1.. SPDX-License-Identifier: GPL-2.0 2 3==== 4FUSE 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 132Only the owner of the mount may read or write these files. 133 134Interrupting filesystem operations 135################################## 136 137If a process issuing a FUSE filesystem request is interrupted, the 138following will happen: 139 140 - If the request is not yet sent to userspace AND the signal is 141 fatal (SIGKILL or unhandled fatal signal), then the request is 142 dequeued and returns immediately. 143 144 - If the request is not yet sent to userspace AND the signal is not 145 fatal, then an interrupted flag is set for the request. When 146 the request has been successfully transferred to userspace and 147 this flag is set, an INTERRUPT request is queued. 148 149 - If the request is already sent to userspace, then an INTERRUPT 150 request is queued. 151 152INTERRUPT requests take precedence over other requests, so the 153userspace filesystem will receive queued INTERRUPTs before any others. 154 155The userspace filesystem may ignore the INTERRUPT requests entirely, 156or may honor them by sending a reply to the *original* request, with 157the error set to EINTR. 158 159It is also possible that there's a race between processing the 160original request and its INTERRUPT request. There are two possibilities: 161 162 1. The INTERRUPT request is processed before the original request is 163 processed 164 165 2. The INTERRUPT request is processed after the original request has 166 been answered 167 168If the filesystem cannot find the original request, it should wait for 169some timeout and/or a number of new requests to arrive, after which it 170should reply to the INTERRUPT request with an EAGAIN error. In case 1711) the INTERRUPT request will be requeued. In case 2) the INTERRUPT 172reply will be ignored. 173 174Aborting a filesystem connection 175================================ 176 177It is possible to get into certain situations where the filesystem is 178not responding. Reasons for this may be: 179 180 a) Broken userspace filesystem implementation 181 182 b) Network connection down 183 184 c) Accidental deadlock 185 186 d) Malicious deadlock 187 188(For more on c) and d) see later sections) 189 190In either of these cases it may be useful to abort the connection to 191the filesystem. There are several ways to do this: 192 193 - Kill the filesystem daemon. Works in case of a) and b) 194 195 - Kill the filesystem daemon and all users of the filesystem. Works 196 in all cases except some malicious deadlocks 197 198 - Use forced umount (umount -f). Works in all cases but only if 199 filesystem is still attached (it hasn't been lazy unmounted) 200 201 - Abort filesystem through the FUSE control filesystem. Most 202 powerful method, always works. 203 204How do non-privileged mounts work? 205================================== 206 207Since the mount() system call is a privileged operation, a helper 208program (fusermount) is needed, which is installed setuid root. 209 210The implication of providing non-privileged mounts is that the mount 211owner must not be able to use this capability to compromise the 212system. Obvious requirements arising from this are: 213 214 A) mount owner should not be able to get elevated privileges with the 215 help of the mounted filesystem 216 217 B) mount owner should not get illegitimate access to information from 218 other users' and the super user's processes 219 220 C) mount owner should not be able to induce undesired behavior in 221 other users' or the super user's processes 222 223How are requirements fulfilled? 224=============================== 225 226 A) The mount owner could gain elevated privileges by either: 227 228 1. creating a filesystem containing a device file, then opening this device 229 230 2. creating a filesystem containing a suid or sgid application, then executing this application 231 232 The solution is not to allow opening device files and ignore 233 setuid and setgid bits when executing programs. To ensure this 234 fusermount always adds "nosuid" and "nodev" to the mount options 235 for non-privileged mounts. 236 237 B) If another user is accessing files or directories in the 238 filesystem, the filesystem daemon serving requests can record the 239 exact sequence and timing of operations performed. This 240 information is otherwise inaccessible to the mount owner, so this 241 counts as an information leak. 242 243 The solution to this problem will be presented in point 2) of C). 244 245 C) There are several ways in which the mount owner can induce 246 undesired behavior in other users' processes, such as: 247 248 1) mounting a filesystem over a file or directory which the mount 249 owner could otherwise not be able to modify (or could only 250 make limited modifications). 251 252 This is solved in fusermount, by checking the access 253 permissions on the mountpoint and only allowing the mount if 254 the mount owner can do unlimited modification (has write 255 access to the mountpoint, and mountpoint is not a "sticky" 256 directory) 257 258 2) Even if 1) is solved the mount owner can change the behavior 259 of other users' processes. 260 261 i) It can slow down or indefinitely delay the execution of a 262 filesystem operation creating a DoS against the user or the 263 whole system. For example a suid application locking a 264 system file, and then accessing a file on the mount owner's 265 filesystem could be stopped, and thus causing the system 266 file to be locked forever. 267 268 ii) It can present files or directories of unlimited length, or 269 directory structures of unlimited depth, possibly causing a 270 system process to eat up diskspace, memory or other 271 resources, again causing *DoS*. 272 273 The solution to this as well as B) is not to allow processes 274 to access the filesystem, which could otherwise not be 275 monitored or manipulated by the mount owner. Since if the 276 mount owner can ptrace a process, it can do all of the above 277 without using a FUSE mount, the same criteria as used in 278 ptrace can be used to check if a process is allowed to access 279 the filesystem or not. 280 281 Note that the *ptrace* check is not strictly necessary to 282 prevent C/2/i, it is enough to check if mount owner has enough 283 privilege to send signal to the process accessing the 284 filesystem, since *SIGSTOP* can be used to get a similar effect. 285 286I think these limitations are unacceptable? 287=========================================== 288 289If a sysadmin trusts the users enough, or can ensure through other 290measures, that system processes will never enter non-privileged 291mounts, it can relax the last limitation in several ways: 292 293 - With the 'user_allow_other' config option. If this config option is 294 set, the mounting user can add the 'allow_other' mount option which 295 disables the check for other users' processes. 296 297 User namespaces have an unintuitive interaction with 'allow_other': 298 an unprivileged user - normally restricted from mounting with 299 'allow_other' - could do so in a user namespace where they're 300 privileged. If any process could access such an 'allow_other' mount 301 this would give the mounting user the ability to manipulate 302 processes in user namespaces where they're unprivileged. For this 303 reason 'allow_other' restricts access to users in the same userns 304 or a descendant. 305 306 - With the 'allow_sys_admin_access' module option. If this option is 307 set, super user's processes have unrestricted access to mounts 308 irrespective of allow_other setting or user namespace of the 309 mounting user. 310 311Note that both of these relaxations expose the system to potential 312information leak or *DoS* as described in points B and C/2/i-ii in the 313preceding section. 314 315Kernel - userspace interface 316============================ 317 318The following diagram shows how a filesystem operation (in this 319example unlink) is performed in FUSE. :: 320 321 322 | "rm /mnt/fuse/file" | FUSE filesystem daemon 323 | | 324 | | >sys_read() 325 | | >fuse_dev_read() 326 | | >request_wait() 327 | | [sleep on fc->waitq] 328 | | 329 | >sys_unlink() | 330 | >fuse_unlink() | 331 | [get request from | 332 | fc->unused_list] | 333 | >request_send() | 334 | [queue req on fc->pending] | 335 | [wake up fc->waitq] | [woken up] 336 | >request_wait_answer() | 337 | [sleep on req->waitq] | 338 | | <request_wait() 339 | | [remove req from fc->pending] 340 | | [copy req to read buffer] 341 | | [add req to fc->processing] 342 | | <fuse_dev_read() 343 | | <sys_read() 344 | | 345 | | [perform unlink] 346 | | 347 | | >sys_write() 348 | | >fuse_dev_write() 349 | | [look up req in fc->processing] 350 | | [remove from fc->processing] 351 | | [copy write buffer to req] 352 | [woken up] | [wake up req->waitq] 353 | | <fuse_dev_write() 354 | | <sys_write() 355 | <request_wait_answer() | 356 | <request_send() | 357 | [add request to | 358 | fc->unused_list] | 359 | <fuse_unlink() | 360 | <sys_unlink() | 361 362.. note:: Everything in the description above is greatly simplified 363 364There are a couple of ways in which to deadlock a FUSE filesystem. 365Since we are talking about unprivileged userspace programs, 366something must be done about these. 367 368**Scenario 1 - Simple deadlock**:: 369 370 | "rm /mnt/fuse/file" | FUSE filesystem daemon 371 | | 372 | >sys_unlink("/mnt/fuse/file") | 373 | [acquire inode semaphore | 374 | for "file"] | 375 | >fuse_unlink() | 376 | [sleep on req->waitq] | 377 | | <sys_read() 378 | | >sys_unlink("/mnt/fuse/file") 379 | | [acquire inode semaphore 380 | | for "file"] 381 | | *DEADLOCK* 382 383The solution for this is to allow the filesystem to be aborted. 384 385**Scenario 2 - Tricky deadlock** 386 387 388This one needs a carefully crafted filesystem. It's a variation on 389the above, only the call back to the filesystem is not explicit, 390but is caused by a pagefault. :: 391 392 | Kamikaze filesystem thread 1 | Kamikaze filesystem thread 2 393 | | 394 | [fd = open("/mnt/fuse/file")] | [request served normally] 395 | [mmap fd to 'addr'] | 396 | [close fd] | [FLUSH triggers 'magic' flag] 397 | [read a byte from addr] | 398 | >do_page_fault() | 399 | [find or create page] | 400 | [lock page] | 401 | >fuse_readpage() | 402 | [queue READ request] | 403 | [sleep on req->waitq] | 404 | | [read request to buffer] 405 | | [create reply header before addr] 406 | | >sys_write(addr - headerlength) 407 | | >fuse_dev_write() 408 | | [look up req in fc->processing] 409 | | [remove from fc->processing] 410 | | [copy write buffer to req] 411 | | >do_page_fault() 412 | | [find or create page] 413 | | [lock page] 414 | | * DEADLOCK * 415 416The solution is basically the same as above. 417 418An additional problem is that while the write buffer is being copied 419to the request, the request must not be interrupted/aborted. This is 420because the destination address of the copy may not be valid after the 421request has returned. 422 423This is solved with doing the copy atomically, and allowing abort 424while the page(s) belonging to the write buffer are faulted with 425get_user_pages(). The 'req->locked' flag indicates when the copy is 426taking place, and abort is delayed until this flag is unset. 427