linux-kernel - Re: [Xen-devel] [PATCH] Solved the Xen PV/KASLR riddle

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Date:	Fri, 29 Aug 2014 10:55:58 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	Stefan Bader <stefan.bader@...onical.com>,
	Kees Cook <keescook@...omium.org>,
	David Vrabel <david.vrabel@...rix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] Solved the Xen PV/KASLR riddle

On Fri, Aug 29, 2014 at 03:44:06PM +0100, Jan Beulich wrote:
> >>> On 29.08.14 at 16:27, <stefan.bader@...onical.com> wrote:
> > Sure. Btw, someone also contacted me saying they have the same problem 
> > without
> > changing the layout but having really big initrd (500M). While that feels 
> > like
> > it should be impossible (if the kernel+initrd+xen stuff has to fix the 512M
> > kernel image size area then). But if it can happen, then surely it does 
> > cause
> > mappings to be where the module space starts then.
> 
> Since the initrd doesn't really need to be mapped into the (limited)
> virtual address space a pv guest starts with, we specifically got
> 
> /*
>  * Whether or not the guest can deal with being passed an initrd not
>  * mapped through its initial page tables.
>  */
> #define XEN_ELFNOTE_MOD_START_PFN 16
> 
> to deal with that situation. The hypervisor side for Dom0 is in place,
> and the kernel side works in our (classic) kernels. Whether it got
> implemented for DomU meanwhile I don't know; I'm pretty certain
> pv-ops kernels don't support it so far.

Correct - Not implemented. Here is what I had mentioned in the past:
(see http://lists.xen.org/archives/html/xen-devel/2014-03/msg00580.html)


XEN_ELFNOTE_INIT_P2M, XEN_ELFNOTE_MOD_START_PFN - I had been looking
    at that but I can't figure out a nice way of implementing this
    without the usage of SPARSEMAP_VMAP virtual addresses - which is how
    the classic Xen does it. But then - I don't know who is using huge PV
    guests - as the PVHVM does a fine job? But then with PVH, now you can
    boot with large amount of memory (1TB?) - so some of these issues
    would go away? Except the 'large ramdisk' as that would eat in the
    MODULES_VADDR I think? Needs more thinking.

.. and then I left it and to my suprise saw on Luis's slides that
Jurgen is going to take a look at that (500GB support).

> 
> Jan
> 
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