docs: filesystems: add fuse-passthrough.rst

Add a documentation about FUSE passthrough.

It's mainly about why FUSE passthrough needs CAP_SYS_ADMIN.

Link: https://lore.kernel.org/all/4b64a41c-6167-4c02-8bae-3021270ca519@fastmail.fm/T/#mc73e04df56b8830b1d7b06b5d9f22e594fba423e
Link: https://lore.kernel.org/linux-fsdevel/CAOQ4uxhAY1m7ubJ3p-A3rSufw_53WuDRMT1Zqe_OC0bP_Fb3Zw@mail.gmail.com/
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Chen Linxuan <chenlinxuan@uniontech.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Documentation/filesystems/fuse-passthrough.rst

@@ -0,0 +1,133 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+================
+FUSE Passthrough
+================
+
+Introduction
+============
+
+FUSE (Filesystem in Userspace) passthrough is a feature designed to improve the
+performance of FUSE filesystems for I/O operations. Typically, FUSE operations
+involve communication between the kernel and a userspace FUSE daemon, which can
+incur overhead. Passthrough allows certain operations on a FUSE file to bypass
+the userspace daemon and be executed directly by the kernel on an underlying
+"backing file".
+
+This is achieved by the FUSE daemon registering a file descriptor (pointing to
+the backing file on a lower filesystem) with the FUSE kernel module. The kernel
+then receives an identifier (``backing_id``) for this registered backing file.
+When a FUSE file is subsequently opened, the FUSE daemon can, in its response to
+the ``OPEN`` request, include this ``backing_id`` and set the
+``FOPEN_PASSTHROUGH`` flag. This establishes a direct link for specific
+operations.
+
+Currently, passthrough is supported for operations like ``read(2)``/``write(2)``
+(via ``read_iter``/``write_iter``), ``splice(2)``, and ``mmap(2)``.
+
+Enabling Passthrough
+====================
+
+To use FUSE passthrough:
+
+  1. The FUSE filesystem must be compiled with ``CONFIG_FUSE_PASSTHROUGH``
+     enabled.
+  2. The FUSE daemon, during the ``FUSE_INIT`` handshake, must negotiate the
+     ``FUSE_PASSTHROUGH`` capability and specify its desired
+     ``max_stack_depth``.
+  3. The (privileged) FUSE daemon uses the ``FUSE_DEV_IOC_BACKING_OPEN`` ioctl
+     on its connection file descriptor (e.g., ``/dev/fuse``) to register a
+     backing file descriptor and obtain a ``backing_id``.
+  4. When handling an ``OPEN`` or ``CREATE`` request for a FUSE file, the daemon
+     replies with the ``FOPEN_PASSTHROUGH`` flag set in
+     ``fuse_open_out::open_flags`` and provides the corresponding ``backing_id``
+     in ``fuse_open_out::backing_id``.
+  5. The FUSE daemon should eventually call ``FUSE_DEV_IOC_BACKING_CLOSE`` with
+     the ``backing_id`` to release the kernel's reference to the backing file
+     when it's no longer needed for passthrough setups.
+
+Privilege Requirements
+======================
+
+Setting up passthrough functionality currently requires the FUSE daemon to
+possess the ``CAP_SYS_ADMIN`` capability. This requirement stems from several
+security and resource management considerations that are actively being
+discussed and worked on. The primary reasons for this restriction are detailed
+below.
+
+Resource Accounting and Visibility
+----------------------------------
+
+The core mechanism for passthrough involves the FUSE daemon opening a file
+descriptor to a backing file and registering it with the FUSE kernel module via
+the ``FUSE_DEV_IOC_BACKING_OPEN`` ioctl. This ioctl returns a ``backing_id``
+associated with a kernel-internal ``struct fuse_backing`` object, which holds a
+reference to the backing ``struct file``.
+
+A significant concern arises because the FUSE daemon can close its own file
+descriptor to the backing file after registration. The kernel, however, will
+still hold a reference to the ``struct file`` via the ``struct fuse_backing``
+object as long as it's associated with a ``backing_id`` (or subsequently, with
+an open FUSE file in passthrough mode).
+
+This behavior leads to two main issues for unprivileged FUSE daemons:
+
+  1. **Invisibility to lsof and other inspection tools**: Once the FUSE
+     daemon closes its file descriptor, the open backing file held by the kernel
+     becomes "hidden." Standard tools like ``lsof``, which typically inspect
+     process file descriptor tables, would not be able to identify that this
+     file is still open by the system on behalf of the FUSE filesystem. This
+     makes it difficult for system administrators to track resource usage or
+     debug issues related to open files (e.g., preventing unmounts).
+
+  2. **Bypassing RLIMIT_NOFILE**: The FUSE daemon process is subject to
+     resource limits, including the maximum number of open file descriptors
+     (``RLIMIT_NOFILE``). If an unprivileged daemon could register backing files
+     and then close its own FDs, it could potentially cause the kernel to hold
+     an unlimited number of open ``struct file`` references without these being
+     accounted against the daemon's ``RLIMIT_NOFILE``. This could lead to a
+     denial-of-service (DoS) by exhausting system-wide file resources.
+
+The ``CAP_SYS_ADMIN`` requirement acts as a safeguard against these issues,
+restricting this powerful capability to trusted processes.
+
+**NOTE**: ``io_uring`` solves this similar issue by exposing its "fixed files",
+which are visible via ``fdinfo`` and accounted under the registering user's
+``RLIMIT_NOFILE``.
+
+Filesystem Stacking and Shutdown Loops
+--------------------------------------
+
+Another concern relates to the potential for creating complex and problematic
+filesystem stacking scenarios if unprivileged users could set up passthrough.
+A FUSE passthrough filesystem might use a backing file that resides:
+
+  * On the *same* FUSE filesystem.
+  * On another filesystem (like OverlayFS) which itself might have an upper or
+    lower layer that is a FUSE filesystem.
+
+These configurations could create dependency loops, particularly during
+filesystem shutdown or unmount sequences, leading to deadlocks or system
+instability. This is conceptually similar to the risks associated with the
+``LOOP_SET_FD`` ioctl, which also requires ``CAP_SYS_ADMIN``.
+
+To mitigate this, FUSE passthrough already incorporates checks based on
+filesystem stacking depth (``sb->s_stack_depth`` and ``fc->max_stack_depth``).
+For example, during the ``FUSE_INIT`` handshake, the FUSE daemon can negotiate
+the ``max_stack_depth`` it supports. When a backing file is registered via
+``FUSE_DEV_IOC_BACKING_OPEN``, the kernel checks if the backing file's
+filesystem stack depth is within the allowed limit.
+
+The ``CAP_SYS_ADMIN`` requirement provides an additional layer of security,
+ensuring that only privileged users can create these potentially complex
+stacking arrangements.
+
+General Security Posture
+------------------------
+
+As a general principle for new kernel features that allow userspace to instruct
+the kernel to perform direct operations on its behalf based on user-provided
+file descriptors, starting with a higher privilege requirement (like
+``CAP_SYS_ADMIN``) is a conservative and common security practice. This allows
+the feature to be used and tested while further security implications are
+evaluated and addressed.

Documentation/filesystems/index.rst

@@ -99,6 +99,7 @@ Documentation for filesystem implementations.
    fuse
    fuse-io
    fuse-io-uring
+   fuse-passthrough
    inotify
    isofs
    nilfs2