linux-kernel - Re: [PATCH] [RFC][WIP] namespace.c: Allow some unprivileged proc mounts when not fully visible

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Date:   Thu, 5 Apr 2018 01:34:22 +1000
From:   Aleksa Sarai <asarai@...e.de>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Alban Crequy <alban.crequy@...il.com>,
        Alban Crequy <alban@...volk.io>,
        Dongsu Park <dongsu@...volk.io>,
        Iago Lopez Galeiras <iago@...volk.io>,
        Stephen J Day <stephen.day@...ker.com>,
        Michael Crosby <crosbymichael@...il.com>,
        Jess Frazelle <acidburn@...rosoft.com>,
        Akihiro Suda <suda.akihiro@....ntt.co.jp>,
        Daniel J Walsh <dwalsh@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        containers@...ts.linux-foundation.org
Subject: Re: [PATCH] [RFC][WIP] namespace.c: Allow some unprivileged proc
 mounts when not fully visible

On 2018-04-04, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> > The following commands show my problem:
> >
> > $ sudo docker run -ti --rm --cap-add=SYS_ADMIN busybox sh -c 'unshare -U -r -p -m -f mount -t proc proc /home && echo ok'
> > mount: permission denied (are you root?)
> >
> > $ sudo docker run -ti --rm --cap-add=SYS_ADMIN busybox sh -c 'mkdir -p /unmasked-proc && mount -t proc proc /unmasked-proc && unshare -U -r -p -m -f mount -t proc proc /home && echo ok'
> > ok
> 
> Actually this does not show your problem because it does not reveal why
> you need to mount proc.
> 
> That is a ``Doctor it hurts when I do this'' example where the Doctor
> will reasonably tell you ``Don't do that then''.

The context is that people want to run unprivileged runc inside a Docker
container, and mounting proc is part of setting up a container. But
Docker (and runc) have masks for /proc to stop containers from being
able to touch things like /proc/scsi and so on. The other possibility is
to give people an escape-hatch when setting up a container which
basically says "make this container slightly less secure so that I can
run containers inside it".

However I share your concern about the layer mixing with inheriting the
masks for the new procfs mount (what if you have a mount over a
particular process, now the mask is masking something completely
different, and a bunch of other possible problems).

> > For my use case, I will need to support at least the following entries:
> >
> > $ sudo docker run -ti --rm busybox sh -c 'mount|grep /proc/'
> > proc on /proc/asound type proc (ro,nosuid,nodev,noexec,relatime)
> > proc on /proc/bus type proc (ro,nosuid,nodev,noexec,relatime)
> > proc on /proc/fs type proc (ro,nosuid,nodev,noexec,relatime)
> > proc on /proc/irq type proc (ro,nosuid,nodev,noexec,relatime)
> > proc on /proc/sys type proc (ro,nosuid,nodev,noexec,relatime)
> > proc on /proc/sysrq-trigger type proc (ro,nosuid,nodev,noexec,relatime)
> > tmpfs on /proc/kcore type tmpfs (rw,context="...",nosuid,mode=755)
> > tmpfs on /proc/latency_stats type tmpfs (rw,context="...",nosuid,mode=755)
> > tmpfs on /proc/timer_list type tmpfs (rw,context="...",nosuid,mode=755)
> > tmpfs on /proc/sched_debug type tmpfs (rw,context="...",nosuid,mode=755)
> > tmpfs on /proc/scsi type tmpfs (ro,seclabel,relatime)
> 
> It looks like a cruft free cousin of proc that is just processes would
> be applicable to your usecase.

I think a procfs that only has processes would be a massive improvement
for a bunch of other reasons too. :D

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

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