

See mount.fuse(8).

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 it's 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 (fusermount3) 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
    fusermount3 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 fusermount3, 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.