[PATCH v2 0/3] virtiofsd: allow virtiofsd to run in a container

[Stefan Hajnoczi <stefanha@redhat.com>]

v2:
 * Update virtiofsd.rst documentation on sandboxing modes
 * Change syntax to -o sandbox=namespace|chroot
 * Add comment explaining that unshare(CLONE_FS) has no visible side-effect
   while single-threaded
 * xfstests and pjdfstest pass. Did not run tests on overlayfs because required
   xattrs do not work without CAP_SYS_ADMIN.

Mrunal and Dan: This patch series adds a sandboxing mode where virtiofsd relies
on the container runtime for isolation. It only does
chroot("path/to/shared-dir"), seccomp, and drops Linux capabilities. Previously
it created a new mount, pid, and net namespace but cannot do this without
CAP_SYS_ADMIN when run inside a container. pivot_root("path/to/shared-dir") has
been replaced with chroot("path/to/shared-dir"), again because CAP_SYS_ADMIN is
unavailable. The point of the chroot() is to prevent escapes from the shared
directory during path traversal. Does this ring any alarm bells or does it
sound sane?

Container runtimes handle namespace setup and remove privileges needed by
virtiofsd to perform sandboxing. Luckily the container environment already
provides most of the sandbox that virtiofsd needs for security.

Introduce a new "virtiofsd -o sandbox=chroot" option that uses chroot(2)
instead of namespaces. This option allows virtiofsd to work inside a container.

Please see the individual patches for details on the changes and security
implications.

Given that people are starting to attempt running virtiofsd in containers I
think this should go into QEMU 5.1.

Stefan Hajnoczi (3):
  virtiofsd: drop CAP_DAC_READ_SEARCH
  virtiofsd: add container-friendly -o sandbox=chroot option
  virtiofsd: probe unshare(CLONE_FS) and print an error

 tools/virtiofsd/fuse_virtio.c    | 16 +++++++++
 tools/virtiofsd/helper.c         |  8 +++++
 tools/virtiofsd/passthrough_ll.c | 58 ++++++++++++++++++++++++++++++--
 docs/tools/virtiofsd.rst         | 32 ++++++++++++++----
 4 files changed, 104 insertions(+), 10 deletions(-)

-- 
2.26.2