linux-kernel - Re: [PATCH v2] proc: "mount -o lookup=" support

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Date:   Wed, 19 Jan 2022 18:31:07 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Alexey Gladkov <legion@...nel.org>
Cc:     Alexey Dobriyan <adobriyan@...il.com>, viro@...iv.linux.org.uk,
        ebiederm@...ssion.com, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        stephen.s.brennan@...cle.com, cyphar@...har.com
Subject: Re: [PATCH v2] proc: "mount -o lookup=" support

On Wed, Jan 19, 2022 at 06:15:22PM +0100, Alexey Gladkov wrote:
> On Wed, Jan 19, 2022 at 05:24:23PM +0100, Christian Brauner wrote:
> > On Wed, Jan 19, 2022 at 06:48:03PM +0300, Alexey Dobriyan wrote:
> > > From 61376c85daab50afb343ce50b5a97e562bc1c8d3 Mon Sep 17 00:00:00 2001
> > > From: Alexey Dobriyan <adobriyan@...il.com>
> > > Date: Mon, 22 Nov 2021 20:41:06 +0300
> > > Subject: [PATCH 1/1] proc: "mount -o lookup=..." support
> > > 
> > > Docker implements MaskedPaths configuration option
> > > 
> > > 	https://github.com/estesp/docker/blob/9c15e82f19b0ad3c5fe8617a8ec2dddc6639f40a/oci/defaults.go#L97
> > > 
> > > to disable certain /proc files. It overmounts them with /dev/null.
> > > 
> > > Implement proper mount option which selectively disables lookup/readdir
> > > in the top level /proc directory so that MaskedPaths doesn't need
> > > to be updated as time goes on.
> > 
> > I might've missed this when this was sent the last time so maybe it was
> > clearly explained in an earlier thread: What's the reason this needs to
> > live in the kernel?
> > 
> > The MaskedPaths entry is optional so runtimes aren't required to block
> > anything by default and this mostly makes sense for workloads that run
> > privileged.
> > 
> > In addition MaskedPaths is a generic option which allows to hide any
> > existing path, not just proc. Even in the very docker-specific defaults
> > /sys/firmware is covered.
> > 
> > I do see clear value in the subset= and hidepid= options. They are
> > generally useful independent of opinionated container workloads. I don't
> > see the same for lookup=.
> > 
> > An alternative I find more sensible is to add a new value for subset=
> > that hides anything(?) that only global root should have read/write
> > access too.
> 
> Or we can allow to change permissions in the procfs only in the direction
> of decreasing (if some file has 644 then allow to set 640 or 600). In this
> case, we will not need to constantly check the whitelist.

I don't fancy any filtering or allowlist approach. I find that rather
inelegant. But if I understand you correctly is that if we were to have
decreasing permissions we could allow a (namespace) procfs-admin to set
permissions so that the relevant files are essentially read-only or not
even readable at all for container workloads. So once you've lowered
perms you can't raise them which ensures even namespace procfs-admin
can't raise them again.
Might work as well. But that implies that we wouldn't need any allowlist
at all afaict.