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Date:   Wed, 1 Apr 2020 17:38:06 +1100
From:   Aleksa Sarai <cyphar@cyphar.com>
To:     Ignat Korchagin <ignat@cloudflare.com>
Cc:     viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@cloudflare.com,
        containers@lists.linux-foundation.org, christian.brauner@ubuntu.com
Subject: Re: [PATCH v2 1/1] mnt: add support for non-rootfs initramfs
Message-ID: <20200401063806.5crx6pnm6vzuc3la@yavin.dot.cyphar.com>
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On 2020-04-01, Aleksa Sarai <cyphar@cyphar.com> wrote:
> On 2020-03-31, Ignat Korchagin <ignat@cloudflare.com> wrote:
> > The main need for this is to support container runtimes on stateless Li=
nux
> > system (pivot_root system call from initramfs).
> >=20
> > 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.
> >=20
> > This, however, breaks different container runtimes, because they usuall=
y 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 t=
he root
> > of the mount tree and can't be unmounted.
> >=20
> > One workaround is to do:
> >=20
> >   mount --bind / /
> >=20
> > However, that defeats one of the purposes of using pivot_root in the cl=
oned
> > containers: get rid of host root filesystem, should the code somehow es=
capes the
> > chroot.
> >=20
> > 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 t=
he outer
> >     layer creates a tmpfs filesystem and unpacks the inner layer, switc=
hes root
> >     and does not forget to properly cleanup the old rootfs
> >   * or we need to use keepinitrd kernel cmdline option, unpack initramf=
s to
> >     rootfs, run a script to create our target tmpfs root, unpack the sa=
me
> >     initramfs there, switch root to it and again properly cleanup the o=
ld root,
> >     thus unpacking the same archive twice and also wasting memory, beca=
use
> >     the kernel stores compressed initramfs image indefinitely.
> >=20
> > 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 t=
o 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 anythi=
ng.
> >=20
> > This also bring the behaviour in line with the older style initrd, wher=
e the
> > initrd is located on some leaf filesystem in the mount tree and rootfs =
remaining
> > empty.
> >=20
> > Signed-off-by: Ignat Korchagin <ignat@cloudflare.com>
>=20
> 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)?
>=20
> 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*).
>=20
> 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.
>=20
> Thanks.
>=20
> [1]: https://github.com/kubernetes/minikube/issues/3512

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

--=20
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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