linux-kernel - Re: [PATCH] proc/sysctl: make protected

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Date:   Thu, 9 Jul 2020 21:31:37 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Julius Hemanth Pitti <jpitti@...co.com>, mingo@...e.hu,
        akpm@...ux-foundation.org
Cc:     yzaikin@...gle.com, mcgrof@...nel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        viro@...iv.linux.org.uk, xe-linux-external@...co.com,
        jannh@...gle.com
Subject: Re: [PATCH] proc/sysctl: make protected_* world readable

On Thu, Jul 09, 2020 at 04:51:15PM -0700, Julius Hemanth Pitti wrote:
> protected_* files have 600 permissions which prevents
> non-superuser from reading them.
> 
> Container like "AWS greengrass" refuse to launch unless
> protected_hardlinks and protected_symlinks are set. When
> containers like these run with "userns-remap" or "--user"
> mapping container's root to non-superuser on host, they
> fail to run due to denied read access to these files.
> 
> As these protections are hardly a secret, and do not
> possess any security risk, making them world readable.
> 
> Though above greengrass usecase needs read access to
> only protected_hardlinks and protected_symlinks files,
> setting all other protected_* files to 644 to keep
> consistency.
> 
> Fixes: 800179c9b8a1 ("fs: add link restrictions")
> Signed-off-by: Julius Hemanth Pitti <jpitti@...co.com>

Acked-by: Kees Cook <keescook@...omium.org>

I had originally proposed it as 0644, but Ingo asked that it have
a more conservative default value[1]. I figured that given the settings
can be discovered easily, it's not worth much. And if there are legit
cases where things are improved, I don't have a problem switching this
back.

Ingo, any thoughts on this now, 8 years later in the age of containers?
:)

(One devil's advocate question: as a workaround, you are able to just
change those files to 0644 after mounting /proc, yes? But regardless,
why get in people's way for no justifiable reason.)

-Kees

[1] https://lore.kernel.org/lkml/20120105091704.GB3249@elte.hu/

-- 
Kees Cook