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diff --git a/Documentation/filesystems/fuse.rst b/Documentation/filesystems/fuse.rst new file mode 100644 index 000000000..1e31e87ae --- /dev/null +++ b/Documentation/filesystems/fuse.rst @@ -0,0 +1,426 @@ +.. SPDX-License-Identifier: GPL-2.0 + +==== +FUSE +==== + +Definitions +=========== + +Userspace filesystem: + A filesystem in which data and metadata are provided by an ordinary + userspace process. The filesystem can be accessed normally through + the kernel interface. + +Filesystem daemon: + The process(es) providing the data and metadata of the filesystem. + +Non-privileged mount (or user mount): + A userspace filesystem mounted by a non-privileged (non-root) user. + The filesystem daemon is running with the privileges of the mounting + user. NOTE: this is not the same as mounts allowed with the "user" + option in /etc/fstab, which is not discussed here. + +Filesystem connection: + A connection between the filesystem daemon and the kernel. The + connection exists until either the daemon dies, or the filesystem is + umounted. Note that detaching (or lazy umounting) the filesystem + does *not* break the connection, in this case it will exist until + the last reference to the filesystem is released. + +Mount owner: + The user who does the mounting. + +User: + The user who is performing filesystem operations. + +What is FUSE? +============= + +FUSE is a userspace filesystem framework. It consists of a kernel +module (fuse.ko), a userspace library (libfuse.*) and a mount utility +(fusermount). + +One of the most important features of FUSE is allowing secure, +non-privileged mounts. This opens up new possibilities for the use of +filesystems. A good example is sshfs: a secure network filesystem +using the sftp protocol. + +The userspace library and utilities are available from the +`FUSE homepage: <https://github.com/libfuse/>`_ + +Filesystem type +=============== + +The filesystem type given to mount(2) can be one of the following: + + fuse + This is the usual way to mount a FUSE filesystem. The first + argument of the mount system call may contain an arbitrary string, + which is not interpreted by the kernel. + + fuseblk + The filesystem is block device based. The first argument of the + mount system call is interpreted as the name of the device. + +Mount options +============= + +fd=N + The file descriptor to use for communication between the userspace + filesystem and the kernel. The file descriptor must have been + obtained by opening the FUSE device ('/dev/fuse'). + +rootmode=M + The file mode of the filesystem's root in octal representation. + +user_id=N + The numeric user id of the mount owner. + +group_id=N + The numeric group id of the mount owner. + +default_permissions + By default FUSE doesn't check file access permissions, the + filesystem is free to implement its access policy or leave it to + the underlying file access mechanism (e.g. in case of network + filesystems). This option enables permission checking, restricting + access based on file mode. It is usually useful together with the + 'allow_other' mount option. + +allow_other + This option overrides the security measure restricting file access + to the user mounting the filesystem. This option is by default only + allowed to root, but this restriction can be removed with a + (userspace) configuration option. + +max_read=N + With this option the maximum size of read operations can be set. + The default is infinite. Note that the size of read requests is + limited anyway to 32 pages (which is 128kbyte on i386). + +blksize=N + Set the block size for the filesystem. The default is 512. This + option is only valid for 'fuseblk' type mounts. + +Control filesystem +================== + +There's a control filesystem for FUSE, which can be mounted by:: + + mount -t fusectl none /sys/fs/fuse/connections + +Mounting it under the '/sys/fs/fuse/connections' directory makes it +backwards compatible with earlier versions. + +Under the fuse control filesystem each connection has a directory +named by a unique number. + +For each connection the following files exist within this directory: + + waiting + The number of requests which are waiting to be transferred to + userspace or being processed by the filesystem daemon. If there is + no filesystem activity and 'waiting' is non-zero, then the + filesystem is hung or deadlocked. + + abort + Writing anything into this file will abort the filesystem + connection. This means that all waiting requests will be aborted an + error returned for all aborted and new requests. + +Only the owner of the mount may read or write these files. + +Interrupting filesystem operations +################################## + +If a process issuing a FUSE filesystem request is interrupted, the +following will happen: + + - If the request is not yet sent to userspace AND the signal is + fatal (SIGKILL or unhandled fatal signal), then the request is + dequeued and returns immediately. + + - If the request is not yet sent to userspace AND the signal is not + fatal, then an interrupted flag is set for the request. When + the request has been successfully transferred to userspace and + this flag is set, an INTERRUPT request is queued. + + - If the request is already sent to userspace, then an INTERRUPT + request is queued. + +INTERRUPT requests take precedence over other requests, so the +userspace filesystem will receive queued INTERRUPTs before any others. + +The userspace filesystem may ignore the INTERRUPT requests entirely, +or may honor them by sending a reply to the *original* request, with +the error set to EINTR. + +It is also possible that there's a race between processing the +original request and its INTERRUPT request. There are two possibilities: + + 1. The INTERRUPT request is processed before the original request is + processed + + 2. The INTERRUPT request is processed after the original request has + been answered + +If the filesystem cannot find the original request, it should wait for +some timeout and/or a number of new requests to arrive, after which it +should reply to the INTERRUPT request with an EAGAIN error. In case +1) the INTERRUPT request will be requeued. In case 2) the INTERRUPT +reply will be ignored. + +Aborting a filesystem connection +================================ + +It is possible to get into certain situations where the filesystem is +not responding. Reasons for this may be: + + a) Broken userspace filesystem implementation + + b) Network connection down + + c) Accidental deadlock + + d) Malicious deadlock + +(For more on c) and d) see later sections) + +In either of these cases it may be useful to abort the connection to +the filesystem. There are several ways to do this: + + - Kill the filesystem daemon. Works in case of a) and b) + + - Kill the filesystem daemon and all users of the filesystem. Works + in all cases except some malicious deadlocks + + - Use forced umount (umount -f). Works in all cases but only if + filesystem is still attached (it hasn't been lazy unmounted) + + - Abort filesystem through the FUSE control filesystem. Most + powerful method, always works. + +How do non-privileged mounts work? +================================== + +Since the mount() system call is a privileged operation, a helper +program (fusermount) is needed, which is installed setuid root. + +The implication of providing non-privileged mounts is that the mount +owner must not be able to use this capability to compromise the +system. Obvious requirements arising from this are: + + A) mount owner should not be able to get elevated privileges with the + help of the mounted filesystem + + B) mount owner should not get illegitimate access to information from + other users' and the super user's processes + + C) mount owner should not be able to induce undesired behavior in + other users' or the super user's processes + +How are requirements fulfilled? +=============================== + + A) The mount owner could gain elevated privileges by either: + + 1. creating a filesystem containing a device file, then opening this device + + 2. creating a filesystem containing a suid or sgid application, then executing this application + + The solution is not to allow opening device files and ignore + setuid and setgid bits when executing programs. To ensure this + fusermount always adds "nosuid" and "nodev" to the mount options + for non-privileged mounts. + + B) If another user is accessing files or directories in the + filesystem, the filesystem daemon serving requests can record the + exact sequence and timing of operations performed. This + information is otherwise inaccessible to the mount owner, so this + counts as an information leak. + + The solution to this problem will be presented in point 2) of C). + + C) There are several ways in which the mount owner can induce + undesired behavior in other users' processes, such as: + + 1) mounting a filesystem over a file or directory which the mount + owner could otherwise not be able to modify (or could only + make limited modifications). + + This is solved in fusermount, by checking the access + permissions on the mountpoint and only allowing the mount if + the mount owner can do unlimited modification (has write + access to the mountpoint, and mountpoint is not a "sticky" + directory) + + 2) Even if 1) is solved the mount owner can change the behavior + of other users' processes. + + i) It can slow down or indefinitely delay the execution of a + filesystem operation creating a DoS against the user or the + whole system. For example a suid application locking a + system file, and then accessing a file on the mount owner's + filesystem could be stopped, and thus causing the system + file to be locked forever. + + ii) It can present files or directories of unlimited length, or + directory structures of unlimited depth, possibly causing a + system process to eat up diskspace, memory or other + resources, again causing *DoS*. + + The solution to this as well as B) is not to allow processes + to access the filesystem, which could otherwise not be + monitored or manipulated by the mount owner. Since if the + mount owner can ptrace a process, it can do all of the above + without using a FUSE mount, the same criteria as used in + ptrace can be used to check if a process is allowed to access + the filesystem or not. + + Note that the *ptrace* check is not strictly necessary to + prevent C/2/i, it is enough to check if mount owner has enough + privilege to send signal to the process accessing the + filesystem, since *SIGSTOP* can be used to get a similar effect. + +I think these limitations are unacceptable? +=========================================== + +If a sysadmin trusts the users enough, or can ensure through other +measures, that system processes will never enter non-privileged +mounts, it can relax the last limitation in several ways: + + - With the 'user_allow_other' config option. If this config option is + set, the mounting user can add the 'allow_other' mount option which + disables the check for other users' processes. + + User namespaces have an unintuitive interaction with 'allow_other': + an unprivileged user - normally restricted from mounting with + 'allow_other' - could do so in a user namespace where they're + privileged. If any process could access such an 'allow_other' mount + this would give the mounting user the ability to manipulate + processes in user namespaces where they're unprivileged. For this + reason 'allow_other' restricts access to users in the same userns + or a descendant. + + - With the 'allow_sys_admin_access' module option. If this option is + set, super user's processes have unrestricted access to mounts + irrespective of allow_other setting or user namespace of the + mounting user. + +Note that both of these relaxations expose the system to potential +information leak or *DoS* as described in points B and C/2/i-ii in the +preceding section. + +Kernel - userspace interface +============================ + +The following diagram shows how a filesystem operation (in this +example unlink) is performed in FUSE. :: + + + | "rm /mnt/fuse/file" | FUSE filesystem daemon + | | + | | >sys_read() + | | >fuse_dev_read() + | | >request_wait() + | | [sleep on fc->waitq] + | | + | >sys_unlink() | + | >fuse_unlink() | + | [get request from | + | fc->unused_list] | + | >request_send() | + | [queue req on fc->pending] | + | [wake up fc->waitq] | [woken up] + | >request_wait_answer() | + | [sleep on req->waitq] | + | | <request_wait() + | | [remove req from fc->pending] + | | [copy req to read buffer] + | | [add req to fc->processing] + | | <fuse_dev_read() + | | <sys_read() + | | + | | [perform unlink] + | | + | | >sys_write() + | | >fuse_dev_write() + | | [look up req in fc->processing] + | | [remove from fc->processing] + | | [copy write buffer to req] + | [woken up] | [wake up req->waitq] + | | <fuse_dev_write() + | | <sys_write() + | <request_wait_answer() | + | <request_send() | + | [add request to | + | fc->unused_list] | + | <fuse_unlink() | + | <sys_unlink() | + +.. note:: Everything in the description above is greatly simplified + +There are a couple of ways in which to deadlock a FUSE filesystem. +Since we are talking about unprivileged userspace programs, +something must be done about these. + +**Scenario 1 - Simple deadlock**:: + + | "rm /mnt/fuse/file" | FUSE filesystem daemon + | | + | >sys_unlink("/mnt/fuse/file") | + | [acquire inode semaphore | + | for "file"] | + | >fuse_unlink() | + | [sleep on req->waitq] | + | | <sys_read() + | | >sys_unlink("/mnt/fuse/file") + | | [acquire inode semaphore + | | for "file"] + | | *DEADLOCK* + +The solution for this is to allow the filesystem to be aborted. + +**Scenario 2 - Tricky deadlock** + + +This one needs a carefully crafted filesystem. It's a variation on +the above, only the call back to the filesystem is not explicit, +but is caused by a pagefault. :: + + | Kamikaze filesystem thread 1 | Kamikaze filesystem thread 2 + | | + | [fd = open("/mnt/fuse/file")] | [request served normally] + | [mmap fd to 'addr'] | + | [close fd] | [FLUSH triggers 'magic' flag] + | [read a byte from addr] | + | >do_page_fault() | + | [find or create page] | + | [lock page] | + | >fuse_readpage() | + | [queue READ request] | + | [sleep on req->waitq] | + | | [read request to buffer] + | | [create reply header before addr] + | | >sys_write(addr - headerlength) + | | >fuse_dev_write() + | | [look up req in fc->processing] + | | [remove from fc->processing] + | | [copy write buffer to req] + | | >do_page_fault() + | | [find or create page] + | | [lock page] + | | * DEADLOCK * + +The solution is basically the same as above. + +An additional problem is that while the write buffer is being copied +to the request, the request must not be interrupted/aborted. This is +because the destination address of the copy may not be valid after the +request has returned. + +This is solved with doing the copy atomically, and allowing abort +while the page(s) belonging to the write buffer are faulted with +get_user_pages(). The 'req->locked' flag indicates when the copy is +taking place, and abort is delayed until this flag is unset. |