Building domains as a lesser user (was Re: [Xen-devel] boot loaders for domain != 0)

[Anthony Liguori <anthony@xxxxxxxxxxxxx>]

> Then libext2 would have to run as a non-root user, and feed its output 
> to a root process doing the actual domain building, assuming that 
> there is no way of making the domain builder or libz choke on the 
> kernel image that is...
> For real security, all this stuff has to be happen within the domU. In 
> a perfect world, privileged code should never read user-supplied data, 
> but given that this world is not perfect, you could relax that to not 
> reading any variable-length user-supplied data.
I've been thinking about this and it seems to get worse and worse the 
more I think about it.  Pushing loading off to domU isn't much better 
because you still need to load a boot loader of some sort.  At what 
point do we then have to implement support for loading the boot loader 
from domU's device (in order to support exotic boot scenarios like 
booting from a CD, BOOTP, etc.).
As an alternative, I was trying to see if there was a way do create a 
domain as a non-root user.  Since root can set up the shared memory 
segments, it seems like the builder should be able to drop to a lesser 
user.  It could even enter a chroot() so that the only potential attack 
vector is a syscall exploit (which are rare and well-known enough that 
that seems to be acceptable).
That would kind of take some of the pressure off of the domain creator 
too.  Does this seem like something that's feasible?
Regards,
Anthony Liguori

> Given that both the (perhaps compressed) ELF image and the Ext2 
> filesystem contain variable-length data, neither should be read by 
> code in dom0.
> Jacob
>
>
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--
Anthony Liguori
anthony@xxxxxxxxxxxxx



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