linux-kernel - Re: [PATCH] Allow restricting permissions in /proc/sys

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Date:   Tue, 12 Nov 2019 15:19:00 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Topi Miettinen <toiwoton@...il.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:FILESYSTEMS (VFS and infrastructure)" 
        <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] Allow restricting permissions in /proc/sys

On Tue, Nov 05, 2019 at 09:35:46AM +0200, Topi Miettinen wrote:
> On 5.11.2019 1.41, Eric W. Biederman wrote:
> > My sense is that if there is any kind of compelling reason to make
> > world-readable values not world-readable, and it doesn't break anything
> > (except malicious applications) than a kernel patch is probably the way
> > to go.
> 
> With kernel patch, do you propose to change individual sysctls to not
> world-readable? That surely would help everybody instead of just those who
> care enough to change /proc/sys permissions. I guess it would also be more
> effort by an order of magnitude or two to convince each owner of a sysctl to
> accept the change.

I would think of this as a two-stage process: provide a mechanism to
tighten permissions arbitrarily so that it is easier to gather evidence
about which could have their default changed in the future.

> These code paths have not changed much or at all since the initial version
> in 2007, so I suppose the maintenance burden has not been overwhelming.
> 
> By the way, /proc/sys still allows changing the {a,c,m}time. I think those
> are not backed anywhere, so they probably suffer from same caching problems
> as my first version of the patch.

Is a v2 of this patch needed? It wasn't clear to me if the inode modes
were incorrectly cached...?

-- 
Kees Cook