libfuse

Warning: unresolved security issue

Be aware that FUSE has an unresolved security bug (bug #15): if the default_permissions mount option is not used, the results of the first permission check performed by the file system for a directory entry will be re-used for subsequent accesses as long as the inode of the accessed entry is present in the kernel cache - even if the permissions have since changed, and even if the subsequent access is made by a different user.

This bug needs to be fixed in the Linux kernel and has been known since 2006 but unfortunately no fix has been applied yet. If you depend on correct permission handling for FUSE file systems, the only workaround is to use default_permissions (which does not currently support ACLs), or to completely disable caching of directory entry attributes. Alternatively, the severity of the bug can be somewhat reduced by not using the allow_other mount option.

About

FUSE (Filesystem in Userspace) is an interface for userspace programs to export a filesystem to the Linux kernel. The FUSE project consists of two components: the fuse kernel module (maintained in the regular kernel repositories) and the libfuse userspace library (maintained in this repository). libfuse provides the reference implementation for communicating with the FUSE kernel module.

A FUSE file system is typically implemented as a standalone application that links with libfuse. libfuse provides functions to mount the file system, unmount it, read requests from the kernel, and send responses back. libfuse offers two APIs: a "high-level", synchronous API, and a "low-level" asynchronous API. In both cases, incoming requests from the kernel are passed to the main program using callbacks. When using the high-level API, the callbacks may work with file names and paths instead of inodes, and processing of a request finishes when the callback function returns. When using the low-level API, the callbacks must work with inodes and responses must be sent explicitly using a separate set of API functions.

Installation

You can download libfuse from https://github.com/libfuse/libfuse/releases. After extracting the tarball, build and install with

./configure
make -j8
make install

To run some self tests, you need a Python 3 environment with the py.test module installed. To run the tests, execute

python3 -m pytest test/

You may also need to add /usr/local/lib to /etc/ld.so.conf and/or run ldconfig. If you're building from the git repository (instead of using a release tarball), you also need to run ./makeconf.sh to create the configure script.

You'll also need a fuse kernel module (Linux kernels 2.6.14 or later contain FUSE support).

Security implications

If you run make install, the fusermount3 program is installed set-user-id to root. This is done to allow normal users to mount their own filesystem implementations.

There must however be some limitations, in order to prevent Bad User from doing nasty things. Currently those limitations are:

• The user can only mount on a mountpoint, for which it has write permission

• The mountpoint is not a sticky directory which isn't owned by the user (like /tmp usually is)

• No other user (including root) can access the contents of the mounted filesystem (though this can be relaxed by allowing the use of the allow_other and allow_root mount options in fuse.conf)

Building your own filesystem

FUSE comes with several example file systems in the examples directory. For example, the passthrough examples mirror the contents of the root directory under the mountpoint. Start from there and adapt the code!

The documentation of the API functions and necessary callbacks is mostly contained in the files include/fuse.h (for the high-level API) and include/fuse_lowlevel.h (for the low-level API). An autogenerated html version of the API is available in the doc/html directory and at http://libfuse.github.io/doxygen.

Getting Help

If you need help, please ask on the fuse-devel@lists.sourceforge.net mailing list (subscribe at https://lists.sourceforge.net/lists/listinfo/fuse-devel).

Please report any bugs on the GitHub issue tracker at https://github.com/libfuse/libfuse/issues.