[RFC] AUFS disabled for 12.04

[Kees Cook]

Hi Gary,

On Thu, Mar 01, 2012 at 05:08:36PM -0500, Gary Poster wrote:
> aufs was reliable for us on Oneiric when creating ephemeral lxc
> instances based on an underlying template.  The most recent
> overlayfs issue that we discovered is today's
> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/944386
> 
> The summary is that, within an overlayfs, this fails:
> 
> gary at garubtosh:~/tmp$ touch 3
> gary at garubtosh:~/tmp$ chmod 0444 3
> gary at garubtosh:~/tmp$ ln 3 4
> ln: failed to create hard link `4' => `3': Operation not permitted
> 
> That error makes xvfb unable to start, in this particular case.
> 
> I'd feel a *lot* more comfortable if aufs were still around, in case
> we end up not finding the next overlayfs bug until it is too late
> for our project's delivery.

At the cost of some security, you can disable this specific restriction via
the sysctl knob:
 echo 0 > /proc/sys/kernel/yama/protected_nonaccess_hardlinks

After the failure, you can see the (unhelpful) report in dmesg:
[  354.737598] non-accessible hardlink creation was attempted by: ln (fsuid 1000)

However, I think the bug is actually with overlayfs. A similar problem was
seen with aufs, but Andy fixed those.

Based on the situation needed to reproduce it (non-writable), I think the
condition that is failing in the hardlink restriction logic is this line:
    cred->fsuid != inode->i_uid
But that makes no sense to me.

Andy, is unionfs lying about the i_uid in the filesystem?

-Kees

-- 
Kees Cook