1.. SPDX-License-Identifier: GPL-2.0 2 3============= 4FUSE Overview 5============= 6 7Definitions 8=========== 9 10Userspace 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 15Filesystem daemon: 16 The process(es) providing the data and metadata of the filesystem. 17 18Non-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 24Filesystem 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 31Mount owner: 32 The user who does the mounting. 33 34User: 35 The user who is performing filesystem operations. 36 37What is FUSE? 38============= 39 40FUSE is a userspace filesystem framework. It consists of a kernel 41module (fuse.ko), a userspace library (libfuse.*) and a mount utility 42(fusermount). 43 44One of the most important features of FUSE is allowing secure, 45non-privileged mounts. This opens up new possibilities for the use of 46filesystems. A good example is sshfs: a secure network filesystem 47using the sftp protocol. 48 49The userspace library and utilities are available from the 50`FUSE homepage: <https://github.com/libfuse/>`_ 51 52Filesystem type 53=============== 54 55The 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 66Mount options 67============= 68 69fd=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 74rootmode=M 75 The file mode of the filesystem's root in octal representation. 76 77user_id=N 78 The numeric user id of the mount owner. 79 80group_id=N 81 The numeric group id of the mount owner. 82 83default_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 91allow_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 97max_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 102blksize=N 103 Set the block size for the filesystem. The default is 512. This 104 option is only valid for 'fuseblk' type mounts. 105 106Control filesystem 107================== 108 109There's a control filesystem for FUSE, which can be mounted by:: 110 111 mount -t fusectl none /sys/fs/fuse/connections 112 113Mounting it under the '/sys/fs/fuse/connections' directory makes it 114backwards compatible with earlier versions. 115 116Under the fuse control filesystem each connection has a directory 117named by a unique number. 118 119For 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 max_background 133 The maximum number of background requests that can be outstanding 134 at a time. When the number of background requests reaches this limit, 135 further requests will be blocked until some are completed, potentially 136 causing I/O operations to stall. 137 138 congestion_threshold 139 The threshold of background requests at which the kernel considers 140 the filesystem to be congested. When the number of background requests 141 exceeds this value, the kernel will skip asynchronous readahead 142 operations, reducing read-ahead optimizations but preserving essential 143 I/O, as well as suspending non-synchronous writeback operations 144 (WB_SYNC_NONE), delaying page cache flushing to the filesystem. 145 146Only the owner of the mount may read or write these files. 147 148Interrupting filesystem operations 149################################## 150 151If a process issuing a FUSE filesystem request is interrupted, the 152following will happen: 153 154 - 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 - 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 - If the request is already sent to userspace, then an INTERRUPT 164 request is queued. 165 166INTERRUPT requests take precedence over other requests, so the 167userspace filesystem will receive queued INTERRUPTs before any others. 168 169The userspace filesystem may ignore the INTERRUPT requests entirely, 170or may honor them by sending a reply to the *original* request, with 171the error set to EINTR. 172 173It is also possible that there's a race between processing the 174original 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 182If the filesystem cannot find the original request, it should wait for 183some timeout and/or a number of new requests to arrive, after which it 184should reply to the INTERRUPT request with an EAGAIN error. In case 1851) the INTERRUPT request will be requeued. In case 2) the INTERRUPT 186reply will be ignored. 187 188Aborting a filesystem connection 189================================ 190 191It is possible to get into certain situations where the filesystem is 192not 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 204In either of these cases it may be useful to abort the connection to 205the 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 218How do non-privileged mounts work? 219================================== 220 221Since the mount() system call is a privileged operation, a helper 222program (fusermount) is needed, which is installed setuid root. 223 224The implication of providing non-privileged mounts is that the mount 225owner must not be able to use this capability to compromise the 226system. 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 237How 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 this device 243 244 2. creating a filesystem containing a suid or sgid application, then executing this application 245 246 The solution is not to allow opening device files and ignore 247 setuid and setgid bits when executing programs. To ensure this 248 fusermount always adds "nosuid" and "nodev" to the mount options 249 for non-privileged mounts. 250 251 B) If another user is accessing files or directories in the 252 filesystem, the filesystem daemon serving requests can record the 253 exact sequence and timing of operations performed. This 254 information is otherwise inaccessible to the mount owner, so this 255 counts as an information leak. 256 257 The solution to this problem will be presented in point 2) of C). 258 259 C) There are several ways in which the mount owner can induce 260 undesired behavior in other users' processes, such as: 261 262 1) mounting a filesystem over a file or directory which the mount 263 owner could otherwise not be able to modify (or could only 264 make limited modifications). 265 266 This is solved in fusermount, by checking the access 267 permissions on the mountpoint and only allowing the mount if 268 the mount owner can do unlimited modification (has write 269 access to the mountpoint, and mountpoint is not a "sticky" 270 directory) 271 272 2) Even if 1) is solved the mount owner can change the behavior 273 of other users' processes. 274 275 i) It can slow down or indefinitely delay the execution of a 276 filesystem operation creating a DoS against the user or the 277 whole system. For example a suid application locking a 278 system file, and then accessing a file on the mount owner's 279 filesystem could be stopped, and thus causing the system 280 file to be locked forever. 281 282 ii) It can present files or directories of unlimited length, or 283 directory structures of unlimited depth, possibly causing a 284 system process to eat up diskspace, memory or other 285 resources, again causing *DoS*. 286 287 The solution to this as well as B) is not to allow processes 288 to access the filesystem, which could otherwise not be 289 monitored or manipulated by the mount owner. Since if the 290 mount owner can ptrace a process, it can do all of the above 291 without using a FUSE mount, the same criteria as used in 292 ptrace can be used to check if a process is allowed to access 293 the filesystem or not. 294 295 Note that the *ptrace* check is not strictly necessary to 296 prevent C/2/i, it is enough to check if mount owner has enough 297 privilege to send signal to the process accessing the 298 filesystem, since *SIGSTOP* can be used to get a similar effect. 299 300I think these limitations are unacceptable? 301=========================================== 302 303If a sysadmin trusts the users enough, or can ensure through other 304measures, that system processes will never enter non-privileged 305mounts, it can relax the last limitation in several ways: 306 307 - With the 'user_allow_other' config option. If this config option is 308 set, the mounting user can add the 'allow_other' mount option which 309 disables the check for other users' processes. 310 311 User namespaces have an unintuitive interaction with 'allow_other': 312 an unprivileged user - normally restricted from mounting with 313 'allow_other' - could do so in a user namespace where they're 314 privileged. If any process could access such an 'allow_other' mount 315 this would give the mounting user the ability to manipulate 316 processes in user namespaces where they're unprivileged. For this 317 reason 'allow_other' restricts access to users in the same userns 318 or a descendant. 319 320 - With the 'allow_sys_admin_access' module option. If this option is 321 set, super user's processes have unrestricted access to mounts 322 irrespective of allow_other setting or user namespace of the 323 mounting user. 324 325Note that both of these relaxations expose the system to potential 326information leak or *DoS* as described in points B and C/2/i-ii in the 327preceding section. 328 329Kernel - userspace interface 330============================ 331 332The following diagram shows how a filesystem operation (in this 333example unlink) is performed in FUSE. :: 334 335 336 | "rm /mnt/fuse/file" | FUSE filesystem daemon 337 | | 338 | | >sys_read() 339 | | >fuse_dev_read() 340 | | >request_wait() 341 | | [sleep on fc->waitq] 342 | | 343 | >sys_unlink() | 344 | >fuse_unlink() | 345 | [get request from | 346 | fc->unused_list] | 347 | >request_send() | 348 | [queue req on fc->pending] | 349 | [wake up fc->waitq] | [woken up] 350 | >request_wait_answer() | 351 | [sleep on req->waitq] | 352 | | <request_wait() 353 | | [remove req from fc->pending] 354 | | [copy req to read buffer] 355 | | [add req to fc->processing] 356 | | <fuse_dev_read() 357 | | <sys_read() 358 | | 359 | | [perform unlink] 360 | | 361 | | >sys_write() 362 | | >fuse_dev_write() 363 | | [look up req in fc->processing] 364 | | [remove from fc->processing] 365 | | [copy write buffer to req] 366 | [woken up] | [wake up req->waitq] 367 | | <fuse_dev_write() 368 | | <sys_write() 369 | <request_wait_answer() | 370 | <request_send() | 371 | [add request to | 372 | fc->unused_list] | 373 | <fuse_unlink() | 374 | <sys_unlink() | 375 376.. note:: Everything in the description above is greatly simplified 377 378There are a couple of ways in which to deadlock a FUSE filesystem. 379Since we are talking about unprivileged userspace programs, 380something must be done about these. 381 382**Scenario 1 - Simple deadlock**:: 383 384 | "rm /mnt/fuse/file" | FUSE filesystem daemon 385 | | 386 | >sys_unlink("/mnt/fuse/file") | 387 | [acquire inode semaphore | 388 | for "file"] | 389 | >fuse_unlink() | 390 | [sleep on req->waitq] | 391 | | <sys_read() 392 | | >sys_unlink("/mnt/fuse/file") 393 | | [acquire inode semaphore 394 | | for "file"] 395 | | *DEADLOCK* 396 397The solution for this is to allow the filesystem to be aborted. 398 399**Scenario 2 - Tricky deadlock** 400 401 402This one needs a carefully crafted filesystem. It's a variation on 403the above, only the call back to the filesystem is not explicit, 404but is caused by a pagefault. :: 405 406 | Kamikaze filesystem thread 1 | Kamikaze filesystem thread 2 407 | | 408 | [fd = open("/mnt/fuse/file")] | [request served normally] 409 | [mmap fd to 'addr'] | 410 | [close fd] | [FLUSH triggers 'magic' flag] 411 | [read a byte from addr] | 412 | >do_page_fault() | 413 | [find or create page] | 414 | [lock page] | 415 | >fuse_readpage() | 416 | [queue READ request] | 417 | [sleep on req->waitq] | 418 | | [read request to buffer] 419 | | [create reply header before addr] 420 | | >sys_write(addr - headerlength) 421 | | >fuse_dev_write() 422 | | [look up req in fc->processing] 423 | | [remove from fc->processing] 424 | | [copy write buffer to req] 425 | | >do_page_fault() 426 | | [find or create page] 427 | | [lock page] 428 | | * DEADLOCK * 429 430The solution is basically the same as above. 431 432An additional problem is that while the write buffer is being copied 433to the request, the request must not be interrupted/aborted. This is 434because the destination address of the copy may not be valid after the 435request has returned. 436 437This is solved with doing the copy atomically, and allowing abort 438while the page(s) belonging to the write buffer are faulted with 439get_user_pages(). The 'req->locked' flag indicates when the copy is 440taking place, and abort is delayed until this flag is unset. 441