1.. SPDX-License-Identifier: GPL-2.0 2============== 3FUSE 4============== 5 6Definitions 7=========== 8 9Userspace filesystem: 10 A filesystem in which data and metadata are provided by an ordinary 11 userspace process. The filesystem can be accessed normally through 12 the kernel interface. 13 14Filesystem daemon: 15 The process(es) providing the data and metadata of the filesystem. 16 17Non-privileged mount (or user mount): 18 A userspace filesystem mounted by a non-privileged (non-root) user. 19 The filesystem daemon is running with the privileges of the mounting 20 user. NOTE: this is not the same as mounts allowed with the "user" 21 option in /etc/fstab, which is not discussed here. 22 23Filesystem connection: 24 A connection between the filesystem daemon and the kernel. The 25 connection exists until either the daemon dies, or the filesystem is 26 umounted. Note that detaching (or lazy umounting) the filesystem 27 does *not* break the connection, in this case it will exist until 28 the last reference to the filesystem is released. 29 30Mount owner: 31 The user who does the mounting. 32 33User: 34 The user who is performing filesystem operations. 35 36What is FUSE? 37============= 38 39FUSE is a userspace filesystem framework. It consists of a kernel 40module (fuse.ko), a userspace library (libfuse.*) and a mount utility 41(fusermount). 42 43One of the most important features of FUSE is allowing secure, 44non-privileged mounts. This opens up new possibilities for the use of 45filesystems. A good example is sshfs: a secure network filesystem 46using the sftp protocol. 47 48The userspace library and utilities are available from the 49`FUSE homepage: <http://fuse.sourceforge.net/>`_ 50 51Filesystem type 52=============== 53 54The filesystem type given to mount(2) can be one of the following: 55 56 fuse 57 This is the usual way to mount a FUSE filesystem. The first 58 argument of the mount system call may contain an arbitrary string, 59 which is not interpreted by the kernel. 60 61 fuseblk 62 The filesystem is block device based. The first argument of the 63 mount system call is interpreted as the name of the device. 64 65Mount options 66============= 67 68fd=N 69 The file descriptor to use for communication between the userspace 70 filesystem and the kernel. The file descriptor must have been 71 obtained by opening the FUSE device ('/dev/fuse'). 72 73rootmode=M 74 The file mode of the filesystem's root in octal representation. 75 76user_id=N 77 The numeric user id of the mount owner. 78 79group_id=N 80 The numeric group id of the mount owner. 81 82default_permissions 83 By default FUSE doesn't check file access permissions, the 84 filesystem is free to implement its access policy or leave it to 85 the underlying file access mechanism (e.g. in case of network 86 filesystems). This option enables permission checking, restricting 87 access based on file mode. It is usually useful together with the 88 'allow_other' mount option. 89 90allow_other 91 This option overrides the security measure restricting file access 92 to the user mounting the filesystem. This option is by default only 93 allowed to root, but this restriction can be removed with a 94 (userspace) configuration option. 95 96max_read=N 97 With this option the maximum size of read operations can be set. 98 The default is infinite. Note that the size of read requests is 99 limited anyway to 32 pages (which is 128kbyte on i386). 100 101blksize=N 102 Set the block size for the filesystem. The default is 512. This 103 option is only valid for 'fuseblk' type mounts. 104 105Control filesystem 106================== 107 108There's a control filesystem for FUSE, which can be mounted by:: 109 110 mount -t fusectl none /sys/fs/fuse/connections 111 112Mounting it under the '/sys/fs/fuse/connections' directory makes it 113backwards compatible with earlier versions. 114 115Under the fuse control filesystem each connection has a directory 116named by a unique number. 117 118For each connection the following files exist within this directory: 119 120 waiting 121 The number of requests which are waiting to be transferred to 122 userspace or being processed by the filesystem daemon. If there is 123 no filesystem activity and 'waiting' is non-zero, then the 124 filesystem is hung or deadlocked. 125 126 abort 127 Writing anything into this file will abort the filesystem 128 connection. This means that all waiting requests will be aborted an 129 error returned for all aborted and new requests. 130 131Only the owner of the mount may read or write these files. 132 133Interrupting filesystem operations 134################################## 135 136If a process issuing a FUSE filesystem request is interrupted, the 137following will happen: 138 139 - If the request is not yet sent to userspace AND the signal is 140 fatal (SIGKILL or unhandled fatal signal), then the request is 141 dequeued and returns immediately. 142 143 - If the request is not yet sent to userspace AND the signal is not 144 fatal, then an interrupted flag is set for the request. When 145 the request has been successfully transferred to userspace and 146 this flag is set, an INTERRUPT request is queued. 147 148 - If the request is already sent to userspace, then an INTERRUPT 149 request is queued. 150 151INTERRUPT requests take precedence over other requests, so the 152userspace filesystem will receive queued INTERRUPTs before any others. 153 154The userspace filesystem may ignore the INTERRUPT requests entirely, 155or may honor them by sending a reply to the *original* request, with 156the error set to EINTR. 157 158It is also possible that there's a race between processing the 159original request and its INTERRUPT request. There are two possibilities: 160 161 1. The INTERRUPT request is processed before the original request is 162 processed 163 164 2. The INTERRUPT request is processed after the original request has 165 been answered 166 167If the filesystem cannot find the original request, it should wait for 168some timeout and/or a number of new requests to arrive, after which it 169should reply to the INTERRUPT request with an EAGAIN error. In case 1701) the INTERRUPT request will be requeued. In case 2) the INTERRUPT 171reply will be ignored. 172 173Aborting a filesystem connection 174================================ 175 176It is possible to get into certain situations where the filesystem is 177not responding. Reasons for this may be: 178 179 a) Broken userspace filesystem implementation 180 181 b) Network connection down 182 183 c) Accidental deadlock 184 185 d) Malicious deadlock 186 187(For more on c) and d) see later sections) 188 189In either of these cases it may be useful to abort the connection to 190the filesystem. There are several ways to do this: 191 192 - Kill the filesystem daemon. Works in case of a) and b) 193 194 - Kill the filesystem daemon and all users of the filesystem. Works 195 in all cases except some malicious deadlocks 196 197 - Use forced umount (umount -f). Works in all cases but only if 198 filesystem is still attached (it hasn't been lazy unmounted) 199 200 - Abort filesystem through the FUSE control filesystem. Most 201 powerful method, always works. 202 203How do non-privileged mounts work? 204================================== 205 206Since the mount() system call is a privileged operation, a helper 207program (fusermount) is needed, which is installed setuid root. 208 209The implication of providing non-privileged mounts is that the mount 210owner must not be able to use this capability to compromise the 211system. Obvious requirements arising from this are: 212 213 A) mount owner should not be able to get elevated privileges with the 214 help of the mounted filesystem 215 216 B) mount owner should not get illegitimate access to information from 217 other users' and the super user's processes 218 219 C) mount owner should not be able to induce undesired behavior in 220 other users' or the super user's processes 221 222How are requirements fulfilled? 223=============================== 224 225 A) The mount owner could gain elevated privileges by either: 226 227 1. creating a filesystem containing a device file, then opening this device 228 229 2. creating a filesystem containing a suid or sgid application, then executing this application 230 231 The solution is not to allow opening device files and ignore 232 setuid and setgid bits when executing programs. To ensure this 233 fusermount always adds "nosuid" and "nodev" to the mount options 234 for non-privileged mounts. 235 236 B) If another user is accessing files or directories in the 237 filesystem, the filesystem daemon serving requests can record the 238 exact sequence and timing of operations performed. This 239 information is otherwise inaccessible to the mount owner, so this 240 counts as an information leak. 241 242 The solution to this problem will be presented in point 2) of C). 243 244 C) There are several ways in which the mount owner can induce 245 undesired behavior in other users' processes, such as: 246 247 1) mounting a filesystem over a file or directory which the mount 248 owner could otherwise not be able to modify (or could only 249 make limited modifications). 250 251 This is solved in fusermount, by checking the access 252 permissions on the mountpoint and only allowing the mount if 253 the mount owner can do unlimited modification (has write 254 access to the mountpoint, and mountpoint is not a "sticky" 255 directory) 256 257 2) Even if 1) is solved the mount owner can change the behavior 258 of other users' processes. 259 260 i) It can slow down or indefinitely delay the execution of a 261 filesystem operation creating a DoS against the user or the 262 whole system. For example a suid application locking a 263 system file, and then accessing a file on the mount owner's 264 filesystem could be stopped, and thus causing the system 265 file to be locked forever. 266 267 ii) It can present files or directories of unlimited length, or 268 directory structures of unlimited depth, possibly causing a 269 system process to eat up diskspace, memory or other 270 resources, again causing *DoS*. 271 272 The solution to this as well as B) is not to allow processes 273 to access the filesystem, which could otherwise not be 274 monitored or manipulated by the mount owner. Since if the 275 mount owner can ptrace a process, it can do all of the above 276 without using a FUSE mount, the same criteria as used in 277 ptrace can be used to check if a process is allowed to access 278 the filesystem or not. 279 280 Note that the *ptrace* check is not strictly necessary to 281 prevent B/2/i, it is enough to check if mount owner has enough 282 privilege to send signal to the process accessing the 283 filesystem, since *SIGSTOP* can be used to get a similar effect. 284 285I think these limitations are unacceptable? 286=========================================== 287 288If a sysadmin trusts the users enough, or can ensure through other 289measures, that system processes will never enter non-privileged 290mounts, it can relax the last limitation with a 'user_allow_other' 291config option. If this config option is set, the mounting user can 292add the 'allow_other' mount option which disables the check for other 293users' processes. 294 295Kernel - userspace interface 296============================ 297 298The following diagram shows how a filesystem operation (in this 299example unlink) is performed in FUSE. :: 300 301 302 | "rm /mnt/fuse/file" | FUSE filesystem daemon 303 | | 304 | | >sys_read() 305 | | >fuse_dev_read() 306 | | >request_wait() 307 | | [sleep on fc->waitq] 308 | | 309 | >sys_unlink() | 310 | >fuse_unlink() | 311 | [get request from | 312 | fc->unused_list] | 313 | >request_send() | 314 | [queue req on fc->pending] | 315 | [wake up fc->waitq] | [woken up] 316 | >request_wait_answer() | 317 | [sleep on req->waitq] | 318 | | <request_wait() 319 | | [remove req from fc->pending] 320 | | [copy req to read buffer] 321 | | [add req to fc->processing] 322 | | <fuse_dev_read() 323 | | <sys_read() 324 | | 325 | | [perform unlink] 326 | | 327 | | >sys_write() 328 | | >fuse_dev_write() 329 | | [look up req in fc->processing] 330 | | [remove from fc->processing] 331 | | [copy write buffer to req] 332 | [woken up] | [wake up req->waitq] 333 | | <fuse_dev_write() 334 | | <sys_write() 335 | <request_wait_answer() | 336 | <request_send() | 337 | [add request to | 338 | fc->unused_list] | 339 | <fuse_unlink() | 340 | <sys_unlink() | 341 342.. note:: Everything in the description above is greatly simplified 343 344There are a couple of ways in which to deadlock a FUSE filesystem. 345Since we are talking about unprivileged userspace programs, 346something must be done about these. 347 348**Scenario 1 - Simple deadlock**:: 349 350 | "rm /mnt/fuse/file" | FUSE filesystem daemon 351 | | 352 | >sys_unlink("/mnt/fuse/file") | 353 | [acquire inode semaphore | 354 | for "file"] | 355 | >fuse_unlink() | 356 | [sleep on req->waitq] | 357 | | <sys_read() 358 | | >sys_unlink("/mnt/fuse/file") 359 | | [acquire inode semaphore 360 | | for "file"] 361 | | *DEADLOCK* 362 363The solution for this is to allow the filesystem to be aborted. 364 365**Scenario 2 - Tricky deadlock** 366 367 368This one needs a carefully crafted filesystem. It's a variation on 369the above, only the call back to the filesystem is not explicit, 370but is caused by a pagefault. :: 371 372 | Kamikaze filesystem thread 1 | Kamikaze filesystem thread 2 373 | | 374 | [fd = open("/mnt/fuse/file")] | [request served normally] 375 | [mmap fd to 'addr'] | 376 | [close fd] | [FLUSH triggers 'magic' flag] 377 | [read a byte from addr] | 378 | >do_page_fault() | 379 | [find or create page] | 380 | [lock page] | 381 | >fuse_readpage() | 382 | [queue READ request] | 383 | [sleep on req->waitq] | 384 | | [read request to buffer] 385 | | [create reply header before addr] 386 | | >sys_write(addr - headerlength) 387 | | >fuse_dev_write() 388 | | [look up req in fc->processing] 389 | | [remove from fc->processing] 390 | | [copy write buffer to req] 391 | | >do_page_fault() 392 | | [find or create page] 393 | | [lock page] 394 | | * DEADLOCK * 395 396The solution is basically the same as above. 397 398An additional problem is that while the write buffer is being copied 399to the request, the request must not be interrupted/aborted. This is 400because the destination address of the copy may not be valid after the 401request has returned. 402 403This is solved with doing the copy atomically, and allowing abort 404while the page(s) belonging to the write buffer are faulted with 405get_user_pages(). The 'req->locked' flag indicates when the copy is 406taking place, and abort is delayed until this flag is unset. 407