From: Chen Linxuan <chenlinxuan@uniontech.com>

Add a documentation about FUSE passthrough.

It's mainly about why FUSE passthrough needs CAP_SYS_ADMIN.

Some related discussions:

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/

Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Bernd Schubert <bernd.schubert@fastmail.fm>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Chen Linxuan <chenlinxuan@uniontech.com>
---
 Documentation/filesystems/fuse-passthrough.rst | 133 +++++++++++++++++++++++++
 Documentation/filesystems/index.rst            |   1 +
 2 files changed, 134 insertions(+)

diff --git a/Documentation/filesystems/fuse-passthrough.rst b/Documentation/filesystems/fuse-passthrough.rst
new file mode 100644
index 0000000000000000000000000000000000000000..2b0e7c2da54acde4d48fd91ecece27256c4e04fd
--- /dev/null
+++ b/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.
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index a9cf8e950b15ad68a021d5f214b07f58d752f4e3..2913f4f2e00ccc466563aba5692e2f95699cb674 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -99,6 +99,7 @@ Documentation for filesystem implementations.
    fuse
    fuse-io
    fuse-io-uring
+   fuse-passthrough
    inotify
    isofs
    nilfs2

-- 
2.43.0

On Wed, May 7, 2025 at 10:42 AM Chen Linxuan via B4 Relay
<devnull+chenlinxuan.uniontech.com@kernel.org> wrote:
>
> From: Chen Linxuan <chenlinxuan@uniontech.com>
>
> Add a documentation about FUSE passthrough.
>
> It's mainly about why FUSE passthrough needs CAP_SYS_ADMIN.
>
> Some related discussions:
>
> 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/
>

For future reference, those links are usually part of the tail part
(without newline)
but don't worry about it - this can be fixed when applying the patch.

Thanks,
Amir.

> Cc: Amir Goldstein <amir73il@gmail.com>
> Cc: Bernd Schubert <bernd.schubert@fastmail.fm>
> Reviewed-by: Amir Goldstein <amir73il@gmail.com>
> Signed-off-by: Chen Linxuan <chenlinxuan@uniontech.com>
> ---
>  Documentation/filesystems/fuse-passthrough.rst | 133 +++++++++++++++++++++++++
>  Documentation/filesystems/index.rst            |   1 +
>  2 files changed, 134 insertions(+)
>
> diff --git a/Documentation/filesystems/fuse-passthrough.rst b/Documentation/filesystems/fuse-passthrough.rst
> new file mode 100644
> index 0000000000000000000000000000000000000000..2b0e7c2da54acde4d48fd91ecece27256c4e04fd
> --- /dev/null
> +++ b/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.
> diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
> index a9cf8e950b15ad68a021d5f214b07f58d752f4e3..2913f4f2e00ccc466563aba5692e2f95699cb674 100644
> --- a/Documentation/filesystems/index.rst
> +++ b/Documentation/filesystems/index.rst
> @@ -99,6 +99,7 @@ Documentation for filesystem implementations.
>     fuse
>     fuse-io
>     fuse-io-uring
> +   fuse-passthrough
>     inotify
>     isofs
>     nilfs2
>
> --
> 2.43.0
>
>