linux-kernel - Re: NFS4 authentification / fsuid

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Date:	Thu, 6 Sep 2007 13:59:50 +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, 6 Sep 2007, Satyam Sharma wrote:
> 
> On Thu, 30 Aug 2007, J. Bruce Fields wrote:
> 
> > On Thu, Aug 30, 2007 at 11:04:00AM -0400, Trond Myklebust wrote:
> > 
> > > 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
    ^^^^^^^^^^^^^^

Oh and btw, note that we're talking of the (lack of) security of a
"running kernel" here -- because across reboots, there is /really/
*absolutely* no such thing as "kernelspace security" because the superuser
will simply switch the vmlinuz itself ...


> 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 ;-)
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