libfuse

Warning: unresolved security issue

Be aware that FUSE has an unresolved security bug
(bug #15): the
permission check for accessing a cached directory is only done once
when the directory entry is first loaded into the cache. Subsequent
accesses will re-use the results of the first check, even if the
directory 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 completely disable caching of directory
entries. 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

./configure
make -j8
make install

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).

For more details see the file INSTALL

Security implications

If you run make install, the fusermount 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 fusexmp example mirrors 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/main/issues.

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