Re: [Xen-users] pygrub vs. pv-grub

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

At 11:48 -0400 7/5/12, Chris Dickson wrote:
> Hello all, I've been using pygrub successfully as my bootloader but 
> I recently ran across this and I'm wondering if anyone has any 
> insight:
> <http://wiki.xensource.com/xenwiki/PvGrub>http://wiki.xensource.com/xenwiki/PvGrub
See the thread titled "Where does PyGrub run?" from teh archives for 
last month.
> This says that pv-grub is a replacement for pygrub that loads the 
> kernel and initrd from within the domU safely. As far as I knew, 
> pygrub does this as well in my recent Xen 4.1 installation, however 
> I know that pygrub has to read the kernel and initrd out of the domU 
> for a moment to boot it.
Indeed, you have hit the nail on the head.
PyGrub copies the DomU kernel and initrd from the DomU filesystem 
image to Dom0 and then creates a new domain using that kernel. This 
means that PyGrub manipulates the DomU filesystem and files from 
within Dom0 which is a potential security issue if someone can find a 
flaw in the code and craft (for example) a malicious filesystem or 
menu.lst.
PvGrub executes within the newly created DomU environment.

If you read through the previous thread you'll see that it's possible 
to setup guests with a read-only recovery partition so that it's not 
possible for a user to make their VM unbootable with PvGrub.
>  My Xen 4.1 installation does not seem to come with any pv-grub gzip 
> files as are shown in the documentation. Has pygrub since replaced 
> pv-grub, or is pv-grub still in existence and if so what is the 
> difference?
AFAIK both are still current programs. However I vaguely recall there 
being some licensing issue that means PvGrub is not included in some 
distros (Debian being one).
Ah, now I look it up I see it's probably more a case of "not been 
packaged yet" for Debian :
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=588839
http://xen.1045712.n5.nabble.com/pv-grub-removed-td3046506.html

--
Simon Hobson

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