Re: [Xen-devel] Xen PV domain regression with KASLR enabled (kernel 3.16)

Konrad Rzeszutek Wilk
2014-08-12T19:07:40Z

On Tue, Aug 12, 2014 at 11:53:03AM -0700, Kees Cook wrote:
> On Tue, Aug 12, 2014 at 11:05 AM, Stefan Bader
> <stefan.bader@canonical.com> wrote:
> > On 12.08.2014 19:28, Kees Cook wrote:
> >> On Fri, Aug 8, 2014 at 7:35 AM, Stefan Bader <stefan.bader@canonical.com> wrote:
> >>> On 08.08.2014 14:43, David Vrabel wrote:
> >>>> On 08/08/14 12:20, Stefan Bader wrote:
> >>>>> Unfortunately I have not yet figured out why this happens, but can confirm by
> >>>>> compiling with or without CONFIG_RANDOMIZE_BASE being set that without KASLR all
> >>>>> is ok, but with it enabled there are issues (actually a dom0 does not even boot
> >>>>> as a follow up error).
> >>>>>
> >>>>> Details can be seen in [1] but basically this is always some portion of a
> >>>>> vmalloc allocation failing after hitting a freshly allocated PTE space not being
> >>>>> PTE_NONE (usually from a module load triggered by systemd-udevd). In the
> >>>>> non-dom0 case this repeats many times but ends in a guest that allows login. In
> >>>>> the dom0 case there is a more fatal error at some point causing a crash.
> >>>>>
> >>>>> I have not tried this for a normal PV guest but for dom0 it also does not help
> >>>>> to add "nokaslr" to the kernel command-line.
> >>>>
> >>>> Maybe it's overlapping with regions of the virtual address space
> >>>> reserved for Xen? What the the VA that fails?
> >>>>
> >>>> David
> >>>>
> >>> Yeah, there is some code to avoid some regions of memory (like initrd). Maybe
> >>> missing p2m tables? I probably need to add debugging to find the failing VA (iow
> >>> not sure whether it might be somewhere in the stacktraces in the report).
> >>>
> >>> The kernel-command line does not seem to be looked at. It should put something
> >>> into dmesg and that never shows up. Also today's random feature is other PV
> >>> guests crashing after a bit somewhere in the check_for_corruption area...
> >>
> >> Right now, the kaslr code just deals with initrd, cmdline, etc. If
> >> there are other reserved regions that aren't listed in the e820, it'll
> >> need to locate and skip them.
> >>
> >> -Kees
> >>
> > Making my little steps towards more understanding I figured out that it isn't
> > the code that does the relocation. Even with that completely disabled there were
> > the vmalloc issues. What causes it seems to be the default of the upper limit
> > and that this changes the split between kernel and modules to 1G+1G instead of
> > 512M+1.5G. That is the reason why nokaslr has no effect.
>
> Oh! That's very interesting. There must be some assumption in Xen
> about the kernel VM layout then?

No. I think most of the changes that look at PTE and PMDs are are all
in arch/x86/xen/mmu.c. I wonder if this is xen_cleanhighmap being
too aggressive
>
> -Kees
>
> --
> Kees Cook
> Chrome OS Security
>
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