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Miklos Szeredi334f4852005-09-09 13:10:27 -07001Definitions
2~~~~~~~~~~~
3
4Userspace 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
10Filesystem daemon:
11
12 The process(es) providing the data and metadata of the filesystem.
13
14Non-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
Miklos Szeredibafa9652006-06-25 05:48:51 -070021Filesystem 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
Miklos Szeredi334f4852005-09-09 13:10:27 -070029Mount owner:
30
31 The user who does the mounting.
32
33User:
34
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 FUSE
50homepage:
51
52 http://fuse.sourceforge.net/
53
Miklos Szeredid6392f82006-12-06 20:35:44 -080054Filesystem type
55~~~~~~~~~~~~~~~
56
57The 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
Miklos Szeredi334f4852005-09-09 13:10:27 -070070Mount 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 it's 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
Alexey Dobriyan91f6e542006-12-29 16:50:08 -080097 access based on file mode. It is usually useful together with the
98 'allow_other' mount option.
Miklos Szeredi334f4852005-09-09 13:10:27 -070099
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
Miklos Szeredi334f4852005-09-09 13:10:27 -0700107'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
Miklos Szeredid8091612006-12-06 20:35:48 -0800113'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
Miklos Szeredibafa9652006-06-25 05:48:51 -0700118Control filesystem
119~~~~~~~~~~~~~~~~~~
Miklos Szeredibacac382006-01-16 22:14:47 -0800120
Miklos Szeredibafa9652006-06-25 05:48:51 -0700121There's a control filesystem for FUSE, which can be mounted by:
Miklos Szeredibacac382006-01-16 22:14:47 -0800122
Miklos Szeredibafa9652006-06-25 05:48:51 -0700123 mount -t fusectl none /sys/fs/fuse/connections
Miklos Szeredibacac382006-01-16 22:14:47 -0800124
Miklos Szeredibafa9652006-06-25 05:48:51 -0700125Mounting it under the '/sys/fs/fuse/connections' directory makes it
126backwards compatible with earlier versions.
Miklos Szeredibacac382006-01-16 22:14:47 -0800127
Miklos Szeredibafa9652006-06-25 05:48:51 -0700128Under the fuse control filesystem each connection has a directory
129named by a unique number.
130
131For each connection the following files exist within this directory:
Miklos Szeredibacac382006-01-16 22:14:47 -0800132
133 'waiting'
134
Matt LaPlantefa00e7e2006-11-30 04:55:36 +0100135 The number of requests which are waiting to be transferred to
Miklos Szeredibacac382006-01-16 22:14:47 -0800136 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
Miklos Szeredibafa9652006-06-25 05:48:51 -0700146Only the owner of the mount may read or write these files.
Miklos Szeredibacac382006-01-16 22:14:47 -0800147
Miklos Szeredia4d27e72006-06-25 05:48:54 -0700148Interrupting filesystem operations
149~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
150
151If a process issuing a FUSE filesystem request is interrupted, the
152following 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
Matt LaPlantefa00e7e2006-11-30 04:55:36 +0100160 the request has been successfully transferred to userspace and
Miklos Szeredia4d27e72006-06-25 05:48:54 -0700161 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
166INTERRUPT requests take precedence over other requests, so the
167userspace filesystem will receive queued INTERRUPTs before any others.
168
169The userspace filesystem may ignore the INTERRUPT requests entirely,
170or may honor them by sending a reply to the _original_ request, with
171the error set to EINTR.
172
173It is also possible that there's a race between processing the
174original request and it's 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
182If the filesystem cannot find the original request, it should wait for
183some timeout and/or a number of new requests to arrive, after which it
184should reply to the INTERRUPT request with an EAGAIN error. In case
1851) the INTERRUPT request will be requeued. In case 2) the INTERRUPT
186reply will be ignored.
187
Miklos Szeredibacac382006-01-16 22:14:47 -0800188Aborting a filesystem connection
189~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
190
191It is possible to get into certain situations where the filesystem is
192not 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
204In either of these cases it may be useful to abort the connection to
205the 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
Miklos Szeredibafa9652006-06-25 05:48:51 -0700215 - Abort filesystem through the FUSE control filesystem. Most
216 powerful method, always works.
Miklos Szeredibacac382006-01-16 22:14:47 -0800217
Miklos Szeredi334f4852005-09-09 13:10:27 -0700218How do non-privileged mounts work?
219~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
220
221Since the mount() system call is a privileged operation, a helper
222program (fusermount) is needed, which is installed setuid root.
223
224The implication of providing non-privileged mounts is that the mount
225owner must not be able to use this capability to compromise the
226system. 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
237How 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
302I think these limitations are unacceptable?
303~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
304
305If a sysadmin trusts the users enough, or can ensure through other
306measures, that system processes will never enter non-privileged
307mounts, it can relax the last limitation with a "user_allow_other"
308config option. If this config option is set, the mounting user can
309add the "allow_other" mount option which disables the check for other
310users' processes.
311
312Kernel - userspace interface
313~~~~~~~~~~~~~~~~~~~~~~~~~~~~
314
315The following diagram shows how a filesystem operation (in this
316example unlink) is performed in FUSE.
317
318NOTE: 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
360There are a couple of ways in which to deadlock a FUSE filesystem.
361Since we are talking about unprivileged userspace programs,
362something must be done about these.
363
364Scenario 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
Miklos Szeredi51eb01e2006-06-25 05:48:50 -0700380The solution for this is to allow the filesystem to be aborted.
Miklos Szeredi334f4852005-09-09 13:10:27 -0700381
382Scenario 2 - Tricky deadlock
383----------------------------
384
385This one needs a carefully crafted filesystem. It's a variation on
386the above, only the call back to the filesystem is not explicit,
387but 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
Miklos Szeredi51eb01e2006-06-25 05:48:50 -0700413Solution is basically the same as above.
Miklos Szeredi334f4852005-09-09 13:10:27 -0700414
Miklos Szeredia4d27e72006-06-25 05:48:54 -0700415An additional problem is that while the write buffer is being copied
416to the request, the request must not be interrupted/aborted. This is
417because the destination address of the copy may not be valid after the
418request has returned.
Miklos Szeredi334f4852005-09-09 13:10:27 -0700419
Miklos Szeredi51eb01e2006-06-25 05:48:50 -0700420This is solved with doing the copy atomically, and allowing abort
421while the page(s) belonging to the write buffer are faulted with
422get_user_pages(). The 'req->locked' flag indicates when the copy is
423taking place, and abort is delayed until this flag is unset.