Message-Id: <20250507-fuse-passthrough-doc-v1-2-cc06af79c722@uniontech.com>
Date: Wed, 07 May 2025 13:16:42 +0800
From: Chen Linxuan via B4 Relay <devnull+chenlinxuan.uniontech.com@...nel.org>
To: Miklos Szeredi <miklos@...redi.hu>, Jonathan Corbet <corbet@....net>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
 linux-doc@...r.kernel.org, Chen Linxuan <chenlinxuan@...ontech.com>, 
 Amir Goldstein <amir73il@...il.com>, 
 Bernd Schubert <bernd.schubert@...tmail.fm>
Subject: [PATCH 2/2] docs: filesystems: add fuse-passthrough.rst

From: Chen Linxuan <chenlinxuan@...ontech.com>

Add a documentation about FUSE passthrough.

It's mainly about why FUSE passthrough needs CAP_SYS_ADMIN.

Cc: Amir Goldstein <amir73il@...il.com>
Cc: Bernd Schubert <bernd.schubert@...tmail.fm>
Signed-off-by: Chen Linxuan <chenlinxuan@...ontech.com>
---
 Documentation/filesystems/fuse-passthrough.rst | 139 +++++++++++++++++++++++++
 1 file changed, 139 insertions(+)

diff --git a/Documentation/filesystems/fuse-passthrough.rst b/Documentation/filesystems/fuse-passthrough.rst
new file mode 100644
index 0000000000000000000000000000000000000000..f7c3b3ac08c255906ed7c909229107ff15cdb223
--- /dev/null
+++ b/Documentation/filesystems/fuse-passthrough.rst
@@ -0,0 +1,139 @@
+.. 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
+introduce 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. As noted in the
+kernel code (``fs/fuse/passthrough.c`` in ``fuse_backing_open()``):
+
+Discussions suggest that exposing information about these backing files, perhaps
+through a dedicated interface under ``/sys/fs/fuse/connections/``, could be a
+step towards relaxing this capability. This would be analogous to how
+``io_uring`` exposes its "fixed files", which are also 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. As Amir Goldstein mentioned in one of the discussions,
+there was "no proof that this is the only potential security risk" when the
+initial privilege checks were put in place.
+

-- 
2.43.0

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