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Based on kernel version 3.13. Page generated on 2014-01-20 22:02 EST.

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	(fusermount).
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	  http://fuse.sourceforge.net/
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	'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	
118	Control filesystem
119	~~~~~~~~~~~~~~~~~~
120	
121	There's a control filesystem for FUSE, which can be mounted by:
122	
123	  mount -t fusectl none /sys/fs/fuse/connections
124	
125	Mounting it under the '/sys/fs/fuse/connections' directory makes it
126	backwards compatible with earlier versions.
127	
128	Under the fuse control filesystem each connection has a directory
129	named by a unique number.
130	
131	For 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	
146	Only the owner of the mount may read or write these files.
147	
148	Interrupting filesystem operations
149	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
150	
151	If a process issuing a FUSE filesystem request is interrupted, the
152	following 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	
166	INTERRUPT requests take precedence over other requests, so the
167	userspace filesystem will receive queued INTERRUPTs before any others.
168	
169	The userspace filesystem may ignore the INTERRUPT requests entirely,
170	or may honor them by sending a reply to the _original_ request, with
171	the error set to EINTR.
172	
173	It is also possible that there's a race between processing the
174	original 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	
182	If the filesystem cannot find the original request, it should wait for
183	some timeout and/or a number of new requests to arrive, after which it
184	should reply to the INTERRUPT request with an EAGAIN error.  In case
185	1) the INTERRUPT request will be requeued.  In case 2) the INTERRUPT
186	reply will be ignored.
187	
188	Aborting a filesystem connection
189	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
190	
191	It is possible to get into certain situations where the filesystem is
192	not 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	
204	In either of these cases it may be useful to abort the connection to
205	the 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	
218	How do non-privileged mounts work?
219	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
220	
221	Since the mount() system call is a privileged operation, a helper
222	program (fusermount) is needed, which is installed setuid root.
223	
224	The implication of providing non-privileged mounts is that the mount
225	owner must not be able to use this capability to compromise the
226	system.  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	
237	How 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	
302	I think these limitations are unacceptable?
303	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
304	
305	If a sysadmin trusts the users enough, or can ensure through other
306	measures, that system processes will never enter non-privileged
307	mounts, it can relax the last limitation with a "user_allow_other"
308	config option.  If this config option is set, the mounting user can
309	add the "allow_other" mount option which disables the check for other
310	users' processes.
311	
312	Kernel - userspace interface
313	~~~~~~~~~~~~~~~~~~~~~~~~~~~~
314	
315	The following diagram shows how a filesystem operation (in this
316	example unlink) is performed in FUSE.
317	
318	NOTE: 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	
360	There are a couple of ways in which to deadlock a FUSE filesystem.
361	Since we are talking about unprivileged userspace programs,
362	something must be done about these.
363	
364	Scenario 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	
380	The solution for this is to allow the filesystem to be aborted.
381	
382	Scenario 2 - Tricky deadlock
383	----------------------------
384	
385	This one needs a carefully crafted filesystem.  It's a variation on
386	the above, only the call back to the filesystem is not explicit,
387	but 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	
413	Solution is basically the same as above.
414	
415	An additional problem is that while the write buffer is being copied
416	to the request, the request must not be interrupted/aborted.  This is
417	because the destination address of the copy may not be valid after the
418	request has returned.
419	
420	This is solved with doing the copy atomically, and allowing abort
421	while the page(s) belonging to the write buffer are faulted with
422	get_user_pages().  The 'req->locked' flag indicates when the copy is
423	taking place, and abort is delayed until this flag is unset.
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