[PATCH v5 00/11] FUSE mounts from non-init user namespaces

[Eric W. Biederman]

Dongsu Park <dongsu at kinvolk.io> writes:

> This patchset v5 is based on work by Seth Forshee and Eric Biederman.
> The latest patchset was v4:
> https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1132206.html
>
> At the moment, filesystems backed by physical medium can only be mounted
> by real root in the initial user namespace. This restriction exists
> because if it's allowed for root user in non-init user namespaces to
> mount the filesystem, then it effectively allows the user to control the
> underlying source of the filesystem. In case of FUSE, the source would
> mean any underlying device.
>
> However, in many use cases such as containers, it's necessary to allow
> filesystems to be mounted from non-init user namespaces. Goal of this
> patchset is to allow FUSE filesystems to be mounted from non-init user
> namespaces. Support for other filesystems like ext4 are not in the
> scope of this patchset.
>
> Let me describe how to test mounting from non-init user namespaces. It's
> assumed that tests are done via sshfs, a userspace filesystem based on
> FUSE with ssh as backend. Testing system is Fedora 27.

In general I am for this work, and more bodies and more eyes on it is
generally better.

I will review this after the New Year, I am out for the holidays right
now.

Eric


>
> ====
> $ sudo dnf install -y sshfs
> $ sudo mkdir -p /mnt/userns
>
> ### workaround to get the sshfs permission checks
> $ sudo chown -R $UID:$UID /etc/ssh/ssh_config.d /usr/share/crypto-policies
>
> $ unshare -U -r -m
> # sshfs root at localhost: /mnt/userns
>
> ### You can see sshfs being mounted from a non-init user namespace
> # mount | grep sshfs
> root at localhost: on /mnt/userns type fuse.sshfs
> (rw,nosuid,nodev,relatime,user_id=0,group_id=0)
>
> # touch /mnt/userns/test
> # ls -l /mnt/userns/test
> -rw-r--r-- 1 root root 0 Dec 11 19:01 /mnt/userns/test
> ====
>
> Open another terminal, check the mountpoint from outside the namespace.
>
> ====
> $ grep userns /proc/$(pidof sshfs)/mountinfo
> 131 102 0:35 / /mnt/userns rw,nosuid,nodev,relatime - fuse.sshfs
> root at localhost: rw,user_id=0,group_id=0
> ====
>
> After all tests are done, you can unmount the filesystem
> inside the namespace.
>
> ====
> # fusermount -u /mnt/userns
> ====
>
> Changes since v4:
>  * Remove other parts like ext4 to keep the patchset minimal for FUSE
>  * Add and change commit messages
>  * Describe how to test non-init user namespaces
>
> TODO:
>  * Think through potential security implications. There are 2 patches
>    being prepared for security issues. One is "ima: define a new policy
>    option named force" by Mimi Zohar, which adds an option to specify
>    that the results should not be cached:
>    https://marc.info/?l=linux-integrity&m=151275680115856&w=2
>    The other one is to basically prevent FUSE results from being cached,
>    which is still in progress.
>
>  * Test IMA/LSMs. Details are written in
>    https://github.com/kinvolk/fuse-userns-patches/blob/master/tests/TESTING_INTEGRITY.md
>
> Patches 1-2 deal with an additional flag of lookup_bdev() to check for
> additional inode permission.
>
> Patches 3-7 allow the superblock owner to change ownership of inodes, and
> deal with additional capability checks w.r.t user namespaces.
>
> Patches 8-10 allow FUSE filesystems to be mounted outside of the init
> user namespace.
>
> Patch 11 handles a corner case of non-root users in EVM.
>
> The patchset is also available in our github repo:
>   https://github.com/kinvolk/linux/tree/dongsu/fuse-userns-v5-1
>
>
> Eric W. Biederman (1):
>   fs: Allow superblock owner to change ownership of inodes
>
> Seth Forshee (10):
>   block_dev: Support checking inode permissions in lookup_bdev()
>   mtd: Check permissions towards mtd block device inode when mounting
>   fs: Don't remove suid for CAP_FSETID for userns root
>   fs: Allow superblock owner to access do_remount_sb()
>   capabilities: Allow privileged user in s_user_ns to set security.*
>     xattrs
>   fs: Allow CAP_SYS_ADMIN in s_user_ns to freeze and thaw filesystems
>   fuse: Support fuse filesystems outside of init_user_ns
>   fuse: Restrict allow_other to the superblock's namespace or a
>     descendant
>   fuse: Allow user namespace mounts
>   evm: Don't update hmacs in user ns mounts
>
>  drivers/md/bcache/super.c           |  2 +-
>  drivers/md/dm-table.c               |  2 +-
>  drivers/mtd/mtdsuper.c              |  6 +++++-
>  fs/attr.c                           | 34 ++++++++++++++++++++++++++--------
>  fs/block_dev.c                      | 13 ++++++++++---
>  fs/fuse/cuse.c                      |  3 ++-
>  fs/fuse/dev.c                       | 11 ++++++++---
>  fs/fuse/dir.c                       | 16 ++++++++--------
>  fs/fuse/fuse_i.h                    |  6 +++++-
>  fs/fuse/inode.c                     | 35 +++++++++++++++++++++--------------
>  fs/inode.c                          |  6 ++++--
>  fs/ioctl.c                          |  4 ++--
>  fs/namespace.c                      |  4 ++--
>  fs/proc/base.c                      |  7 +++++++
>  fs/proc/generic.c                   |  7 +++++++
>  fs/proc/proc_sysctl.c               |  7 +++++++
>  fs/quota/quota.c                    |  2 +-
>  include/linux/fs.h                  |  2 +-
>  kernel/user_namespace.c             |  1 +
>  security/commoncap.c                |  8 ++++++--
>  security/integrity/evm/evm_crypto.c |  3 ++-
>  21 files changed, 127 insertions(+), 52 deletions(-)