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

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Date:   Wed, 29 Jul 2020 00:03:32 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Julius Hemanth Pitti <jpitti@...co.com>, mingo@...e.hu,
        akpm@...ux-foundation.org, yzaikin@...gle.com,
        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 09:31:37PM -0700, Kees Cook wrote:
> 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.

If we're going to to do this, can we please document why these are
"protected" then?

  Luis

> 
> 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