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334f485d MS |
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 | ||
bafa9654 MS |
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 | ||
334f485d MS |
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 | ||
d6392f87 MS |
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 | ||
334f485d MS |
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 | |
a33f3224 | 94 | filesystem is free to implement its access policy or leave it to |
334f485d MS |
95 | the underlying file access mechanism (e.g. in case of network |
96 | filesystems). This option enables permission checking, restricting | |
91f6e54b AD |
97 | access based on file mode. It is usually useful together with the |
98 | 'allow_other' mount option. | |
334f485d MS |
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 | ||
334f485d MS |
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 | ||
d8091614 MS |
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 | ||
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118 | Control filesystem |
119 | ~~~~~~~~~~~~~~~~~~ | |
120 | ||
121 | There's a control filesystem for FUSE, which can be mounted by: | |
bacac382 | 122 | |
bafa9654 | 123 | mount -t fusectl none /sys/fs/fuse/connections |
bacac382 | 124 | |
bafa9654 MS |
125 | Mounting it under the '/sys/fs/fuse/connections' directory makes it |
126 | backwards compatible with earlier versions. | |
bacac382 | 127 | |
bafa9654 MS |
128 | Under the fuse control filesystem each connection has a directory |
129 | named by a unique number. | |
bacac382 | 130 | |
bafa9654 | 131 | For each connection the following files exist within this directory: |
bacac382 MS |
132 | |
133 | 'waiting' | |
134 | ||
fa00e7e1 | 135 | The number of requests which are waiting to be transferred to |
bacac382 MS |
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 | ||
bafa9654 | 146 | Only the owner of the mount may read or write these files. |
bacac382 | 147 | |
a4d27e75 MS |
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 | |
fa00e7e1 | 160 | the request has been successfully transferred to userspace and |
a4d27e75 MS |
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 | |
a33f3224 | 174 | original request and its INTERRUPT request. There are two possibilities: |
a4d27e75 MS |
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 | ||
bacac382 MS |
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 | ||
bafa9654 MS |
215 | - Abort filesystem through the FUSE control filesystem. Most |
216 | powerful method, always works. | |
bacac382 | 217 | |
334f485d MS |
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 | ||
51eb01e7 | 380 | The solution for this is to allow the filesystem to be aborted. |
334f485d MS |
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 | ||
51eb01e7 | 413 | Solution is basically the same as above. |
334f485d | 414 | |
a4d27e75 MS |
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. | |
334f485d | 419 | |
51eb01e7 MS |
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. |