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

Kees Cook
2014-08-12T18:53:07Z

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?

-Kees

--
Kees Cook
Chrome OS Security