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    1 Definitions
    2 ~~~~~~~~~~~
    3 
    4 Userspace 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 
   10 Filesystem daemon:
   11 
   12   The process(es) providing the data and metadata of the filesystem.
   13 
   14 Non-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 
   21 Filesystem 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 
   29 Mount owner:
   30 
   31   The user who does the mounting.
   32 
   33 User:
   34 
   35   The user who is performing filesystem operations.
   36 
   37 What is FUSE?
   38 ~~~~~~~~~~~~~
   39 
   40 FUSE is a userspace filesystem framework.  It consists of a kernel
   41 module (fuse.ko), a userspace library (libfuse.*) and a mount utility
   42 (fusermount3).
   43 
   44 One of the most important features of FUSE is allowing secure,
   45 non-privileged mounts.  This opens up new possibilities for the use of
   46 filesystems.  A good example is sshfs: a secure network filesystem
   47 using the sftp protocol.
   48 
   49 The userspace library and utilities are available from the FUSE
   50 homepage:
   51 
   52   https://github.com/libfuse/libfuse/
   53 
   54 Filesystem type
   55 ~~~~~~~~~~~~~~~
   56 
   57 The 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 
   70 Mount options
   71 ~~~~~~~~~~~~~
   72 
   73 See mount.fuse(8).
   74 
   75 Control filesystem
   76 ~~~~~~~~~~~~~~~~~~
   77 
   78 There's a control filesystem for FUSE, which can be mounted by:
   79 
   80   mount -t fusectl none /sys/fs/fuse/connections
   81 
   82 Mounting it under the '/sys/fs/fuse/connections' directory makes it
   83 backwards compatible with earlier versions.
   84 
   85 Under the fuse control filesystem each connection has a directory
   86 named by a unique number.
   87 
   88 For each connection the following files exist within this directory:
   89 
   90  'waiting'
   91 
   92   The number of requests which are waiting to be transferred to
   93   userspace or being processed by the filesystem daemon.  If there is
   94   no filesystem activity and 'waiting' is non-zero, then the
   95   filesystem is hung or deadlocked.
   96 
   97  'abort'
   98 
   99   Writing anything into this file will abort the filesystem
  100   connection.  This means that all waiting requests will be aborted an
  101   error returned for all aborted and new requests.
  102 
  103 Only the owner of the mount may read or write these files.
  104 
  105 Interrupting filesystem operations
  106 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  107 
  108 If a process issuing a FUSE filesystem request is interrupted, the
  109 following will happen:
  110 
  111   1) If the request is not yet sent to userspace AND the signal is
  112      fatal (SIGKILL or unhandled fatal signal), then the request is
  113      dequeued and returns immediately.
  114 
  115   2) If the request is not yet sent to userspace AND the signal is not
  116      fatal, then an 'interrupted' flag is set for the request.  When
  117      the request has been successfully transferred to userspace and
  118      this flag is set, an INTERRUPT request is queued.
  119 
  120   3) If the request is already sent to userspace, then an INTERRUPT
  121      request is queued.
  122 
  123 INTERRUPT requests take precedence over other requests, so the
  124 userspace filesystem will receive queued INTERRUPTs before any others.
  125 
  126 The userspace filesystem may ignore the INTERRUPT requests entirely,
  127 or may honor them by sending a reply to the _original_ request, with
  128 the error set to EINTR.
  129 
  130 It is also possible that there's a race between processing the
  131 original request and it's INTERRUPT request.  There are two possibilities:
  132 
  133   1) The INTERRUPT request is processed before the original request is
  134      processed
  135 
  136   2) The INTERRUPT request is processed after the original request has
  137      been answered
  138 
  139 If the filesystem cannot find the original request, it should wait for
  140 some timeout and/or a number of new requests to arrive, after which it
  141 should reply to the INTERRUPT request with an EAGAIN error.  In case
  142 1) the INTERRUPT request will be requeued.  In case 2) the INTERRUPT
  143 reply will be ignored.
  144 
  145 Aborting a filesystem connection
  146 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  147 
  148 It is possible to get into certain situations where the filesystem is
  149 not responding.  Reasons for this may be:
  150 
  151   a) Broken userspace filesystem implementation
  152 
  153   b) Network connection down
  154 
  155   c) Accidental deadlock
  156 
  157   d) Malicious deadlock
  158 
  159 (For more on c) and d) see later sections)
  160 
  161 In either of these cases it may be useful to abort the connection to
  162 the filesystem.  There are several ways to do this:
  163 
  164   - Kill the filesystem daemon.  Works in case of a) and b)
  165 
  166   - Kill the filesystem daemon and all users of the filesystem.  Works
  167     in all cases except some malicious deadlocks
  168 
  169   - Use forced umount (umount -f).  Works in all cases but only if
  170     filesystem is still attached (it hasn't been lazy unmounted)
  171 
  172   - Abort filesystem through the FUSE control filesystem.  Most
  173     powerful method, always works.
  174 
  175 How do non-privileged mounts work?
  176 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  177 
  178 Since the mount() system call is a privileged operation, a helper
  179 program (fusermount3) is needed, which is installed setuid root.
  180 
  181 The implication of providing non-privileged mounts is that the mount
  182 owner must not be able to use this capability to compromise the
  183 system.  Obvious requirements arising from this are:
  184 
  185  A) mount owner should not be able to get elevated privileges with the
  186     help of the mounted filesystem
  187 
  188  B) mount owner should not get illegitimate access to information from
  189     other users' and the super user's processes
  190 
  191  C) mount owner should not be able to induce undesired behavior in
  192     other users' or the super user's processes
  193 
  194 How are requirements fulfilled?
  195 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  196 
  197  A) The mount owner could gain elevated privileges by either:
  198 
  199      1) creating a filesystem containing a device file, then opening
  200 	this device
  201 
  202      2) creating a filesystem containing a suid or sgid application,
  203 	then executing this application
  204 
  205     The solution is not to allow opening device files and ignore
  206     setuid and setgid bits when executing programs.  To ensure this
  207     fusermount3 always adds "nosuid" and "nodev" to the mount options
  208     for non-privileged mounts.
  209 
  210  B) If another user is accessing files or directories in the
  211     filesystem, the filesystem daemon serving requests can record the
  212     exact sequence and timing of operations performed.  This
  213     information is otherwise inaccessible to the mount owner, so this
  214     counts as an information leak.
  215 
  216     The solution to this problem will be presented in point 2) of C).
  217 
  218  C) There are several ways in which the mount owner can induce
  219     undesired behavior in other users' processes, such as:
  220 
  221      1) mounting a filesystem over a file or directory which the mount
  222         owner could otherwise not be able to modify (or could only
  223         make limited modifications).
  224 
  225         This is solved in fusermount3, by checking the access
  226         permissions on the mountpoint and only allowing the mount if
  227         the mount owner can do unlimited modification (has write
  228         access to the mountpoint, and mountpoint is not a "sticky"
  229         directory)
  230 
  231      2) Even if 1) is solved the mount owner can change the behavior
  232         of other users' processes.
  233 
  234          i) It can slow down or indefinitely delay the execution of a
  235            filesystem operation creating a DoS against the user or the
  236            whole system.  For example a suid application locking a
  237            system file, and then accessing a file on the mount owner's
  238            filesystem could be stopped, and thus causing the system
  239            file to be locked forever.
  240 
  241          ii) It can present files or directories of unlimited length, or
  242            directory structures of unlimited depth, possibly causing a
  243            system process to eat up diskspace, memory or other
  244            resources, again causing DoS.
  245 
  246 	The solution to this as well as B) is not to allow processes
  247 	to access the filesystem, which could otherwise not be
  248 	monitored or manipulated by the mount owner.  Since if the
  249 	mount owner can ptrace a process, it can do all of the above
  250 	without using a FUSE mount, the same criteria as used in
  251 	ptrace can be used to check if a process is allowed to access
  252 	the filesystem or not.
  253 
  254 	Note that the ptrace check is not strictly necessary to
  255 	prevent B/2/i, it is enough to check if mount owner has enough
  256 	privilege to send signal to the process accessing the
  257 	filesystem, since SIGSTOP can be used to get a similar effect.
  258 
  259 I think these limitations are unacceptable?
  260 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  261 
  262 If a sysadmin trusts the users enough, or can ensure through other
  263 measures, that system processes will never enter non-privileged
  264 mounts, it can relax the last limitation with a "user_allow_other"
  265 config option.  If this config option is set, the mounting user can
  266 add the "allow_other" mount option which disables the check for other
  267 users' processes.
  268 
  269 Kernel - userspace interface
  270 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  271 
  272 The following diagram shows how a filesystem operation (in this
  273 example unlink) is performed in FUSE.
  274 
  275 NOTE: everything in this description is greatly simplified
  276 
  277  |  "rm /mnt/fuse/file"               |  FUSE filesystem daemon
  278  |                                    |
  279  |                                    |  >sys_read()
  280  |                                    |    >fuse_dev_read()
  281  |                                    |      >request_wait()
  282  |                                    |        [sleep on fc->waitq]
  283  |                                    |
  284  |  >sys_unlink()                     |
  285  |    >fuse_unlink()                  |
  286  |      [get request from             |
  287  |       fc->unused_list]             |
  288  |      >request_send()               |
  289  |        [queue req on fc->pending]  |
  290  |        [wake up fc->waitq]         |        [woken up]
  291  |        >request_wait_answer()      |
  292  |          [sleep on req->waitq]     |
  293  |                                    |      <request_wait()
  294  |                                    |      [remove req from fc->pending]
  295  |                                    |      [copy req to read buffer]
  296  |                                    |      [add req to fc->processing]
  297  |                                    |    <fuse_dev_read()
  298  |                                    |  <sys_read()
  299  |                                    |
  300  |                                    |  [perform unlink]
  301  |                                    |
  302  |                                    |  >sys_write()
  303  |                                    |    >fuse_dev_write()
  304  |                                    |      [look up req in fc->processing]
  305  |                                    |      [remove from fc->processing]
  306  |                                    |      [copy write buffer to req]
  307  |          [woken up]                |      [wake up req->waitq]
  308  |                                    |    <fuse_dev_write()
  309  |                                    |  <sys_write()
  310  |        <request_wait_answer()      |
  311  |      <request_send()               |
  312  |      [add request to               |
  313  |       fc->unused_list]             |
  314  |    <fuse_unlink()                  |
  315  |  <sys_unlink()                     |
  316 
  317 There are a couple of ways in which to deadlock a FUSE filesystem.
  318 Since we are talking about unprivileged userspace programs,
  319 something must be done about these.
  320 
  321 Scenario 1 -  Simple deadlock
  322 -----------------------------
  323 
  324  |  "rm /mnt/fuse/file"               |  FUSE filesystem daemon
  325  |                                    |
  326  |  >sys_unlink("/mnt/fuse/file")     |
  327  |    [acquire inode semaphore        |
  328  |     for "file"]                    |
  329  |    >fuse_unlink()                  |
  330  |      [sleep on req->waitq]         |
  331  |                                    |  <sys_read()
  332  |                                    |  >sys_unlink("/mnt/fuse/file")
  333  |                                    |    [acquire inode semaphore
  334  |                                    |     for "file"]
  335  |                                    |    *DEADLOCK*
  336 
  337 The solution for this is to allow the filesystem to be aborted.
  338 
  339 Scenario 2 - Tricky deadlock
  340 ----------------------------
  341 
  342 This one needs a carefully crafted filesystem.  It's a variation on
  343 the above, only the call back to the filesystem is not explicit,
  344 but is caused by a pagefault.
  345 
  346  |  Kamikaze filesystem thread 1      |  Kamikaze filesystem thread 2
  347  |                                    |
  348  |  [fd = open("/mnt/fuse/file")]     |  [request served normally]
  349  |  [mmap fd to 'addr']               |
  350  |  [close fd]                        |  [FLUSH triggers 'magic' flag]
  351  |  [read a byte from addr]           |
  352  |    >do_page_fault()                |
  353  |      [find or create page]         |
  354  |      [lock page]                   |
  355  |      >fuse_readpage()              |
  356  |         [queue READ request]       |
  357  |         [sleep on req->waitq]      |
  358  |                                    |  [read request to buffer]
  359  |                                    |  [create reply header before addr]
  360  |                                    |  >sys_write(addr - headerlength)
  361  |                                    |    >fuse_dev_write()
  362  |                                    |      [look up req in fc->processing]
  363  |                                    |      [remove from fc->processing]
  364  |                                    |      [copy write buffer to req]
  365  |                                    |        >do_page_fault()
  366  |                                    |           [find or create page]
  367  |                                    |           [lock page]
  368  |                                    |           * DEADLOCK *
  369 
  370 Solution is basically the same as above.
  371 
  372 An additional problem is that while the write buffer is being copied
  373 to the request, the request must not be interrupted/aborted.  This is
  374 because the destination address of the copy may not be valid after the
  375 request has returned.
  376 
  377 This is solved with doing the copy atomically, and allowing abort
  378 while the page(s) belonging to the write buffer are faulted with
  379 get_user_pages().  The 'req->locked' flag indicates when the copy is
  380 taking place, and abort is delayed until this flag is unset.