linux-kernel - Re: NFS4 authentification / fsuid

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Message-Id: <1189142049.6681.29.camel@heimdal.trondhjem.org>
Date:	Fri, 07 Sep 2007 07:14:09 +0200
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Kyle Moffett <mrmacman_g4@....com>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	Satyam Sharma <satyam@...radead.org>,
	Jan Engelhardt <jengelh@...putergmbh.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: NFS4 authentification / fsuid

On Thu, 2007-09-06 at 20:56 -0400, Kyle Moffett wrote:
> On Sep 06, 2007, at 19:35:14, Trond Myklebust wrote:
> > On Thu, 2007-09-06 at 19:30 -0400, Kyle Moffett wrote:
> >> Actually, that's a fairly simple problem (barring disassembling  
> >> the system and attaching a hardware debugger).  You encrypt the  
> >> root filesystem and require a password to boot (See: LUKS).   
> >> Debian has built-in support for installing onto fs-on-LVM-on-crypt- 
> >> on-RAID, and it works quite well on all the laptops I use  
> >> regularly.  It's not even much of a speed penalty; once you take  
> >> the overhead of hitting a 5400RPM laptop drive you can chew  
> >> thousands of cycles of CPU without anybody noticing (much).  Then  
> >> all you have to do is burn a copy of your /boot with bootloader  
> >> onto some read-only media (like a finalized CDROM/DVDROM) and  
> >> you're set to go.
> >
> > Disconnect battery, and watch boot password go 'poof!'.
> 
> Umm, I did say "encrypt the root filesystem", didn't I?  Booting my  
> laptops this way follows this procedure:
>    1) Enter BIOS boot menu
>    2) Insert /boot CDROM
>    3) Select the "CDROM" entry
>    4) Wait for kernel to start and run through initramfs
>    5) Type password into the initramfs prompt so that it can DECRYPT  
> THE ROOT FILESYSTEM
>    6) Continue to boot the system.
> 
> Under this setup, tinkering with my BIOS does virtually nothing; the  
> only avenues of attack are strictly of the "Install a hardware  
> keylogger" variety.  Without my "boot" password you are looking at a  
> block device which appears to be little more than a random bit- 
> bucket, using AES-256 encryption.  If you can break that by  
> disconnecting the BIOS battery a lot of governments would be very  
> interested in the exact procedure. :-D  Furthermore if I think that  
> the hardware has been compromised I can pull out the HDD and my CDROM  
> and take them to a trusted computer to gain access to my data.
> 
> That said, a useful BIOS password helps keep somebody from casually  
> setting a supervisor password or mucking with the critical-to-boot  
> settings and making _me_ unplug the battery.
> 
> Cheers,
> Kyle Moffett

So an attacker will instead install a hardware keylogger, or swap out
your boot cdrom with a compromised but almost identical boot cdrom
instead, or mod your bios, ...

A fully self-certifying system that can prevent any attack is _very_
hard to achieve. Just ask apple (iPhone) or any games console vendor...

Trond

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