Subject: Re: [vserver] Copy-on-write Hard Links, Shared Libraries, Prelink and Memory
From: Gordan Bobic <gordan@bobich.net>
Date: Thu, 10 Jun 2010 21:13:56 +0100

On 06/10/2010 08:50 PM, Herbert Poetzl wrote:

>> Essentially - if it is not safe to do this between the host and
>> a guest, how come it is safe to do between guests?
>
> because the host (context) has all priviledges and
> can manipulate all the guests (and usually their filesystem
> without any restriction)
>
> so, the danger is not that the guest will modify a host
> binary and use that for some kind of exploit, the danger
> is more that you accidentially drop the required security
> mechanisms while accessing those files (from the host)
> and the guest could exploit this to mess with the host
> binaries ... or it could simply mess up the guest by
> involuntarily changing guest files (along with host changes)

What exactly are you referring to with "security mechanisms" in this 
context?

> the guests are all limited in their capabilities and will
> not be able to do such things, but the host context is not
> limited at all, i.e. everything goes :)

I get that, I'm just curious how sharing a hard-link between host and 
guest could be used to compromise the host. At the moment, I cannot 
quite see the attack vector.

> besides that, having a few hundred megabytes of host
> files/binaries are usually acceptable ...

A few hundred MB of disk space isn't a big issue. A few hundred MB of 
RAM, however, is - I'm trying to implement something on a very low power 
machine (N450 Atom, has to be passively cooled) which is limited to 2GB 
of RAM, and I need to deploy about 3-4VMs in it. Hence why I am trying 
to scrape a bit more off the bottom of the barrel. :)

Gordan