[Virtio-fs] [PATCH for-5.1 2/3] virtiofsd: add container-friendly -o chroot sandboxing option

[Daniel P. Berrangé]

On Wed, Jul 22, 2020 at 02:02:05PM +0100, Stefan Hajnoczi wrote:
> virtiofsd cannot run in an unprivileged container because CAP_SYS_ADMIN
> is required to create namespaces.
> 
> Introduce a weaker sandbox that is sufficient in container environments
> because the container runtime already sets up namespaces. Use chroot to
> restrict path traversal to the shared directory.
> 
> virtiofsd loses the following:
> 
> 1. Mount namespace. The process chroots to the shared directory but
>    leaves the mounts in place. Seccomp rejects mount(2)/umount(2)
>    syscalls.
> 
> 2. Pid namespace. This should be fine because virtiofsd is the only
>    process running in the container.
> 
> 3. Network namespace. This should be fine because seccomp already
>    rejects the connect(2) syscall, but an additional layer of security
>    is lost. Container runtime-specific network security policies can be
>    used drop network traffic (except for the vhost-user UNIX domain
>    socket).

IIUC this relies on the fact that the container will still have 
CAP_SYS_CHROOT IOW, we still don't have a solution for running
virtiofsd as an unprivileged user.

Bearing in mind possibly need for future improvements to handle
unprivileged users....

> Signed-off-by: Stefan Hajnoczi <stefanha at redhat.com>
> ---
>  tools/virtiofsd/helper.c         |  3 +++
>  tools/virtiofsd/passthrough_ll.c | 44 ++++++++++++++++++++++++++++++--
>  2 files changed, 45 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/virtiofsd/helper.c b/tools/virtiofsd/helper.c
> index 3105b6c23a..7421c9ca1a 100644
> --- a/tools/virtiofsd/helper.c
> +++ b/tools/virtiofsd/helper.c
> @@ -151,6 +151,9 @@ void fuse_cmdline_help(void)
>             "    -o cache=<mode>            cache mode. could be one of \"auto, "
>             "always, none\"\n"
>             "                               default: auto\n"
> +           "    -o chroot|no_chroot        use container-friendly chroot instead\n"
> +           "                               of stronger mount namespace sandbox\n"
> +           "                               default: false\n"

A simple boolean like this feels like it lacks future-proofing.

Should we have an option that takes an enumerated arg string:

   -o sandbox=[namespace|chroot|....]

so we can add more options later with the same syntax.


Regards,
Daniel
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