linux-kernel - Re: Leaking Path in XFS's ioctl interface(missing LSM check)

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Date:   Tue, 2 Oct 2018 06:08:16 +1000 (AEST)
From:   James Morris <jmorris@...ei.org>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
cc:     Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Dave Chinner <david@...morbit.com>, TongZhang <ztong@...edu>,
        linux-xfs@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        linux-security-module@...r.kernel.org,
        Wenbo Shen <shenwenbosmile@...il.com>
Subject: Re: Leaking Path in XFS's ioctl interface(missing LSM check)

On Mon, 1 Oct 2018, Darrick J. Wong wrote:

> If we /did/ replace CAP_SYS_ADMIN checking with a pile of LSM hooks,

Not sure we'd need a pile of hooks, what about just "read" and "write" 
storage admin?

Or even two new capabilities along these lines, which we convert existing 
CAP_SYS_ADMIN etc. to?

> how do we make sure that we (XFS) avoid breaking existing XFS tools?  I 
> guess the default compatibility handler for all of those new hooks would 
> be "return capable(CAP_SYS_ADMIN) ? 0 : -EPERM;" and then other LSMs 
> could restrict that further if so configured.
> 
> Seriously, I don't know that much about how LSMs actually perform
> security checks -- it looks like a number of them can be active
> simultaneously, and we just call each of them in a chain until one of
> them denies permission or we run out of LSMs and allow it?
> 

Correct.

> FWIW I didn't have a particular security or threat model in mind when I
> made the above list; all I did was break up the existing CAP_SYS_ADMIN
> into rough functional areas.  Maybe it makes sense, but maybe I'm
> rambling. :)
> 
> --D
> 
> > You can build that model where for example only an administrative
> > login from a trusted console may launch processes to do that
> > management.
> > 
> > Or you could - if things were not going around the LSM hooks.
> > 
> > Alan
> 

-- 
James Morris
<jmorris@...ei.org>