On Wed, May 7, 2025 at 7:17 AM Chen Linxuan via B4 Relay <devnull+chenlinxuan.uniontech.com@xxxxxxxxxx> wrote: > > From: Chen Linxuan <chenlinxuan@xxxxxxxxxxxxx> > > Add a documentation about FUSE passthrough. > > It's mainly about why FUSE passthrough needs CAP_SYS_ADMIN. > Hi Chen, Thank you for this contribution! Very good summary. with minor nits below fix you may add to both patches: Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx> > Cc: Amir Goldstein <amir73il@xxxxxxxxx> > Cc: Bernd Schubert <bernd.schubert@xxxxxxxxxxx> > Signed-off-by: Chen Linxuan <chenlinxuan@xxxxxxxxxxxxx> > --- > 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()``): As Bagas commented, I don't see the need to reference comments in the code here. > + > +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 I am not sure this is helpful to have this "maybe this is how we will solve it" documented here. the idea was to document the concerns and the reasons for CAP_SYS_ADMIN. Now that you documented them, you can work on the solution and document the solution here. > +``io_uring`` exposes its "fixed files", which are also visible via ``fdinfo`` > +and accounted under the registering user's ``RLIMIT_NOFILE``. If you want, you can leave this as a NOTE about how io_uring solves a similar issue. > + > +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. > + I don't think that referencing those discussions is useful. They are too messy. The idea of the doc is clarity. It's fine to have Link: in the commit message tail for git history sake. You could instead write that a documented security model is needed before CAP_SYS_ADMIN can be relaxed. or add nothing at all, because you already documented the concerns. Thanks, Amir.
