Re: [PATCH v2 1/1] mnt: add support for non-rootfs initramfs

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2020-03-31, Ignat Korchagin <ignat@cloudflare.com> wrote:
> The main need for this is to support container runtimes on stateless Linux
> system (pivot_root system call from initramfs).
> 
> Normally, the task of initramfs is to mount and switch to a "real" root
> filesystem. However, on stateless systems (booting over the network) it is just
> convenient to have your "real" filesystem as initramfs from the start.
> 
> This, however, breaks different container runtimes, because they usually use
> pivot_root system call after creating their mount namespace. But pivot_root does
> not work from initramfs, because initramfs runs form rootfs, which is the root
> of the mount tree and can't be unmounted.
> 
> One workaround is to do:
> 
>   mount --bind / /
> 
> However, that defeats one of the purposes of using pivot_root in the cloned
> containers: get rid of host root filesystem, should the code somehow escapes the
> chroot.
> 
> There is a way to solve this problem from userspace, but it is much more
> cumbersome:
>   * either have to create a multilayered archive for initramfs, where the outer
>     layer creates a tmpfs filesystem and unpacks the inner layer, switches root
>     and does not forget to properly cleanup the old rootfs
>   * or we need to use keepinitrd kernel cmdline option, unpack initramfs to
>     rootfs, run a script to create our target tmpfs root, unpack the same
>     initramfs there, switch root to it and again properly cleanup the old root,
>     thus unpacking the same archive twice and also wasting memory, because
>     the kernel stores compressed initramfs image indefinitely.
> 
> With this change we can ask the kernel (by specifying nonroot_initramfs kernel
> cmdline option) to create a "leaf" tmpfs mount for us and switch root to it
> before the initramfs handling code, so initramfs gets unpacked directly into
> the "leaf" tmpfs with rootfs being empty and no need to clean up anything.
> 
> This also bring the behaviour in line with the older style initrd, where the
> initrd is located on some leaf filesystem in the mount tree and rootfs remaining
> empty.
> 
> Signed-off-by: Ignat Korchagin <ignat@cloudflare.com>

I know this is a bit of a stretch, but I thought I'd ask -- is it
possible to solve the problem with pivot_root(2) without requiring this
workaround (and an additional cmdline option)?

From the container runtime side of things, most runtimes do support
working on initramfs but it requires disabling pivot_root(2) support (in
the runc world this is --no-pivot-root). We would love to be able to
remove support for disabling pivot_root(2) because lots of projects have
been shipping with pivot_root(2) disabled (such as minikube until
recently[1]) -- which opens such systems to quite a few breakout and
other troubling exploits (obviously they also ship without using user
namespaces *sigh*).

But requiring a new cmdline option might dissuade people from switching.
If there was a way to fix the underlying restriction on pivot_root(2),
I'd be much happier with that as a solution.

Thanks.

[1]: https://github.com/kubernetes/minikube/issues/3512

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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