1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 ==== 3 ====
4 FUSE 4 FUSE
5 ==== 5 ====
6 6
7 Definitions 7 Definitions
8 =========== 8 ===========
9 9
10 Userspace filesystem: 10 Userspace filesystem:
11 A filesystem in which data and metadata are 11 A filesystem in which data and metadata are provided by an ordinary
12 userspace process. The filesystem can be ac 12 userspace process. The filesystem can be accessed normally through
13 the kernel interface. 13 the kernel interface.
14 14
15 Filesystem daemon: 15 Filesystem daemon:
16 The process(es) providing the data and metad 16 The process(es) providing the data and metadata of the filesystem.
17 17
18 Non-privileged mount (or user mount): 18 Non-privileged mount (or user mount):
19 A userspace filesystem mounted by a non-priv 19 A userspace filesystem mounted by a non-privileged (non-root) user.
20 The filesystem daemon is running with the pr 20 The filesystem daemon is running with the privileges of the mounting
21 user. NOTE: this is not the same as mounts 21 user. NOTE: this is not the same as mounts allowed with the "user"
22 option in /etc/fstab, which is not discussed 22 option in /etc/fstab, which is not discussed here.
23 23
24 Filesystem connection: 24 Filesystem connection:
25 A connection between the filesystem daemon a 25 A connection between the filesystem daemon and the kernel. The
26 connection exists until either the daemon di 26 connection exists until either the daemon dies, or the filesystem is
27 umounted. Note that detaching (or lazy umou 27 umounted. Note that detaching (or lazy umounting) the filesystem
28 does *not* break the connection, in this cas 28 does *not* break the connection, in this case it will exist until
29 the last reference to the filesystem is rele 29 the last reference to the filesystem is released.
30 30
31 Mount owner: 31 Mount owner:
32 The user who does the mounting. 32 The user who does the mounting.
33 33
34 User: 34 User:
35 The user who is performing filesystem operat 35 The user who is performing filesystem operations.
36 36
37 What is FUSE? 37 What is FUSE?
38 ============= 38 =============
39 39
40 FUSE is a userspace filesystem framework. It 40 FUSE is a userspace filesystem framework. It consists of a kernel
41 module (fuse.ko), a userspace library (libfuse 41 module (fuse.ko), a userspace library (libfuse.*) and a mount utility
42 (fusermount). 42 (fusermount).
43 43
44 One of the most important features of FUSE is 44 One of the most important features of FUSE is allowing secure,
45 non-privileged mounts. This opens up new poss 45 non-privileged mounts. This opens up new possibilities for the use of
46 filesystems. A good example is sshfs: a secur 46 filesystems. A good example is sshfs: a secure network filesystem
47 using the sftp protocol. 47 using the sftp protocol.
48 48
49 The userspace library and utilities are availa 49 The userspace library and utilities are available from the
50 `FUSE homepage: <https://github.com/libfuse/>` 50 `FUSE homepage: <https://github.com/libfuse/>`_
51 51
52 Filesystem type 52 Filesystem type
53 =============== 53 ===============
54 54
55 The filesystem type given to mount(2) can be o 55 The filesystem type given to mount(2) can be one of the following:
56 56
57 fuse 57 fuse
58 This is the usual way to mount a FUSE fi 58 This is the usual way to mount a FUSE filesystem. The first
59 argument of the mount system call may co 59 argument of the mount system call may contain an arbitrary string,
60 which is not interpreted by the kernel. 60 which is not interpreted by the kernel.
61 61
62 fuseblk 62 fuseblk
63 The filesystem is block device based. T 63 The filesystem is block device based. The first argument of the
64 mount system call is interpreted as the 64 mount system call is interpreted as the name of the device.
65 65
66 Mount options 66 Mount options
67 ============= 67 =============
68 68
69 fd=N 69 fd=N
70 The file descriptor to use for communication 70 The file descriptor to use for communication between the userspace
71 filesystem and the kernel. The file descrip 71 filesystem and the kernel. The file descriptor must have been
72 obtained by opening the FUSE device ('/dev/f 72 obtained by opening the FUSE device ('/dev/fuse').
73 73
74 rootmode=M 74 rootmode=M
75 The file mode of the filesystem's root in oc 75 The file mode of the filesystem's root in octal representation.
76 76
77 user_id=N 77 user_id=N
78 The numeric user id of the mount owner. 78 The numeric user id of the mount owner.
79 79
80 group_id=N 80 group_id=N
81 The numeric group id of the mount owner. 81 The numeric group id of the mount owner.
82 82
83 default_permissions 83 default_permissions
84 By default FUSE doesn't check file access pe 84 By default FUSE doesn't check file access permissions, the
85 filesystem is free to implement its access p 85 filesystem is free to implement its access policy or leave it to
86 the underlying file access mechanism (e.g. i 86 the underlying file access mechanism (e.g. in case of network
87 filesystems). This option enables permissio 87 filesystems). This option enables permission checking, restricting
88 access based on file mode. It is usually us 88 access based on file mode. It is usually useful together with the
89 'allow_other' mount option. 89 'allow_other' mount option.
90 90
91 allow_other 91 allow_other
92 This option overrides the security measure r 92 This option overrides the security measure restricting file access
93 to the user mounting the filesystem. This o 93 to the user mounting the filesystem. This option is by default only
94 allowed to root, but this restriction can be 94 allowed to root, but this restriction can be removed with a
95 (userspace) configuration option. 95 (userspace) configuration option.
96 96
97 max_read=N 97 max_read=N
98 With this option the maximum size of read op 98 With this option the maximum size of read operations can be set.
99 The default is infinite. Note that the size 99 The default is infinite. Note that the size of read requests is
100 limited anyway to 32 pages (which is 128kbyt 100 limited anyway to 32 pages (which is 128kbyte on i386).
101 101
102 blksize=N 102 blksize=N
103 Set the block size for the filesystem. The 103 Set the block size for the filesystem. The default is 512. This
104 option is only valid for 'fuseblk' type moun 104 option is only valid for 'fuseblk' type mounts.
105 105
106 Control filesystem 106 Control filesystem
107 ================== 107 ==================
108 108
109 There's a control filesystem for FUSE, which c 109 There's a control filesystem for FUSE, which can be mounted by::
110 110
111 mount -t fusectl none /sys/fs/fuse/connectio 111 mount -t fusectl none /sys/fs/fuse/connections
112 112
113 Mounting it under the '/sys/fs/fuse/connection 113 Mounting it under the '/sys/fs/fuse/connections' directory makes it
114 backwards compatible with earlier versions. 114 backwards compatible with earlier versions.
115 115
116 Under the fuse control filesystem each connect 116 Under the fuse control filesystem each connection has a directory
117 named by a unique number. 117 named by a unique number.
118 118
119 For each connection the following files exist 119 For each connection the following files exist within this directory:
120 120
121 waiting 121 waiting
122 The number of requests which are wai 122 The number of requests which are waiting to be transferred to
123 userspace or being processed by the 123 userspace or being processed by the filesystem daemon. If there is
124 no filesystem activity and 'waiting' 124 no filesystem activity and 'waiting' is non-zero, then the
125 filesystem is hung or deadlocked. 125 filesystem is hung or deadlocked.
126 126
127 abort 127 abort
128 Writing anything into this file will 128 Writing anything into this file will abort the filesystem
129 connection. This means that all wai 129 connection. This means that all waiting requests will be aborted an
130 error returned for all aborted and n 130 error returned for all aborted and new requests.
131 131
132 Only the owner of the mount may read or write 132 Only the owner of the mount may read or write these files.
133 133
134 Interrupting filesystem operations 134 Interrupting filesystem operations
135 ################################## 135 ##################################
136 136
137 If a process issuing a FUSE filesystem request 137 If a process issuing a FUSE filesystem request is interrupted, the
138 following will happen: 138 following will happen:
139 139
140 - If the request is not yet sent to userspa 140 - If the request is not yet sent to userspace AND the signal is
141 fatal (SIGKILL or unhandled fatal signal) 141 fatal (SIGKILL or unhandled fatal signal), then the request is
142 dequeued and returns immediately. 142 dequeued and returns immediately.
143 143
144 - If the request is not yet sent to userspa 144 - If the request is not yet sent to userspace AND the signal is not
145 fatal, then an interrupted flag is set fo 145 fatal, then an interrupted flag is set for the request. When
146 the request has been successfully transfe 146 the request has been successfully transferred to userspace and
147 this flag is set, an INTERRUPT request is 147 this flag is set, an INTERRUPT request is queued.
148 148
149 - If the request is already sent to userspa 149 - If the request is already sent to userspace, then an INTERRUPT
150 request is queued. 150 request is queued.
151 151
152 INTERRUPT requests take precedence over other 152 INTERRUPT requests take precedence over other requests, so the
153 userspace filesystem will receive queued INTER 153 userspace filesystem will receive queued INTERRUPTs before any others.
154 154
155 The userspace filesystem may ignore the INTERR 155 The userspace filesystem may ignore the INTERRUPT requests entirely,
156 or may honor them by sending a reply to the *o 156 or may honor them by sending a reply to the *original* request, with
157 the error set to EINTR. 157 the error set to EINTR.
158 158
159 It is also possible that there's a race betwee 159 It is also possible that there's a race between processing the
160 original request and its INTERRUPT request. T 160 original request and its INTERRUPT request. There are two possibilities:
161 161
162 1. The INTERRUPT request is processed before 162 1. The INTERRUPT request is processed before the original request is
163 processed 163 processed
164 164
165 2. The INTERRUPT request is processed after 165 2. The INTERRUPT request is processed after the original request has
166 been answered 166 been answered
167 167
168 If the filesystem cannot find the original req 168 If the filesystem cannot find the original request, it should wait for
169 some timeout and/or a number of new requests t 169 some timeout and/or a number of new requests to arrive, after which it
170 should reply to the INTERRUPT request with an 170 should reply to the INTERRUPT request with an EAGAIN error. In case
171 1) the INTERRUPT request will be requeued. In 171 1) the INTERRUPT request will be requeued. In case 2) the INTERRUPT
172 reply will be ignored. 172 reply will be ignored.
173 173
174 Aborting a filesystem connection 174 Aborting a filesystem connection
175 ================================ 175 ================================
176 176
177 It is possible to get into certain situations 177 It is possible to get into certain situations where the filesystem is
178 not responding. Reasons for this may be: 178 not responding. Reasons for this may be:
179 179
180 a) Broken userspace filesystem implementatio 180 a) Broken userspace filesystem implementation
181 181
182 b) Network connection down 182 b) Network connection down
183 183
184 c) Accidental deadlock 184 c) Accidental deadlock
185 185
186 d) Malicious deadlock 186 d) Malicious deadlock
187 187
188 (For more on c) and d) see later sections) 188 (For more on c) and d) see later sections)
189 189
190 In either of these cases it may be useful to a 190 In either of these cases it may be useful to abort the connection to
191 the filesystem. There are several ways to do 191 the filesystem. There are several ways to do this:
192 192
193 - Kill the filesystem daemon. Works in case 193 - Kill the filesystem daemon. Works in case of a) and b)
194 194
195 - Kill the filesystem daemon and all users o 195 - Kill the filesystem daemon and all users of the filesystem. Works
196 in all cases except some malicious deadloc 196 in all cases except some malicious deadlocks
197 197
198 - Use forced umount (umount -f). Works in a 198 - Use forced umount (umount -f). Works in all cases but only if
199 filesystem is still attached (it hasn't be 199 filesystem is still attached (it hasn't been lazy unmounted)
200 200
201 - Abort filesystem through the FUSE control 201 - Abort filesystem through the FUSE control filesystem. Most
202 powerful method, always works. 202 powerful method, always works.
203 203
204 How do non-privileged mounts work? 204 How do non-privileged mounts work?
205 ================================== 205 ==================================
206 206
207 Since the mount() system call is a privileged 207 Since the mount() system call is a privileged operation, a helper
208 program (fusermount) is needed, which is insta 208 program (fusermount) is needed, which is installed setuid root.
209 209
210 The implication of providing non-privileged mo 210 The implication of providing non-privileged mounts is that the mount
211 owner must not be able to use this capability 211 owner must not be able to use this capability to compromise the
212 system. Obvious requirements arising from thi 212 system. Obvious requirements arising from this are:
213 213
214 A) mount owner should not be able to get elev 214 A) mount owner should not be able to get elevated privileges with the
215 help of the mounted filesystem 215 help of the mounted filesystem
216 216
217 B) mount owner should not get illegitimate ac 217 B) mount owner should not get illegitimate access to information from
218 other users' and the super user's processe 218 other users' and the super user's processes
219 219
220 C) mount owner should not be able to induce u 220 C) mount owner should not be able to induce undesired behavior in
221 other users' or the super user's processes 221 other users' or the super user's processes
222 222
223 How are requirements fulfilled? 223 How are requirements fulfilled?
224 =============================== 224 ===============================
225 225
226 A) The mount owner could gain elevated privil 226 A) The mount owner could gain elevated privileges by either:
227 227
228 1. creating a filesystem containing a devi 228 1. creating a filesystem containing a device file, then opening this device
229 229
230 2. creating a filesystem containing a suid 230 2. creating a filesystem containing a suid or sgid application, then executing this application
231 231
232 The solution is not to allow opening devic 232 The solution is not to allow opening device files and ignore
233 setuid and setgid bits when executing prog 233 setuid and setgid bits when executing programs. To ensure this
234 fusermount always adds "nosuid" and "nodev 234 fusermount always adds "nosuid" and "nodev" to the mount options
235 for non-privileged mounts. 235 for non-privileged mounts.
236 236
237 B) If another user is accessing files or dire 237 B) If another user is accessing files or directories in the
238 filesystem, the filesystem daemon serving 238 filesystem, the filesystem daemon serving requests can record the
239 exact sequence and timing of operations pe 239 exact sequence and timing of operations performed. This
240 information is otherwise inaccessible to t 240 information is otherwise inaccessible to the mount owner, so this
241 counts as an information leak. 241 counts as an information leak.
242 242
243 The solution to this problem will be prese 243 The solution to this problem will be presented in point 2) of C).
244 244
245 C) There are several ways in which the mount 245 C) There are several ways in which the mount owner can induce
246 undesired behavior in other users' process 246 undesired behavior in other users' processes, such as:
247 247
248 1) mounting a filesystem over a file or d 248 1) mounting a filesystem over a file or directory which the mount
249 owner could otherwise not be able to m 249 owner could otherwise not be able to modify (or could only
250 make limited modifications). 250 make limited modifications).
251 251
252 This is solved in fusermount, by check 252 This is solved in fusermount, by checking the access
253 permissions on the mountpoint and only 253 permissions on the mountpoint and only allowing the mount if
254 the mount owner can do unlimited modif 254 the mount owner can do unlimited modification (has write
255 access to the mountpoint, and mountpoi 255 access to the mountpoint, and mountpoint is not a "sticky"
256 directory) 256 directory)
257 257
258 2) Even if 1) is solved the mount owner c 258 2) Even if 1) is solved the mount owner can change the behavior
259 of other users' processes. 259 of other users' processes.
260 260
261 i) It can slow down or indefinitely d 261 i) It can slow down or indefinitely delay the execution of a
262 filesystem operation creating a Do 262 filesystem operation creating a DoS against the user or the
263 whole system. For example a suid 263 whole system. For example a suid application locking a
264 system file, and then accessing a 264 system file, and then accessing a file on the mount owner's
265 filesystem could be stopped, and t 265 filesystem could be stopped, and thus causing the system
266 file to be locked forever. 266 file to be locked forever.
267 267
268 ii) It can present files or directori 268 ii) It can present files or directories of unlimited length, or
269 directory structures of unlimited 269 directory structures of unlimited depth, possibly causing a
270 system process to eat up diskspac 270 system process to eat up diskspace, memory or other
271 resources, again causing *DoS*. 271 resources, again causing *DoS*.
272 272
273 The solution to this as well as B) is 273 The solution to this as well as B) is not to allow processes
274 to access the filesystem, which could 274 to access the filesystem, which could otherwise not be
275 monitored or manipulated by the mount 275 monitored or manipulated by the mount owner. Since if the
276 mount owner can ptrace a process, it c 276 mount owner can ptrace a process, it can do all of the above
277 without using a FUSE mount, the same c 277 without using a FUSE mount, the same criteria as used in
278 ptrace can be used to check if a proce 278 ptrace can be used to check if a process is allowed to access
279 the filesystem or not. 279 the filesystem or not.
280 280
281 Note that the *ptrace* check is not st 281 Note that the *ptrace* check is not strictly necessary to
282 prevent C/2/i, it is enough to check i !! 282 prevent B/2/i, it is enough to check if mount owner has enough
283 privilege to send signal to the proces 283 privilege to send signal to the process accessing the
284 filesystem, since *SIGSTOP* can be use 284 filesystem, since *SIGSTOP* can be used to get a similar effect.
285 285
286 I think these limitations are unacceptable? 286 I think these limitations are unacceptable?
287 =========================================== 287 ===========================================
288 288
289 If a sysadmin trusts the users enough, or can 289 If a sysadmin trusts the users enough, or can ensure through other
290 measures, that system processes will never ent 290 measures, that system processes will never enter non-privileged
291 mounts, it can relax the last limitation in se !! 291 mounts, it can relax the last limitation with a 'user_allow_other'
292 !! 292 config option. If this config option is set, the mounting user can
293 - With the 'user_allow_other' config option. !! 293 add the 'allow_other' mount option which disables the check for other
294 set, the mounting user can add the 'allow_ !! 294 users' processes.
295 disables the check for other users' proces <<
296 <<
297 User namespaces have an unintuitive intera <<
298 an unprivileged user - normally restricted <<
299 'allow_other' - could do so in a user name <<
300 privileged. If any process could access su <<
301 this would give the mounting user the abil <<
302 processes in user namespaces where they're <<
303 reason 'allow_other' restricts access to u <<
304 or a descendant. <<
305 <<
306 - With the 'allow_sys_admin_access' module o <<
307 set, super user's processes have unrestric <<
308 irrespective of allow_other setting or use <<
309 mounting user. <<
310 <<
311 Note that both of these relaxations expose the <<
312 information leak or *DoS* as described in poin <<
313 preceding section. <<
314 295
315 Kernel - userspace interface 296 Kernel - userspace interface
316 ============================ 297 ============================
317 298
318 The following diagram shows how a filesystem o 299 The following diagram shows how a filesystem operation (in this
319 example unlink) is performed in FUSE. :: 300 example unlink) is performed in FUSE. ::
320 301
321 302
322 | "rm /mnt/fuse/file" | FUSE 303 | "rm /mnt/fuse/file" | FUSE filesystem daemon
323 | | 304 | |
324 | | >sys_ 305 | | >sys_read()
325 | | >fu 306 | | >fuse_dev_read()
326 | | > 307 | | >request_wait()
327 | | 308 | | [sleep on fc->waitq]
328 | | 309 | |
329 | >sys_unlink() | 310 | >sys_unlink() |
330 | >fuse_unlink() | 311 | >fuse_unlink() |
331 | [get request from | 312 | [get request from |
332 | fc->unused_list] | 313 | fc->unused_list] |
333 | >request_send() | 314 | >request_send() |
334 | [queue req on fc->pending] | 315 | [queue req on fc->pending] |
335 | [wake up fc->waitq] | 316 | [wake up fc->waitq] | [woken up]
336 | >request_wait_answer() | 317 | >request_wait_answer() |
337 | [sleep on req->waitq] | 318 | [sleep on req->waitq] |
338 | | < 319 | | <request_wait()
339 | | [ 320 | | [remove req from fc->pending]
340 | | [ 321 | | [copy req to read buffer]
341 | | [ 322 | | [add req to fc->processing]
342 | | <fu 323 | | <fuse_dev_read()
343 | | <sys_ 324 | | <sys_read()
344 | | 325 | |
345 | | [perf 326 | | [perform unlink]
346 | | 327 | |
347 | | >sys_ 328 | | >sys_write()
348 | | >fu 329 | | >fuse_dev_write()
349 | | [ 330 | | [look up req in fc->processing]
350 | | [ 331 | | [remove from fc->processing]
351 | | [ 332 | | [copy write buffer to req]
352 | [woken up] | [ 333 | [woken up] | [wake up req->waitq]
353 | | <fu 334 | | <fuse_dev_write()
354 | | <sys_ 335 | | <sys_write()
355 | <request_wait_answer() | 336 | <request_wait_answer() |
356 | <request_send() | 337 | <request_send() |
357 | [add request to | 338 | [add request to |
358 | fc->unused_list] | 339 | fc->unused_list] |
359 | <fuse_unlink() | 340 | <fuse_unlink() |
360 | <sys_unlink() | 341 | <sys_unlink() |
361 342
362 .. note:: Everything in the description above 343 .. note:: Everything in the description above is greatly simplified
363 344
364 There are a couple of ways in which to deadloc 345 There are a couple of ways in which to deadlock a FUSE filesystem.
365 Since we are talking about unprivileged usersp 346 Since we are talking about unprivileged userspace programs,
366 something must be done about these. 347 something must be done about these.
367 348
368 **Scenario 1 - Simple deadlock**:: 349 **Scenario 1 - Simple deadlock**::
369 350
370 | "rm /mnt/fuse/file" | FUSE 351 | "rm /mnt/fuse/file" | FUSE filesystem daemon
371 | | 352 | |
372 | >sys_unlink("/mnt/fuse/file") | 353 | >sys_unlink("/mnt/fuse/file") |
373 | [acquire inode semaphore | 354 | [acquire inode semaphore |
374 | for "file"] | 355 | for "file"] |
375 | >fuse_unlink() | 356 | >fuse_unlink() |
376 | [sleep on req->waitq] | 357 | [sleep on req->waitq] |
377 | | <sys_ 358 | | <sys_read()
378 | | >sys_ 359 | | >sys_unlink("/mnt/fuse/file")
379 | | [ac 360 | | [acquire inode semaphore
380 | | fo 361 | | for "file"]
381 | | *DE 362 | | *DEADLOCK*
382 363
383 The solution for this is to allow the filesyst 364 The solution for this is to allow the filesystem to be aborted.
384 365
385 **Scenario 2 - Tricky deadlock** 366 **Scenario 2 - Tricky deadlock**
386 367
387 368
388 This one needs a carefully crafted filesystem. 369 This one needs a carefully crafted filesystem. It's a variation on
389 the above, only the call back to the filesyste 370 the above, only the call back to the filesystem is not explicit,
390 but is caused by a pagefault. :: 371 but is caused by a pagefault. ::
391 372
392 | Kamikaze filesystem thread 1 | Kamik 373 | Kamikaze filesystem thread 1 | Kamikaze filesystem thread 2
393 | | 374 | |
394 | [fd = open("/mnt/fuse/file")] | [requ 375 | [fd = open("/mnt/fuse/file")] | [request served normally]
395 | [mmap fd to 'addr'] | 376 | [mmap fd to 'addr'] |
396 | [close fd] | [FLUS 377 | [close fd] | [FLUSH triggers 'magic' flag]
397 | [read a byte from addr] | 378 | [read a byte from addr] |
398 | >do_page_fault() | 379 | >do_page_fault() |
399 | [find or create page] | 380 | [find or create page] |
400 | [lock page] | 381 | [lock page] |
401 | >fuse_readpage() | 382 | >fuse_readpage() |
402 | [queue READ request] | 383 | [queue READ request] |
403 | [sleep on req->waitq] | 384 | [sleep on req->waitq] |
404 | | [read 385 | | [read request to buffer]
405 | | [crea 386 | | [create reply header before addr]
406 | | >sys_ 387 | | >sys_write(addr - headerlength)
407 | | >fu 388 | | >fuse_dev_write()
408 | | [ 389 | | [look up req in fc->processing]
409 | | [ 390 | | [remove from fc->processing]
410 | | [ 391 | | [copy write buffer to req]
411 | | 392 | | >do_page_fault()
412 | | 393 | | [find or create page]
413 | | 394 | | [lock page]
414 | | 395 | | * DEADLOCK *
415 396
416 The solution is basically the same as above. 397 The solution is basically the same as above.
417 398
418 An additional problem is that while the write 399 An additional problem is that while the write buffer is being copied
419 to the request, the request must not be interr 400 to the request, the request must not be interrupted/aborted. This is
420 because the destination address of the copy ma 401 because the destination address of the copy may not be valid after the
421 request has returned. 402 request has returned.
422 403
423 This is solved with doing the copy atomically, 404 This is solved with doing the copy atomically, and allowing abort
424 while the page(s) belonging to the write buffe 405 while the page(s) belonging to the write buffer are faulted with
425 get_user_pages(). The 'req->locked' flag indi 406 get_user_pages(). The 'req->locked' flag indicates when the copy is
426 taking place, and abort is delayed until this 407 taking place, and abort is delayed until this flag is unset.
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