linux-kernel - Re: NFS4 authentification / fsuid

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Date:	Thu, 6 Sep 2007 13:44:05 +0530 (IST)
From:	Satyam Sharma <satyam@...radead.org>
To:	"J. Bruce Fields" <bfields@...ldses.org>
cc:	Trond Myklebust <trond.myklebust@....uio.no>,
	Jan Engelhardt <jengelh@...putergmbh.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: NFS4 authentification / fsuid



On Thu, 30 Aug 2007, J. Bruce Fields wrote:

> On Thu, Aug 30, 2007 at 11:04:00AM -0400, Trond Myklebust wrote:
> > With CIFS or other password based protocols (including RPCSEC_GSS)
> 
> Well, rpcsec_gss isn't inherently password based, and you can
> authenticate in some way that doesn't actually give away your password
> (or other long-lived credential).
> 
> > What I'm saying is that the superuser can pretty much do whatever it
> > takes to grab either your kerberos password (e.g. install a keyboard
> > listener), a stored credential (read the contents of your kerberos
> > on-disk credential cache), or s/he can access the cached contents of the
> > file by hunting through /dev/kmem.
> > 
> > IOW: There is no such thing as security on a root-compromised machine.
> 
> And in theory a kernel could provide *some* guarantees against root,
> right?  (Is there some reason a unix-like kernel must provide such
> things as /dev/kmem?)

/dev/kmem was just an example -- IMHO differentiating between kernel and
userspace from a security p.o.v. is always tricky. Like Trond said, there
are very high number of ways in which privileged userspace can compromise
a running kernel if it really wants to do that, root-is-God has always
been *the* major problem with Unix :-)

The only _real_ way a kernel can lock itself completely against malicious
userspace involves trusted tamperproof hardware, but even that only if
you can get yourself to believe such a thing exists in the first place ;-)


Satyam
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