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

[Aleksa Sarai]

On 2020-04-01, Aleksa Sarai <cyphar@xxxxxxxxxx> wrote:
> On 2020-03-31, Ignat Korchagin <ignat@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> 
> 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

(I forgot to add the kernel containers ML to Cc.)

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

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