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

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Date:	Fri, 08 Aug 2014 16:35:46 +0200
From:	Stefan Bader <stefan.bader@...onical.com>
To:	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>
CC:	Kees Cook <keescook@...omium.org>
Subject: Re: [Xen-devel] Xen PV domain regression with KASLR enabled (kernel
 3.16)

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...

-Stefan


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