linux-kernel - Re: [RFC] [PATCH] Add a "nolinks" mount option.

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Message-ID: <20161014150017.GB19539@ZenIV.linux.org.uk>
Date:   Fri, 14 Oct 2016 16:00:17 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Mattias Nissler <mnissler@...omium.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] [PATCH] Add a "nolinks" mount option.

On Fri, Oct 14, 2016 at 03:55:15PM +0100, Al Viro wrote:
> > Setting the "nolinks" mount option helps prevent privileged writers
> > from modifying files unintentionally in case there is an unexpected
> > link along the accessed path. The "nolinks" option is thus useful as a
> > defensive measure against persistent exploits (i.e. a system getting
> > re-exploited after a reboot) for systems that employ a read-only or
> > dm-verity-protected rootfs. These systems prevent non-legit binaries
> > from running after reboot. However, legit code typically still reads
> > from and writes to a writable file system previously under full
> > control of the attacker, who can place symlinks to trick file writes
> > after reboot to target a file of their choice. "nolinks" fundamentally
> > prevents this.
> 
> Which parts of the tree would be on that "protected" rootfs and which would
> you mount with that option?  Description above is rather vague and I'm
> not convinced that it actually buys you anything.  Details, please...

PS: what the hell do restrictions on _following_ symlinks have to _creating_
hardlinks?  I'm trying to imagine a threat model where both would apply or
anything else beyond the word "link" they would have in common...

The one you've described above might have something to do with the first
one (modulo missing description of the setup you have in mind), but it
clearly has nothing to do with the second - attackers could've created
whatever they wanted while the fs had been under their control, after all.
Doesn't make sense...