1 .. SPDX-License-Identifier: GPL-2.0
2
3 ====
4 FUSE
5 ====
6
7 Definitions
8 ===========
9
10 Userspace filesystem:
11 A filesystem in which data and metadata are provided by an ordinary
12 userspace process. The filesystem can be accessed normally through
13 the kernel interface.
14
15 Filesystem daemon:
16 The process(es) providing the data and metadata of the filesystem.
17
18 Non-privileged mount (or user mount):
19 A userspace filesystem mounted by a non-privileged (non-root) user.
20 The filesystem daemon is running with the privileges of the mounting
21 user. NOTE: this is not the same as mounts allowed with the "user"
22 option in /etc/fstab, which is not discussed here.
23
24 Filesystem connection:
25 A connection between the filesystem daemon and the kernel. The
26 connection exists until either the daemon dies, or the filesystem is
27 umounted. Note that detaching (or lazy umounting) the filesystem
28 does *not* break the connection, in this case it will exist until
29 the last reference to the filesystem is released.
30
31 Mount owner:
32 The user who does the mounting.
33
34 User:
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
50 `FUSE homepage: <https://github.com/libfuse/>`_
51
52 Filesystem type
53 ===============
54
55 The filesystem type given to mount(2) can be one of the following:
56
57 fuse
58 This is the usual way to mount a FUSE filesystem. The first
59 argument of the mount system call may contain an arbitrary string,
60 which is not interpreted by the kernel.
61
62 fuseblk
63 The filesystem is block device based. The first argument of the
64 mount system call is interpreted as the name of the device.
65
66 Mount options
67 =============
68
69 fd=N
70 The file descriptor to use for communication between the userspace
71 filesystem and the kernel. The file descriptor must have been
72 obtained by opening the FUSE device ('/dev/fuse').
73
74 rootmode=M
75 The file mode of the filesystem's root in octal representation.
76
77 user_id=N
78 The numeric user id of the mount owner.
79
80 group_id=N
81 The numeric group id of the mount owner.
82
83 default_permissions
84 By default FUSE doesn't check file access permissions, the
85 filesystem is free to implement its access policy or leave it to
86 the underlying file access mechanism (e.g. in case of network
87 filesystems). This option enables permission checking, restricting
88 access based on file mode. It is usually useful together with the
89 'allow_other' mount option.
90
91 allow_other
92 This option overrides the security measure restricting file access
93 to the user mounting the filesystem. This option is by default only
94 allowed to root, but this restriction can be removed with a
95 (userspace) configuration option.
96
97 max_read=N
98 With this option the maximum size of read operations can be set.
99 The default is infinite. Note that the size of read requests is
100 limited anyway to 32 pages (which is 128kbyte on i386).
101
102 blksize=N
103 Set the block size for the filesystem. The default is 512. This
104 option is only valid for 'fuseblk' type mounts.
105
106 Control filesystem
107 ==================
108
109 There's a control filesystem for FUSE, which can be mounted by::
110
111 mount -t fusectl none /sys/fs/fuse/connections
112
113 Mounting it under the '/sys/fs/fuse/connections' directory makes it
114 backwards compatible with earlier versions.
115
116 Under the fuse control filesystem each connection has a directory
117 named by a unique number.
118
119 For each connection the following files exist within this directory:
120
121 waiting
122 The number of requests which are waiting to be transferred to
123 userspace or being processed by the filesystem daemon. If there is
124 no filesystem activity and 'waiting' is non-zero, then the
125 filesystem is hung or deadlocked.
126
127 abort
128 Writing anything into this file will abort the filesystem
129 connection. This means that all waiting requests will be aborted an
130 error returned for all aborted and new requests.
131
132 Only the owner of the mount may read or write these files.
133
134 Interrupting filesystem operations
135 ##################################
136
137 If a process issuing a FUSE filesystem request is interrupted, the
138 following will happen:
139
140 - If the request is not yet sent to userspace AND the signal is
141 fatal (SIGKILL or unhandled fatal signal), then the request is
142 dequeued and returns immediately.
143
144 - If the request is not yet sent to userspace AND the signal is not
145 fatal, then an interrupted flag is set for the request. When
146 the request has been successfully transferred to userspace and
147 this flag is set, an INTERRUPT request is queued.
148
149 - If the request is already sent to userspace, then an INTERRUPT
150 request is queued.
151
152 INTERRUPT requests take precedence over other requests, so the
153 userspace filesystem will receive queued INTERRUPTs before any others.
154
155 The userspace filesystem may ignore the INTERRUPT requests entirely,
156 or may honor them by sending a reply to the *original* request, with
157 the error set to EINTR.
158
159 It is also possible that there's a race between processing the
160 original request and its INTERRUPT request. There are two possibilities:
161
162 1. The INTERRUPT request is processed before the original request is
163 processed
164
165 2. The INTERRUPT request is processed after the original request has
166 been answered
167
168 If the filesystem cannot find the original request, it should wait for
169 some timeout and/or a number of new requests to arrive, after which it
170 should reply to the INTERRUPT request with an EAGAIN error. In case
171 1) the INTERRUPT request will be requeued. In case 2) the INTERRUPT
172 reply will be ignored.
173
174 Aborting a filesystem connection
175 ================================
176
177 It is possible to get into certain situations where the filesystem is
178 not responding. Reasons for this may be:
179
180 a) Broken userspace filesystem implementation
181
182 b) Network connection down
183
184 c) Accidental deadlock
185
186 d) Malicious deadlock
187
188 (For more on c) and d) see later sections)
189
190 In either of these cases it may be useful to abort the connection to
191 the filesystem. There are several ways to do this:
192
193 - Kill the filesystem daemon. Works in case of a) and b)
194
195 - Kill the filesystem daemon and all users of the filesystem. Works
196 in all cases except some malicious deadlocks
197
198 - Use forced umount (umount -f). Works in all cases but only if
199 filesystem is still attached (it hasn't been lazy unmounted)
200
201 - Abort filesystem through the FUSE control filesystem. Most
202 powerful method, always works.
203
204 How do non-privileged mounts work?
205 ==================================
206
207 Since the mount() system call is a privileged operation, a helper
208 program (fusermount) is needed, which is installed setuid root.
209
210 The implication of providing non-privileged mounts is that the mount
211 owner must not be able to use this capability to compromise the
212 system. Obvious requirements arising from this are:
213
214 A) mount owner should not be able to get elevated privileges with the
215 help of the mounted filesystem
216
217 B) mount owner should not get illegitimate access to information from
218 other users' and the super user's processes
219
220 C) mount owner should not be able to induce undesired behavior in
221 other users' or the super user's processes
222
223 How are requirements fulfilled?
224 ===============================
225
226 A) The mount owner could gain elevated privileges by either:
227
228 1. creating a filesystem containing a device file, then opening this device
229
230 2. creating a filesystem containing a suid or sgid application, then executing this application
231
232 The solution is not to allow opening device files and ignore
233 setuid and setgid bits when executing programs. To ensure this
234 fusermount always adds "nosuid" and "nodev" to the mount options
235 for non-privileged mounts.
236
237 B) If another user is accessing files or directories in the
238 filesystem, the filesystem daemon serving requests can record the
239 exact sequence and timing of operations performed. This
240 information is otherwise inaccessible to the mount owner, so this
241 counts as an information leak.
242
243 The solution to this problem will be presented in point 2) of C).
244
245 C) There are several ways in which the mount owner can induce
246 undesired behavior in other users' processes, such as:
247
248 1) mounting a filesystem over a file or directory which the mount
249 owner could otherwise not be able to modify (or could only
250 make limited modifications).
251
252 This is solved in fusermount, by checking the access
253 permissions on the mountpoint and only allowing the mount if
254 the mount owner can do unlimited modification (has write
255 access to the mountpoint, and mountpoint is not a "sticky"
256 directory)
257
258 2) Even if 1) is solved the mount owner can change the behavior
259 of other users' processes.
260
261 i) It can slow down or indefinitely delay the execution of a
262 filesystem operation creating a DoS against the user or the
263 whole system. For example a suid application locking a
264 system file, and then accessing a file on the mount owner's
265 filesystem could be stopped, and thus causing the system
266 file to be locked forever.
267
268 ii) It can present files or directories of unlimited length, or
269 directory structures of unlimited depth, possibly causing a
270 system process to eat up diskspace, memory or other
271 resources, again causing *DoS*.
272
273 The solution to this as well as B) is not to allow processes
274 to access the filesystem, which could otherwise not be
275 monitored or manipulated by the mount owner. Since if the
276 mount owner can ptrace a process, it can do all of the above
277 without using a FUSE mount, the same criteria as used in
278 ptrace can be used to check if a process is allowed to access
279 the filesystem or not.
280
281 Note that the *ptrace* check is not strictly necessary to
282 prevent C/2/i, it is enough to check if mount owner has enough
283 privilege to send signal to the process accessing the
284 filesystem, since *SIGSTOP* can be used to get a similar effect.
285
286 I think these limitations are unacceptable?
287 ===========================================
288
289 If a sysadmin trusts the users enough, or can ensure through other
290 measures, that system processes will never enter non-privileged
291 mounts, it can relax the last limitation in several ways:
292
293 - With the 'user_allow_other' config option. If this config option is
294 set, the mounting user can add the 'allow_other' mount option which
295 disables the check for other users' processes.
296
297 User namespaces have an unintuitive interaction with 'allow_other':
298 an unprivileged user - normally restricted from mounting with
299 'allow_other' - could do so in a user namespace where they're
300 privileged. If any process could access such an 'allow_other' mount
301 this would give the mounting user the ability to manipulate
302 processes in user namespaces where they're unprivileged. For this
303 reason 'allow_other' restricts access to users in the same userns
304 or a descendant.
305
306 - With the 'allow_sys_admin_access' module option. If this option is
307 set, super user's processes have unrestricted access to mounts
308 irrespective of allow_other setting or user namespace of the
309 mounting user.
310
311 Note that both of these relaxations expose the system to potential
312 information leak or *DoS* as described in points B and C/2/i-ii in the
313 preceding section.
314
315 Kernel - userspace interface
316 ============================
317
318 The following diagram shows how a filesystem operation (in this
319 example unlink) is performed in FUSE. ::
320
321
322 | "rm /mnt/fuse/file" | FUSE filesystem daemon
323 | |
324 | | >sys_read()
325 | | >fuse_dev_read()
326 | | >request_wait()
327 | | [sleep on fc->waitq]
328 | |
329 | >sys_unlink() |
330 | >fuse_unlink() |
331 | [get request from |
332 | fc->unused_list] |
333 | >request_send() |
334 | [queue req on fc->pending] |
335 | [wake up fc->waitq] | [woken up]
336 | >request_wait_answer() |
337 | [sleep on req->waitq] |
338 | | <request_wait()
339 | | [remove req from fc->pending]
340 | | [copy req to read buffer]
341 | | [add req to fc->processing]
342 | | <fuse_dev_read()
343 | | <sys_read()
344 | |
345 | | [perform unlink]
346 | |
347 | | >sys_write()
348 | | >fuse_dev_write()
349 | | [look up req in fc->processing]
350 | | [remove from fc->processing]
351 | | [copy write buffer to req]
352 | [woken up] | [wake up req->waitq]
353 | | <fuse_dev_write()
354 | | <sys_write()
355 | <request_wait_answer() |
356 | <request_send() |
357 | [add request to |
358 | fc->unused_list] |
359 | <fuse_unlink() |
360 | <sys_unlink() |
361
362 .. note:: Everything in the description above is greatly simplified
363
364 There are a couple of ways in which to deadlock a FUSE filesystem.
365 Since we are talking about unprivileged userspace programs,
366 something must be done about these.
367
368 **Scenario 1 - Simple deadlock**::
369
370 | "rm /mnt/fuse/file" | FUSE filesystem daemon
371 | |
372 | >sys_unlink("/mnt/fuse/file") |
373 | [acquire inode semaphore |
374 | for "file"] |
375 | >fuse_unlink() |
376 | [sleep on req->waitq] |
377 | | <sys_read()
378 | | >sys_unlink("/mnt/fuse/file")
379 | | [acquire inode semaphore
380 | | for "file"]
381 | | *DEADLOCK*
382
383 The solution for this is to allow the filesystem to be aborted.
384
385 **Scenario 2 - Tricky deadlock**
386
387
388 This one needs a carefully crafted filesystem. It's a variation on
389 the above, only the call back to the filesystem is not explicit,
390 but is caused by a pagefault. ::
391
392 | Kamikaze filesystem thread 1 | Kamikaze filesystem thread 2
393 | |
394 | [fd = open("/mnt/fuse/file")] | [request served normally]
395 | [mmap fd to 'addr'] |
396 | [close fd] | [FLUSH triggers 'magic' flag]
397 | [read a byte from addr] |
398 | >do_page_fault() |
399 | [find or create page] |
400 | [lock page] |
401 | >fuse_readpage() |
402 | [queue READ request] |
403 | [sleep on req->waitq] |
404 | | [read request to buffer]
405 | | [create reply header before addr]
406 | | >sys_write(addr - headerlength)
407 | | >fuse_dev_write()
408 | | [look up req in fc->processing]
409 | | [remove from fc->processing]
410 | | [copy write buffer to req]
411 | | >do_page_fault()
412 | | [find or create page]
413 | | [lock page]
414 | | * DEADLOCK *
415
416 The solution is basically the same as above.
417
418 An additional problem is that while the write buffer is being copied
419 to the request, the request must not be interrupted/aborted. This is
420 because the destination address of the copy may not be valid after the
421 request has returned.
422
423 This is solved with doing the copy atomically, and allowing abort
424 while the page(s) belonging to the write buffer are faulted with
425 get_user_pages(). The 'req->locked' flag indicates when the copy is
426 taking place, and abort is delayed until this flag is unset.
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.