Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 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.