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

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Message-ID: <20220119162423.eqbyefywhtzm22tr@wittgenstein>
Date:   Wed, 19 Jan 2022 17:24:23 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     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,
        legion@...nel.org, cyphar@...har.com
Subject: Re: [PATCH v2] proc: "mount -o lookup=" support

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.